Ever wonder what the energy flow through a net meter looks like when you have solar on your house? No? Well, if you did, take a gander at the graphs above (click for a bigger version). First off – if it looks complicated, in real life, it’s not. I turn on a light switch and the light comes on, just like every other house – clouds or no clouds. The details of the energy flow simply don’t matter in daily life. Sometimes the meter spins backwards, sometimes forwards, and I pay (or get paid for) the net result at the end of the month.
But being the measurement geek that I am, I think it’s cool to see the ins and outs of the energy flow through a day. And this was a “complicated” day, with clouds, and variable loads making a messy scribble on the graphs. Here’s an annotated version:
The top graph represents household load (red) and solar generation (green). The line graphs represent power (watts) at any given time – peaks mean more watts. The filled in areas of similar color represent cumulative energy use or generation (kWh) as the day progresses. You can see that the fridges were cycling on and off while we slept, and see that I made my espresso fairly late, and that we cooked pizza this evening – those are the big and little spikes in the use graph. And you can see it was a semi-cloudy day – rather than a nice bell curve on a cloudless day, it’s got all kinds of dropouts as clouds roll in and out.
The bottom graph shows the net energy and power flow a bit better. The dark blue line shows the net power, negative (net power use) or positive (net production). The filled in lighter blue shows accumulating energy net use, or net generation. At around noon, production caught up with use for the day, and we were net-zero on energy for the day. We built up a bit of a surplus, and then it started declining again in the evening as the sun went down and the stove turned on. By midnight we’ll probably be about even again.
After all that, all it really means in the end is that for this day, we will pay (and get paid) approximately $0.00 for our energy. But we took an interesting path to get there!
By the way, these graphs come from the fantastic and free pvoutput.org.