Yesterday’s post introduced the new graph style I’ve added to HouseMon. As mentioned, I am using step-style graphs for the power (i.e. Watt) based displays. Zooming in a bit on the one shown yesterday, we get:
There’s something really odd going on here, as can be seen in the blocky lines up until about 15:00. The blocks indicate that there are many missing data points in the measurement data (normally there should be several measurements per minute). But in itself these rectangular shapes are in fact very accurate: the area is Watt times duration, which is the amount of energy (usually expressed in Joules, Wh, or kWh).
So rectangles are in fact exactly right for graphing power levels: we want to represent each measurement as an area which matches the amount of energy used. If there are fewer measurements, the rectangles will be wider, but the area will still represent our best estimate of the energy consumption.
But not all is well – here’s a more detailed example:
The cursor in the screenshot indicates that the power usage is 100.1 W at 19:16:18. The problem is that the rectangle drawn is wrong: it should precede the cursor position, not follow it. Because the measurement is the result of sampling power and returning the cumulative power level at the end of that measurement period. So the rectangle which is 100.1 W high should really be drawn from 19:18:00 to 19:18:16, i.e. from the time of the previous measurement.
I have yet to find a graphing system which gets this right.
Tomorrow: another oddity, not in the graph, but related to how power is measured…