I just borrowed a Wattson Power Meter from a friend at work, and while there's nothing special about power meters, the good folks at DIY Kyoto have put a nice touch on this one. [Standard disclaimer: I don't work for them and I haven't received any incentives from them either!]
There has been a trend of wireless power meters for the home, so they can be easily adapted to the consumer market. They solve the problem of running wires around your house - you put the sensor (or current transducer or CT) in your meter box or on a specific appliance, and the display goes somewhere convenient. Wattson has the opportunity to connect 4 CTs: 3 for 3 phases and one for renewable monitoring, or in any other configuration.
But Why? Well there were numerous reasons for me, everyone is different:
Firstly I wanted to see how much my 60L camping fridge cost to run on electricity (it runs on LPG, 240V AC or 12V DC). It turns out it draws less than 100W continuous, which would cost about $160/year on our current tariff (if I calculate correctly). That's assuming the fridge is running full time, but it has a thermostat so the actual cost will depend on the ambient temperature.
Secondly, I have a "solar aware" dishwasher. Essentially it has a thermostat as well to measure the water temperature. If you have solar hot water, you connect the hot pipe (instead of the usual cold) to the dishwasher and it doesn't use it's internal electric heater. I wanted to see if it was cost effective to pay for a plumber to put in a hot feed (and a tap for those cloudy days so I still have warm showers).
I connected Wattson, and turned on the dishwasher (full of dishes of course!). It used about 50W at first, for the actuators I assume. Then about 200W as the water filled and the "sprinklers" started. Well, 200 Watts is nothing I thought. But about 10 minutes in the heater started. The power jumped up to 1.6kW!! That's more than my split system air-conditioner! Luckily it only ran like this for about 20 minutes, but still that's a decent heater!
I calculate about $54 per year just for the dishwasher heater (I can't save the costs of the other actions of the dishwasher - unless I have solar power too!). So it looks like a plumber wouldn't be very cost effective. I'm probably looking at an $80 call out fee plus an hours labour and parts. Close to $200, which would take four years to pay back!
The final stage is to connect Wattson to my meter box, and watch the total energy consumption of my home. Just from today (Saturday with the whole family home) we use about 500W without airconditioners on, and about 3kW with them on! It was interesting to see the different appliances turn on and off (fan - 80W, washing machine - 300W, microwave - 2000W).
Wattson provides a few weeks of storage built-in, and there is software called Holmes (yes, Holmes and Wattson). Holmes is flash based, for Windows or Mac only. Luckily Wattson uses and FTDI usb-serial connection, so it shouldn't be impossible to get some data in Linux. I'll keep you posted with my success!