WO2009044126A2

WO2009044126A2 - System and method for monitoring utility consumption

Info

Publication number: WO2009044126A2
Application number: PCT/GB2008/003314
Authority: WO
Inventors: Ali Mohabbat
Original assignee: Npower Limited; Box Telematics Limited
Priority date: 2007-10-01
Filing date: 2008-10-01
Publication date: 2009-04-09
Also published as: GB2453325A; GB0719102D0; WO2009044126A3

Abstract

A system and method for monitoring utility consumption such as gas, electricity or water comprises a display unit (20) communicable with a sensor unit (10). The sensor unit receives data from a utility meter (30a,30b,30c) and stores it in memory (60), under the control of a microprocessor (40). The data is transmitted to the display unit (20) where a screen (80) displays to a user an amount of utility used, in graphical format, for example. The display unit (20) and sensor unit (10) are each operable in an active mode in which the sensor unit (10) transmits stored data to the display unit (20) at a relatively high data rate with commensurate relatively high power usage. The display unit (20) and sensor unit (10) are also each operable in one or more power saving modes in which the sensor unit (10) transmits data to the display unit (20) less frequently than in the active mode so as to draw less power. The system may also have a second power saving mode in which the screen (80) of the display unit (20) is partly or completely shut down.

Description

System and method for monitoring utility consumption

Field of the Invention

This invention relates to a system and a method for monitoring utility consumption.

Background to the Invention

Consumption of utility services such as electricity, gas or water has traditionally been measured by separate meters located on or close to a consumer's premises. A variety of technologies have been employed to measure and record the quantity of the utility used by the consumer. Normally just the incremental consumption of the product since meter installation is recorded. Utility meters are traditionally located in remote, inaccessible parts of a building (or outside of it) and are thus not easy or convenient for the consumer to read, and may for example require a torch to be visible. Even when read by a consumer, moreover, the incremental nature of the display means that no meaningful determination of utility use can be made without knowledge of a previous reading (and its approximate date) . In order to have any idea of the cost consequence of that reading, furthermore, an approximate tariff (cost per unit used) is also needed. In recent years, a demand has arisen for an arrangement that will allow a consumer to ascertain, more conveniently, utility consumption, such as for example electricity or gas. Several such arrangements are currently available. Typically, the arrangement comprises a small sender unit mounted adjacent to the meter itself, which obtains information on the amount of the utility being used by the premises at any given time, and a second receiver unit which is mains or battery powered and which has an LCD screen that displays utility usage over a user selected period of time. Provided that the receiver unit (typically) is supplied with a tariff, (either fixed, an average or split) , the LCD is able to provide an indication of the typical cost of the utility used as well. Some devices are also able to display a so-called "carbon footprint" which is calculated from a stored value of the typical amount of CO₂ emitted by the supplier per unit of utility metered. One specific utility metering system with a display is described in GB-A-2431754. Although domestic and business power use is variable in magnitude over daily and longer cycles, it tends however to be continuous . In order to ensure integrity and continuity of collected data, therefore, existing devices employ a sender unit which is mains powered from the electricity meter. There typically being no convenient power socket in the vicinity of such electricity meters, power is instead inductively coupled into the sender unit by clamping around the mains power cable into or out of the meter unit. Alternatively, electricity may be generated by a flow of gas in the case that the meter unit is a gas meter.

It is undesirable that any expertise should be necessary to install a device for monitoring energy use. Present devices, requiring clamping- to the cable entering or exiting the meter unit, can however be difficult or even present hazards to consumers particularly where it is difficult to access behind the meter cable to affix the clamp .

In order to maximize the usefulness of utility monitoring arrangements such as these, it is desirable that they be lightweight, readily portable, and easy for a consumer to install without specialist knowledge or tools. US-A-2005/0190074 describes an arrangement for collecting and displaying consumption data from a domestic meter reading system. A battery powered sender unit collects meter data and transmits it to a display unit which may also be battery powered. However such a system suffers from the limited battery life of the power supplies to both sender and display units.

Summary of the Invention Against this background, the present invention provides a system for monitoring utility consumption, comprising: A sensor unit including:

A detector for receiving, from a utility meter, detector data indicative of the quantity of the utility consumed;

A memory for storing the received detector data, or data related to that, as stored data;

A transmitter arranged to transmit output data based upon the stored data; and A power supply for providing power to the sensor unit; and

A display unit capable of communication with the sensor unit, the display unit including:

A receiver for receiving the output data transmitted from the sensor unit;

A power supply for providing power to the display unit; and

Visual display means for displaying, to a user, a quantity representative of or related to an amount of utility consumed, based upon the received output data;

Wherein the display unit and sensor unit are each operable in an active mode in which the sensor unit is arranged to control the transmitter thereof to transmit stored data to the display unit at a first series of time intervals and the display unit is arranged to control the visual display unit thereof to display the said quantity representative of or related to the amount of utility consumed based upon the received output data whilst drawing a first amount of power from the display unit power supply; and wherein the display unit and sensor unit are each operable in at least one power saving mode in which the sensor unit is arranged to control the transmitter thereof to transmit stored data to the display unit less frequently than occurs in the said active mode, and in which the display unit in that at least one power saving mode draws less power from the display unit power supply than is drawn in the said active mode of operation thereof.

Thus the system of the present invention provides for an optimized power use protocol. The display unit is configured to have an optimal compromise between the display of a maximum amount of current and historical utility consumption (which maximises the usefulness and accuracy of the information displayed to a user) on the one hand, and the minimum amount of power required both by the sender unit and the display unit to maximize battery use, for example, on the other hand. This is achieved by the use of multiple (preferably, three or more) modes of operation of the sender unit and the display unit, with one or more of these modes being power saving modes. Optionally the power saving mode(s) is/are initiated by the display unit.

In preferred embodiments, the transmitter transmits collected data from a utility meter, such as an electricity, gas or water supply meter, to the display unit at a first, relatively rapid rate. For example, the transmitter may transmit data every four seconds. In an alternative, first power saving mode, however, the transmitter might transmit data less frequently. In one embodiment, the data collected from the utility meter may be stored in the sensor unit memory and transmitted every fifteen minutes, for example. It will be appreciated that, in a device having an RF transmitter, for example, a significant percentage of the power consumption is employed in powering that transmitter. Thus, by operating the sensor unit transmitter much less frequently in the first power saving mode than in the active mode, the total power required is significantly reduced. This in turn permits the sensor unit to be operable using batteries rather than requiring a mains power supply. The first series of time intervals at which the transmitter transmits stored data in the active mode may be equally spaced. Alternatively, different time intervals may be employed (for example, in the active mode, the transmitter may transmit at t=0s, t=5s, t-30s, t=35s, t=60s etc) provided only, of course, that the average time interval in the active mode is shorter than the average time interval in the first power saving mode. Likewise, although the transmitter may transmit at predetermined intervals in the active and/or first power saving modes, it is to be understood that this is not essential to the operation of the sensor unit in accordance with the present invention. Thus, for example, in an alternative embodiment, the transmitter may only transmit, in the first power saving mode, when the sensor unit determines that it is appropriate to do so. This may be, for example, through determination that the display unit with which the sender unit is in communication is operational. By taking this approach, the sensor unit again minimises power loss by not unnecessarily operating the transmitter thereof when the transmitted data would not be received by the display unit anyway. However, in order to maintain integrity of data, in that case it is preferable that the memory of the sensor unit accumulates data until the sensor unit determines that it is appropriate or desirable to transmit that data. With modern memory availability, a significant but clearly not unlimited amount of data may be accumulated in this fashion. Once the memory limit is reached, the sensor unit can be configured to replace the earliest data on a rolling basis so as to maintain integrity of data for a meaningful period of time prior to transmission of that data, when possible, to the display unit again.

In a particularly preferred embodiment, the receiver of the sensor unit is arranged to receive an "OK to transmit" signal from the display unit, the transmitter being arranged to transmit data to the display unit, in the said first power saving mode, only upon receipt of the "OK to transmit" signal . The system may also be operable in a further power saving mode whereby data is still transmitted to the display unit but at a slower rate/less frequently than in the active mode. In that further power saving mode the display unit may partly or completely shut down power to the visual display means so as to reduce the power drawn from the display unit power supply.

Although the system is operable with a single sensor unit connected to a single utility meter, multiple sensor units may be employed to monitor, for example, separately, gas usage, electricity usage, water usage and so forth. Each separate sensor unit is located adjacent to the respective utility meter but each sensor unit is, separately, in communication with the same display unit in the preferred embodiment. Of course, if multiple utility meters are located relatively close to one another within a dwelling, it would be possible to employ a single sensor unit, or a limited number of sensor units, with multiple inputs to that sensor unit from the different utility meters .

In a preferred embodiment of the present invention, the display unit may have various input means, such as buttons or switches, to allow a user selected variety of different options to be displayed. Furthermore, the display unit may be optionally powered by a battery or batteries, and/or may be powered from a mains power supply. This allows the display unit to be fully portable. One particularly straightforward way of implementing this is to provide a display unit with a battery power supply and a "docking cradle" which is supplied with power from the mains and which can thus power the display unit when docked in the docking cradle. As a further alternative, to minimise power use by the display unit, the microprocessor thereof may be preprogrammed with an auto power down procedure, particularly when the display unit is not provided with a mains power supply. Thus the display of the display unit may automatically disappear after a preprogrammed period of time to minimise battery drain.

The display unit may optionally be provided with a further auxiliary input means such as an Ethernet port to allow communication with other external devices. Additionally or alternatively, the RF transmitter/receiver of the display unit could be adapted to allow the display unit to become a node within a WLAN. This might allow remote interrogation by a utility company, for example, for the purposes of determining an amount of utility used by a particular dwelling without requiring access to that dwelling . In accordance with a second aspect of the present invention there is provided a method of monitoring utility consumption, comprising:

Receiving, at a sensor unit, detector data indicative of the quantity of the utility consumed, from a utility meter; and

Storing the received detector data, or data related to that, as stored data; the method further comprising:

(a) in an active mode: transmitting output data based upon the stored data to a remote display unit at a first series of time intervals; and displaying in a visual display means of the display unit, a quantity representative of, or relative to, an amount of utility used whilst drawing a first amount of power from a power supply of the display unit ;

(b) in at least one power saving mode: transmitting stored data from the sensor unit less frequently than occurs in the said active mode; and causing the display unit in that at least one power saving mode to draw less power from the display unit power supply than is drawn in the said active mode. Other features and advantages of the present invention will become apparent from the following description.

Brief description of the Drawings The present invention may be put into practice in a number of ways and one specific embodiment will now be described by way of example only and with reference to the accompanying figure in which; Figure 1 shows a schematic diagram of a system for the monitoring of utility consumption.

Detailed description of the preferred embodiment Figure 1 shows, schematically, a system for monitoring utility consumption in a household for example. The system comprises a sensor unit 10 and a display unit 20.

The sensor unit 10 is mounted or otherwise located adjacent to a utility meter such as an electricity meter 30b. Modern electricity meters contain a power use detector which is able to provide, for example, a pulse output with the number of pulses over a unit period of time representing the quantity of electricity consumed.

The sensor unit 10 includes a central microprocessor 40 which controls a transmitter 50 and is in communication with a sensor unit memory 60. The sensor unit 10 is powered by a battery power supply 70.

In use, the sensor unit receives data from the utility meter such as the electricity meter 30b and converts it into an instantaneous power value. For example, the sensor unit 10 may include an on chip analogue to digital converter

(ADC) which controls data from the utility meter 30 every second. The measured voltage is then converted to an instantaneous power value using a scaling and conversion formula which does not form part of the present invention and which is preprogrammed or user-programmable (in a manner to be described below) and stored within the sensor unit 10. The data is collected and stored in the memory 60 and/or transmitted to the display unit 20 in a manner to be described in further detail below. The microprocessor 40 may also integrate the data once obtained from the utility meter and may store the integrated data as well or instead within the memory 60.

The display unit 20 is designed to be portable and hand-held. As seen in Figure 1, the display unit 20 has a screen 80 for displaying information relating to utility consumption, a series of buttons 90 surrounding the screen to allow selection of various different options for display, and, internally, a central microprocessor 100 which communicates with the screen 80 and an RF transmitter/ receiver 110. Power is supplied either via a battery power supply 120 or a mains power supply 130 depending upon the location of the display unit at any given time. More particularly, the display unit itself may only be battery powered to maximize portability thereof, with mains electricity supplied via a docking cradle (not shown) connected to an electricity socket. Thus the batteries of the display unit 20 could be rechargeable with the docking cradle supplying the current to charge the batteries from the mains .

The sensor unit and display unit are, in the preferred embodiment, configured to communicate with one another using the MicrelNet 868MHz radio system. The display unit 20 acts as a master/host for the radio network and the sensor unit 10 communicates collected real time data from the utility meter 30 to the display unit 20 acting as a host unit, at a variety of different intervals which are selected to optimise power consumption particularly at the sensor unit 10. The display unit 20, by being configured as a master/host for the radio network, and comprising buttons 90, can allow user setup and management of the system overall .

The system operates in a number of different modes, designed to optimise power consumption of both the sensor unit 10 and the display unit 20. In the case of the sensor unit 10, the battery power supply 70 makes optimization of power use by that sensor unit 10 desirable to avoid constant replacement of batteries. However power use optimization by the display unit 20 is also desirable; when the latter is removed from the docking cradle it has a limited operational life before its batteries expire and need replacing and/or recharging. Even when the display unit 20 is supplied by mains power, however, via the docking cradle for example, it is beneficial that power consumption is optimized/minimized since it is undesirable that a device which has, as one purpose, an intention to increase awareness of energy use by a consumer, should itself draw unnecessary power.

Specifically, the display unit 20 has a plurality of operational (display) modes, and these in turn govern the mode of operation of the sensor unit 10. In a first, active mode of operation of the display unit, the screen 80 may be illuminated and display numbers and/or graphics as shown in Figure 1. The buttons 90 may also be illuminated. In a second, "screensaver" mode, much but not all of the display unit's functionality may be limited; for example in this mode the screen 80 may simply show the time of day faintly and the illumination of the buttons 90 may be switched off. In a third, "sleep" mode, the screen 80 is blank. Thus in the "screensave" and "sleep" modes, the display unit 20 draws less power from the power supplies 120, 130 which is particularly beneficial when the display unit 20 is disconnected from the mains power supply 130.

In still a further (fourth) mode, the display unit 20 may be switched off, either through deliberate action by a user to turn the unit off, or alternatively when the unit does not have access to mains power supply and the battery power supply 120 is drained.

The sensor unit 10 operates in a variety of different modes to optimise power use of the battery power supply 70 thereof. To determine the appropriate mode of use, the sensor unit 10 sends out a comms/synch packet at regular intervals such as every fifteen minutes to ensure that the sensor unit 10 and display unit 20 remain synchronised. The display unit 20 returns an ACK signal via the RF receiver/transmitter 110 to the sensor unit 10 when it correctly receives the comms/synch packet. Assuming that the sensor unit 10 has successfully received an ACK from the display unit 20, it operates in "active" mode. Here, data is collected in or near real time from the utility meter 30 and a maximum value reading is sent every four seconds to the display unit 20 for display there in a manner to be described below.

If the sensor unit 10 correctly receives an ACK but is also informed by the display unit 20 that it is in screensave mode, then the sensor unit 10 enters the second, screensave mode which requires less power to be drawn from the battery power supply 70 over a given period, than is drawn therefrom in the first active mode of operation over a similar time period. The sensor unit may be informed of the change of display mode at the display unit 20 by transmission, from the display unit 20, of a mode switch packet or trigger signal. This may be acknowledged by the sensor unit 10 (which may return another ACK signal to the display unit when a mode switch signal is successfully- received) though such an acknowledgement is not essential. The switch from active to screensave mode is typically automatic and based upon the user not interacting with the display unit 20 (by depressing any buttons 90) for more than a predetermined period of time that may be factory preset but user amendable via the display unit 20. A suitable default may be 10 minutes. In this second, screensave mode, data is still accumulated from the utility meter 30 but is sent less frequently to the display unit (which is, in the screensave mode, no longer displaying real time use of the utility anyway) . Although the sample rate from the utility meter 30 may remain the same, so that the total amount of data sent remains the same (though sent less frequently in the screensave mode) , alternatively, the data collected at the sensor unit 10 from the utility meter 30 may instead be integrated or averaged so that less data is transmitted in the screensave mode than in the active mode. Nevertheless, it will be understood that, in the currently preferred embodiment, data continues to be sent at regular intervals such as every fifteen minutes in the screensave mode.

In the third ("sleep") mode of operation of the display unit 20, where the display unit 20 is on standby and the screen is switched off, still a further data transfer interval is employed. In this standby mode, data is accumulated at the sensor unit 10 but only transmitted every hour, for example. However, again, preferably the data is transmitted on a regular basis (every hour) . As with the second, screensave mode, the sensor unit 10 is instructed to enter the third (sleep) mode of operation by the display unit 20 which sends out a suitable trigger signal to the sender unit 10. Again this may optionally be acknowledged by the sender unit 10. The switch into the sleep mode may be as a result of a further time period having elapsed since the display unit 20 was left engaged with (eg, more than 2 hours since any button 90 was depressed) , or may be during set times (eg 2am to 7am) , or by manual intervention by the user (by selecting the mode through depression of one of the buttons 90) . Again the time periods of inactivity or time of day may be factory preset but user amendable via the buttons 90.

The final, fourth mode of operation of the display unit 20 is entered by the sensor unit 10 when no ACK signal is received. Typically, the sensor unit 10 would send a comms/synch packet every fifteen minutes as described above. If after five attempts no ACK has been received, then the microprocessor 40 of the sensor unit 10 causes the sensor unit 10 to enter "no host" mode. In this mode, the sensor unit 10 accumulates data from the utility meter 30 but does not transmit it. Instead, the sensor unit 10 continues to broadcast a comms/synch packet every fifteen minutes until an ACK is received back at the sensor unit 10. At that point, the sensor unit 10 recognises a host reconnection . Following reconnection to the display unit 20 following the no host mode, the sensor unit 10 sends (with ACK) data stored on an hourly basis since the last successful data transmission. Alternatively, or additionally, of course, the display unit 20 may be provided with an "on/off" button which, when depressed, will shut the display unit 20 down. This will result in no ACK signal being generated so that the sender unit 10 will eventually enter the "no host" mode. In the currently preferred embodiment, the memory 60 of the sensor unit 10 is capable of storing twenty-eight days' worth of utility data, accumulated hourly. In order to maintain data in as useful a form as possible, however, the earliest stored hour is overwritten with the latest stored hour data once the memory becomes full after twenty-eight days. Thus, if the display unit 20 is inoperable for more than twenty-eight days, upon being switched on again, a rolling twenty-eight day window for the preceding twenty- eight days before recommencement of operation of the display unit 20 is sent to the display unit. The data recordings stored in the memory 60 of the sensor unit 10 are time and date stamped to assist with future data upload recording.

In order to ensure integrity of data stored in the memory of the sensor unit 10, the memory 60 is preferably non-volatile.

Thus it will be understood that the system provides for an optimized power use protocol. The display unit 20 is configured to have an optimal compromise between the display of a maximum amount of current and historical utility consumption (which maximises the usefulness and accuracy of the information displayed to a user) on the one hand, and the minimum amount of power required both by the sender unit 10 and the display unit 20 to maximize battery use, for example, on the other hand. This is achieved by the use of multiple (preferably, three or more) modes of operation of the sender unit 10 and the display unit 20, with one or more of these modes being power saving modes initiated by the display unit 20.

The display unit 20 has a screen 80 which is preferably an organic light emitting diode (OLED) . This arrangement minimises power consumption. The screen may be touch sensitive. Additionally or alternatively, surrounding the screen may be provided a plurality of buttons 90 which allow access to various different modes of use of the display unit 20.

In the currently preferred embodiment the display screen 80 is able to display, variously, a clock, the current room temperature, energy consumption as both a number and/or a graph, the cost of fuel in pence per hour or pounds per day, the applicable tariff rate (peak or off-peak for instance), and other functions. Desirably, the display unit 20 is arranged to display, upon the screen, an icon confirming that there is an operable link between the display unit 20 and the sensor unit 10.

The display unit is configured to allow input of data by a user, or other programming. This allows the user to update tariffs (such as the unit rate or the time bands) , to amend the time of day, and so forth. The buttons 90 on the display unit 20 are also able to allow configuration of the display modes described above (active mode, screensave mode and sleep mode) . For example, the user can programme the period, which is set by default to ten minutes, after which the display unit 20 enters "screensave" mode where the screen display powers down and the faint clock is instead displayed. The period after which the system goes into "sleep" (standby) mode may also be user defined but is set at default to two hours. This mode may be entered manually again by pressing one of the buttons 90 on the display unit 20.

In addition to local storage of the programmed information provided by a user to the display unit 20, optionally the display unit 20 may also transmit some or all of that inputted information back to the sensor unit 10 across the radio network. The display unit 20 is capable of displaying energy consumption over a variety of different periods (daily/weekly/monthly etc) .

As a further development, and to improve still further the functionality and usefulness of the display to the user, the sensor unit 10 or the display unit 20 may be configured to store an updatable value representative of the peak instantaneous consumption over a preceding predetermined rolling period. For example the sensor unit 10 may store the maximum instantaneous power supplied to the premises over a preceding 7 day period. The display unit 20 may then display, graphically or numerically, the present instantaneous utility use as a proportion of that stored maximum instantaneous power. For example, the display unit may provide a graphical distinction between low use (0-25% of previous 7 day instantaneous peak) medium use (25-50% thereof) and high use (50% +) , by illuminating different coloured lights on the docking station, by increasing a "halo" around a picture of the globe on the screen, or (when the display unit 20 is set to show the current cost of utility being consumed) by increasing or decreasing the height of a graphically displayed pile of coins.

The display unit 20 maintains data received from the sensor unit for a short period of time so as to prevent loss of standing data or consumption history in the case of loss of power to the display unit. This avoids loss of historic data particularly when the sensor unit 10 has already transmitted that data to the display unit 20 and has thus overwritten that sent data in the sensor unit memory 60. In the case where the sensor unit 10 is configured to receive utility consumption data from multiple different utility meters 20a, 30b, 30c, the buttons 90 on the display unit 20 may also be able to allow the user to select which of the data sources (electricity, gas or water, for instance) is displayed at a given time.

Although a specific embodiment has been described, various modifications will be apparent to the skilled reader. For example, although in Figure 1 a plurality of utility meters 30a, 30b, 30c are shown connecting to a single sensor unit 10, in practice, particularly where the utility meters are spaced apart around a dwelling, in may be preferable to employ separate sensor units, one per utility meter. In that case, each sensor unit 10 operates as a node of the radio network, where the display unit 20 acts as the master unit. For example, in addition to obtaining electricity data from an electricity meter, some modern gas meters also have a sensor which is capable of producing an electrical output based upon the amount of gas consumed. This can, as with the electricity meter operation as described above, be converted into a digital signal for storage as consumption data at the sensor unit 10 for that gas meter 30a. Water meters likewise provide an electrical output indicative of water consumed.

Moreover, although the foregoing embodiment is described in terms of a domestic utility supply, it will readily be understood that the invention is not so limited and that the system could equally be installed so as to provide information to a business user. Indeed with growing business awareness of environmental issues the display unit could be mounted in a business reception area so as to indicate to staff and visitors that their energy consumption/carbon footprint is of concern to the business.

Claims

1. A system for monitoring utility consumption, comprising : a sensor unit including: a detector for receiving, from a utility meter, detector data indicative of the quantity of the utility consumed; a memory for storing the received detector data, or data related to that, as stored data; a transmitter arranged to transmit output data based upon the stored data; and a power supply for providing power to the sensor unit; and a display unit capable of communication with the sensor unit, the display unit including: a receiver for receiving the output data transmitted from the sensor unit; a power supply for providing power to the display unit; and visual display means for displaying, to a user, a quantity representative of or related to an amount of utility consumed, based upon the received output data; wherein the display unit and sensor unit are each operable in an active mode in which the sensor unit is arranged to control the transmitter thereof to transmit stored data to the display unit at a first series of time intervals and the display unit is arranged to control the visual display unit thereof to display the said quantity representative of or related to the amount of utility consumed based upon the received output data whilst drawing a first amount of power from the display unit power supply; and wherein the display unit and sensor unit are each operable in at least one power saving mode in which the sensor unit is arranged to control the transmitter thereof to transmit stored data to the display unit less frequently than occurs in the said active mode, and in which the display unit in that at least one power saving mode draws less power from the display unit power supply than is drawn in the said active mode of operation thereof.

2. The system of claim 1, wherein the sensor unit transmitter is controlled so as to transmit in the active mode at a first series of predefined time intervals, but wherein the transmitter is controlled so as to transmit in a first power saving mode only when the sensor unit determines that it is appropriate to do so.

3. The system of claim 2, wherein the display unit is arranged, in the said first power saving mode, to generate an OK to transmit signal only when the display unit is switched on.

4. The system of claim 3, wherein the display unit comprises a display unit transmitter and the sensor unit comprises a sensor unit receiver and wherein the sensor unit is arranged to control the transmitter thereof to transmit output data only when an OK to transmit signal has been exchanged successfully between the sensor unit and the display unit.

5. The system of claim 4, wherein the sensor unit is arranged to generate an OK to transmit signal and to transmit that to the display unit, but where the display unit is configured to acknowledge receipt of that OK to transmit signal when the display unit is switched on.

6. The system of claim 1, wherein the sensor unit is arranged to transmit stored data to the display unit at a second series of time intervals during a first power saving mode, the average separation of which time intervals is longer than the average separation of the time intervals in the first series thereof in the said active mode.

7. The system of claim 6, wherein the display unit is arranged to switch off the supply of power to the visual display means or a part thereof in the said first power saving mode so as to reduce the amount of power drawn from the display unit power supply.

8. The system of claim 7, wherein the display unit is configured automatically to switch from the said active mode to the said first power saving mode after a non-zero period of time.

9. The system of claim 8, wherein the display unit has a user input means, and wherein the display unit is configured to switch from the said active mode to the said first power saving mode a predefined period after a preceding activation of the user input means .

10. The system of claim 8, wherein the display unit has a user input means, and wherein the display unit is configured to switch from the said active mode to the said first power saving mode upon instruction to do so by activation, by a user, of the user input means.

11. The system of any of claims 2 to 5, wherein the sensor unit is controlled so as to transmit stored data to the display unit at a second series of time intervals during a second power saving mode, the average separation of which time intervals is longer than the average separation of the time intervals in the first series thereof in the said active mode.

12. The sensor unit of any preceding claim, wherein the sensor unit power supply generates power for the sensor unit with one or more batteries.

13. The sensor unit of any of claims 2 to 10 wherein the memory unit is configured to retain the stored data in the first power saving mode until either it is transmitted by the transmitter or a maximum time period is reached and the data has still not been transmitted.

14. The system of any preceding claim, further comprising: a second sensor unit located separately to the first sensor unit proximate a second utility meter, and being arranged to receive detector data indicative of the quantity of the utility used from that second utility meter.

15. The system of any preceding claim, wherein the display unit further comprises a switch for selectively displaying, upon the visual display means of the display unit, one or more of the following representations of utility consumption: (a) the quantity of the utility consumed over a selectable one of a plurality of different time periods; (b) the approximate cost, to the user, of the utility consumption over a specific time period; and (c) where the utility being monitored is a form of energy, the approximate volume of carbon dioxide that has been emitted over a specific time period as a consequence of the consumption of that utility over that time period.

16. The system of any preceding claim, wherein the display unit further comprises an auxiliary input means for allowing the said display unit to communicate with other devices via a communications means such as Ethernet or a Wireless link.

17. A method of monitoring utility consumption, comprising: receiving, at a sensor unit, detector data indicative of the quantity of the utility consumed, from a utility- meter; and storing the received detector data, or data related to that, as stored data; the method further comprising: (a) in an active mode: transmitting output data based upon the stored data to a remote display unit at a first series of time intervals; and displaying in a visual display means of the display unit, a quantity representative of, or relative to, an amount of utility used, whilst drawing a first amount of power from a power supply of the display unit; and (b) in at least one power saving mode: transmitting stored data from the sensor unit less frequently than occurs in the said active mode; and causing the display unit in that at least one power saving mode to draw less power from the display unit power supply than is drawn in the said active mode.

18. The method of claim 17, further comprising: receiving, at the sensor unit, a trigger signal from the display unit; and switching from transmission in the said active mode to transmission in a first power saving mode in response to the received trigger signal.

19. The method of claim 17, further comprising, in a first power saving mode, transmitting output data to the display unit only when the sensor unit determines that the display unit is operable to receive the said output data.

20. The method of any of claims 17 to 19, further comprising: retaining the stored data in the first power saving mode until either it is transmitted by the transmitter or a maximum time period has elapsed and the data has still not been transmitted.