Not reaching 18: A little photovoltaic update

On April 25, I posted “Little milestones,” about reaching two (or three) milestones with our solar photovoltaic system. (Again: “photovoltaic” both because it’s a great polysyllabic word and because there are quite a few solar systems around here that *aren’t* photovoltaic–that heat water for pools and the like rather than generating electricity.)

At the time, the milestones were that we’d reached–or almost reached–17 kWh in one day, 70% generation (that is, we’d generated 70% of all electricity used since the system went live in November 2009) and a “negative tick” on the monthly PG&E statement–the first month we generated more than we used.

I also thought it might take three weeks or so to get from 17 kWh to 18 kWh for a peak day, hoping that we might eventually get to 20 kWh. And noting the difficulty of moving from 70% to 75% to 80%…since each percentage increase gets tougher.

Consider: At 75%, you’ve generated three kWh for each kWh you get from the utility. At 80%, you’ve generated four kWh for each utility-provided kWh. At 85%, you’re approaching six kWh for each utility-provided kWh–and at 90%, you’ve generated nine kWh for each one you get from the utility. That gets tough.

I believe our target was 80% over the course of a year; that’s usually what companies aim for in designing photovoltaic systems.


The 18 kWh day has proved elusive, and I now suspect that unless we fully wash the panels (not just spray them down, which we’ve done), we may never get there (since Sunday was presumably our best chance). Spraying down seems to have done almost no good. Our peak generation seems to run around 2,200 watts, not the 2,350 peak the system should do. That’s the not-so-good news.

On the other hand, as of the last time I checked, we’re at 86%: That is, we’ve generated a little more than six kWh for each kWh we’ve used from the utility. (We peaked at about 680 kWh of PG&E power; that’s down to 380 kWh net as of this morning, since we’ve generated 300 kWh more than we’ve used over the last three months.)

I’m thinking we might get to 90% before the end of summer, depending on how much AC we use (so far, we’ve barely had it on at all, and we have an 18SEER system set at 80F during the day). Will we reach the end of the “true-up year” (end of October, basically) at 90%? Hard to say.

Again, a caution: Photovoltaic results vary wildly depending on suitable roof (or ground) areas, extent of cloud cover where you are, etc., etc., etc…. If a company says you’ll earn back your investment in six years or eight years, I’d triple-check the figures. Energy-efficient usage is still the best way to save money on electricity, and we’d already done a lot of that.

[Are we happy we installed the system? Absolutely.]

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