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Keep track of electricity use with the Cent-a-meter Are you conscious that your electricity bills are higher than you’d like? Would you like to be able to monitor your total household consumption at any time? Now there is an easy way, with the Cent-ameter Wireless Electricity Monitor. T he Cent-a-meter is a small LCD module which can fish-tank and all the gear on standby – nothing much really... Since then, I have seen much larger readings on that sit any-where in your home and it can display your instantaneous power consumption in kilowatts little display and it really does make you conscious of the (kW), greenhouse gas emissions (kilograms/hour) or cost power being used (and its cost!). For example, it can draw attention to a radiator or a in cents/hour. It computes this information from data sent to it by a unit dryer left running long after it needs to. Or lights running connected to your home’s switchboard. The data is sent by in rooms where no-one is present... And it can make you very conscious of just how much an RF link at 433MHz so you can monitor your electricity standby power you are using when nothing at all is being consumption from anywhere in your home. And while this is very convenient, it can be quite used, supposedly. All that electronic gear with remote alarming at times, to see just how much power is being controls really can cost you quite a lot of money to run over the course of a year. used. For example, on the first evening after it had been installed, I was surprised to see the power reading in excess How it works A current transformer is clipped over the main supply of 5kW. Why? Nothing much was going on, no washing machine, lead in your switchboard (there is no actual electrical dishwasher or fridge running was running at the time. But connection). This transformer is connected to a what was running was a microwave 433MHz transmitter module mounted oven in the kitchen and a 2400W radiator in the family room, plus a few lights, Review by LEO SIMPSON just outside the switchboard. It digitises www.siliconchip.com.au November 2003  21 The current transformer, shown in-situ at left and openedout ready for installation above, simply clips over the Active mains wire feeding the main supply meter and/or main switch. While no connection is made to any live wires, the fact that the switchboard needs to be opened up means that a licenced electrican should install the current meter. Care must be taken that the thin cable to the transmitter is not severed or shorted by the fuse box door when closed. Because off-peak hot water operates under a different (usually much lower) tariff, this is not normally measured (however, you could have the Cent-a-meter across the offpeak hot water service only and enter its tariff to find out what that costs you. the reading from the current transformer and sends it as a serial data burst once every six seconds to the LCD receiver module in your house. The LCD module then computes the power consumption and displays it as noted above. Note that only the 240VAC supply current is monitored, not the voltage, so the displayed power is computed with an assumed input voltage. This may be set to 110V, 220V, 230V, 240V or 250V. For example, in my home the mains voltage seems to sit at between 245 and 250VAC so it would be appropriate to set the unit at 250V. The factory (default) setting is 240V. Also note that since the Cent-a-meter does not monitor voltage, it makes no allowance for distortion in the 50Hz mains supply waveform or power factor of the load. It just calculates the product of the measured current with the selected voltage (eg, 240V) and displays the result as power. However, we have been informed by the designer that the current measurement is a true RMS value. The display resolution is .01kW (ie, 10W) and overall accuracy is largely dependent on that of the current transformer. This is specified as <5% for currents between 3A and 71A, <10% for currents between 1A and 3A and not specified for currents below 1A. Nevertheless, the Cent- The three measurement options for the Cent-a-meter: at left, it is showing the power being consumed at that instant (incidentally, by a 1kW electric radiator). In the middle is the amount of greenhouse gas that power useage generates. Finally, at right, the most important figure of the lot – what that power is costing per hour. We used a tarrif of 10.7c/kWh, as shown bottom right. This has recently increased slightly. 22  Silicon Chip www.siliconchip.com.au a-meter is a very useful indicator of instantaneous power demand. To display the electricity cost per hour, a fixed tariff is assumed and again, the factory default setting is 12c per kilowatt-hour. This can be set to your local tariff which for Sydney is presently between 10.95c and 11.35c/kWh (including GST), about the cheapest in Australia. Some other states are much higher, with Adelaide, South Australia, paying as much as 18.88c/kWh. Whether you use the factory default tariff or your local tariff, the Cent-a-meter ignores the lower tariff for the first 1750kWh block (or whatever the level is). It does not need this information because it only displays the present cost of electricity being used; it does not make a calculation for power used to date. Greenhouse gases As already noted, the Cent-a-meter can display greenhouse gas emissions for your current level of power usage. This is assumed to be 1kg of greenhouse gases (CO2 etc) per kilowatt-hour. However, as with the other defaults, you can plug in other values, if required. The LCD module also alternately displays the room temperature and relative humidity and continuously displays the electricity tariff. Monitoring 3-phase power Most home installations will be single-phase and even then they probably won’t be set up to monitor off-peak hot-water electricity consumption since that is normally a much lower tariff. But what if your home uses a 3-phase instantaneous hot-water heater or perhaps a big air-conditioning system? In that case, the Cent-a-meter needs to monitor the current in all three phases and that means extra current transformers are required. The transmitter module has provision for three current transformer inputs for this very reason. By the way, the transmitter module runs from two AA alkaline cells and these are expected to last about 12 months. Three AA cells are used in the LCD module and interestingly, their life can be extended by changing the update rate from once every six seconds to once a minute. Presumably the saving comes about because the internal microcontroller stays “asleep” for longer periods. Range of the transmitter module is stated to be up to 30 metres in the open. Certainly there was no problem with range in my own largish 3-storey home so there should not be any problems in this regard. Overall, the Cent-a-meter is a very well conceived product. Designed in Australia, with patent pending, it is likely to be very popular both here and overseas. It retails for $149 including GST. Installation cost is extra, with the company recommending that a licenced electrician do the job. We expect that Cent-a-meter will be widely available from electrical wholesalers and hardware stores. It is presently available from AGL Energy shops. It is distributed throughout Australia by Gerard Industries Pty Ltd (Clipsal). Further information is available at www.clipsal.com.au and www.centameter.com.au SC www.siliconchip.com.au November 2003  23