Wednesday, August 11, 2010

Accuracy of Wireless Home Electricity Monitors

We often get enquiries about the accuracy of wireless home electricity monitors, so here is a summary of our findings.

Almost all real-time displays available in Australia and New Zealand currently get their data from fitting a CT (Current Transformer) clamp to a cable.

The current flowing through the cable generates a magnetic field. The magnetic field subsequently generates a current in the CT Clamp, which is measured and the data is sent to the display via the transmitter.

As a result, because a CT Clamp cannot take into account such things as Power Factor and varying voltage, it can never be 100% accurate. From experiments with the Current Cost ENVI we ave found it to be 1 – 2% accurate up to 1000W, and 3-8% from 1kW – 10kW (with the accuracy decreasing as the power increases).

The Current Cost ENVI default voltage is 240V, so in areas where the voltage is noticeably different, the accuracy will decrease. However, the ENVI-R (due to be available in October / November) comes with a new transmitter that allows multiple voltages to be set (200V, 210V, 220V, 230V, 240V, 250V, 260V).

With regards to Power Factor, some people make a big fuss about this. However, most household appliances have a Power Factor of close to 1 (the assumed PF in the Current Cost ENVI and most other wireless electricity monitors). It is only in commercial environments where there are large electric motors in operation does this become a real issue.

As an example, the Graham Restaurant in Port Melbourne has an ENVI connected up. Since they’ve connected it, their power consumption has reduced from approx 440kWh per day to 260kWh per day – their monthly bill is usually within 3% of the kWh reading that the ENVI has determined. 260kWh a day is a LOT of power – the average house should be using less than 10kWh per day. http://smartnowenvironmental.blogspot.com/2010/05/turn-off-your-coffee-machines.html

However, all the points above are largely irrelevant in my opinion. If you’re after accurate measurement of power used, you’ve got to spend $thousands for an expensive bit of kit to be installed to do this. The Current Cost ENVI http://www.smartnow.com.au/current_cost_envi_store.php costs $139.95 in Australia, and is compatible with Google PowerMeter Australia and New Zealand http://www.google.com/powermeter/about/about.html. This will tell you clearly that your electric heater is drawing 200 times as much power as your energy efficient light globe, and hence turning lights off in your house is not going to have anywhere near as much impact on your bill as turning off your electric heater / excluding drafts / insulating your house.

It’s all about behvioural change – these electricity monitors provide the information people need to be able to make educated decisions about their power consumption and importantly at this time of rising prices, be able to do something about them in an informed and pragmatic fashion.

6 comments:

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  2. Hey does anyone have the devboard or know where to get one? It seems like in the UK there is (or was) support for the techies who want to hook up other sensors. But in the ROW (of which the USA is part of in this case) it's just buy it but don't look for any really technical discussion beyond just software.

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  4. Thanks for posting this! I'm trying to find information on Electricity Monitors and this has definitely helped me in this process.

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  5. thank you for your post. Accuracy is not very good in home energy monitors. That's why i'm using odenergyhome (www.odenergyhome.com). This device is like a professional one but for homes.

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  6. with the passage of time this information is doing good for me.

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