Equipment Used
This is a simple project to measure the amount of electrical energy I use in my house. The total cost of the hardware used is below 300 USD (The starter kit is a bit expensive, but you can buy the needed parts separately, since you won't need the LCD-stuff for this one :).
I use a starter kit (#1) from Phidgets Inc.. This kit contains a light sensor which is the main component that interfaces with the built-in house electricity meter.
Implementation
The light sensor is duct taped(!) in front of the watt meters LED which blinks 1000 times pr kWh used. This sensor is then connected via the interface kit to one USB-port on my Nokia 770 (Great Linux-device!). Here I run a small program that sends a packet to my server whenever the LED triggers (770 has wifi).
On my server I listen for the packets and by measuring the time between two pulses, I can calculate the amount of Watt-hours. (Which is basically (60/t)*60.) I then have MRTG pick up this value and plot it to a graph as seen below.
The numbers are Watt-hours on a five minutes average. In the graph we can see some spikes that indicates fridge, heating cables etc. The bigger spikes would be kitchen oven or hot water tank.
I've also written a simple desktop application where I can see the usage with a 10-second average. This value is a bit more "jumpy" due to the short measure time.
Code!
And finally (or regrettably) some source code!
- Nokia 770 Program (Phidgets, C. Requires libs from Phidgets.com)
- Simple UDP Server (PHP)
These two can easily be combined to create a more optimized measurement. By calculating the Watt-hours on the Phidgets-device it self, you will eliminate the wifi-lag which might be a couple of milliseconds or even seconds in worst cases. Therefor by reporting Watt-hours directly from the device to a graphing service, the data may be more accurate.
More pictures are available here.
(C) Harald Nesland 2008. Contact.