Solar System: A First Look at the Data

My new solar panels, installed two months ago, have been working hard during the beautiful days of late summer and early fall.

Although the days have gotten shorter, the noon sun faces the panels most directly at this time of year—thanks to my steep roof. I can now report that under a clear sky and direct sunlight, the output of my system is typically about 950 watts. That’s the alternating current coming out of the microinverters, as reported by the monitoring system. For comparison, the nameplate rating on the panels themselves is 280 watts each, or 1120 watts total. I’m not sure how much of the difference between 1120 and 950 is due to atmospheric conditions, and how much is due to the losses in the DC-to-AC conversion.

To get an idea of the variability of the power output, you can look at the data on the Enphase Enlighten site. Here’s a plot of all the data from September on a single horizontal axis (click to enlarge):

This graph shows instantaneous power in watts. To calculate the total energy produced, you need to multiply the power by the time elapsed and then add that up for each time interval (the system records data in five-minute intervals). If the time is expressed in hours, then the energy will be in watt-hours; divide by 1000 to convert to kilowatt-hours (kWh), the power company’s billing unit.

On my system’s best day so far, September 18, its total energy output was 6.5 kWh. On its worst day, just two days earlier, the output was only 0.3 kWh. Fortunately, I live where the skies are not cloudy all day—at least not very often—so the system is averaging about 5 kWh per day.

I use some of that solar-generated electricity as it comes off the panels, but most of it gets pushed onto the grid for my neighbors to use. Then, at night and at other times when I need more power than the panels are producing, I pull what I need off the grid. The power company’s meter, on the back of my house, separately measures the power flowing in both directions, records both amounts of cumulative energy, and blinks between displaying the two amounts:

I took these photos on the morning of October 17, when the incoming energy (since the meter was installed on August 27) had reached 100 kWh (left) and the outgoing energy had reached 200 kWh (right).

By combining the solar monitor data with the net meter readings, I can construct a comprehensive picture of the energy flows through my house. Here’s the picture for the calendar month of September:

During this time period the solar system produced 151 kWh of energy, while the net meter reported that I pushed 114 kWh onto the grid. Therefore I must have used the other 37 kWh directly, as it was being produced. Meanwhile, the net meter reported that I pulled another 58 kWh off the grid, so my total household use was 95 kWh. (My usage is lowest in spring and fall, higher in the summer, and highest in the winter.)

Fortunately, the power company (under direction from the Utah Public Services Commission) lets me accumulate credits for energy pushed onto the grid, and apply them toward future months when I’ll use more energy than I produce. Here’s a copy of my first net-metering bill, covering the end of August and the beginning of September:

As you can see, they actually applied 32 kWh of my credits to the final reading off the old meter (which couldn’t distinguish incoming from outgoing energy, so it “charged” me for some of the energy I produced from August 19-27). Even so, I ended the billing month with 16 kWh of credits, and I have quite a bit more than that now.

I’m still getting billed the $6 “basic charge” that everyone pays for being connected to the grid, plus a $2 “minimum charge” for not using any (net) electricity. (So in effect, the basic charge is really $8 and they give you your first $2 worth of electricity for free. That’s not much electricity, but this practice still bugs me.) Add on the taxes and surcharges and my total bill comes to just over $9.

It’s only fair that I have to pay to be connected to the grid, because I really do depend on it. Here, for example, is a detailed plot of my solar production on the best day so far, with my “typical” electricity use superimposed:

The big spikes are from cooking: a pancake breakfast, toasting bread for the lunch I packed in the morning, and a pretty big meal in the evening. The little bumps that repeat about once an hour are from the refrigerator cycling on and off. There’s a bunch of miscellaneous activity in the evening, mostly from lights and my computer. Last but not least, there’s a baseline of about 40 watts that I'm using 24/7, for my modem, router, clock, smoke alarms, smart thermostat, solar monitor, and the electricity monitor that took this data.

(That electricity monitor is the Efergy Elite Classic and Engage hub system, which I installed soon after the solar panels. It’s a marvelous tool, and I really wish I had installed it earlier. But I also wish I had paid another $25 for the version that measures true power, because my microinverters have a nontrivial power factor that fools the Efergy Elite Classic, especially at night. Unfortunately, even Efergy’s “true power” meter apparently can’t measure the direction of energy flow, so it would give confusing data when my solar panels are active during the day. There are competing brands that lack this drawback but I haven’t tried them. In any case, I’ve had to manipulate my Efergy data quite a bit to produce the “typical” usage graph shown above.)

Because I use so much electricity when sunlight is scarce or absent, I can hardly claim that my home is 100% solar powered. I still depend very much on Rocky Mountain Power’s coal- and gas-fired power plants, which are steadily pumping carbon dioxide into the atmosphere and contributing to global warming. Consequently, I don’t consider my solar panels to be a license to waste electricity. Rather, they’ve inspired me to better understand and minimize my electricity use.

Here, then, is an estimated breakdown of my daily household electricity use, averaged over the seasons:

I obtained these estimates through a variety of measurements using my power company’s meter, my Efergy monitor, and a few handy Kill-a-watt meters. Even so, there’s a lot of guess-work involved in getting these annual averages, especially for seasonal contributions like heating and fans. I’ll have better data on heating after my first winter with the new smart thermostat.

My total household electricity use, as reported earlier, averages about 4 kWh per day. That’s quite a bit lower than the per-capita average here in the U.S., but not so different from most other industrialized countries. Notably absent from my household are such unnecessary luxuries as air conditioning, a second refrigerator or freezer, an electric clothes dryer, a television, or a hot tub.

Not everyone is in a position to invest in rooftop solar panels, but everyone can work to cut their unneeded electricity use—and save money in the process. As Mr. Money Mustache says, “Measure everything, then get angry at waste.”