Energy Use

Solar Panels

Our PV system consists of sixteen roof-mounted Mitsubishi 185 watt panels, for a nominal 2.96 kW DC at the panels. For PV panels, the "rated power" is for 1000 W/m2 solar intensity perpendicular to the panels at 20°C. In most of the U.S., that's roughly the intensity at midday on a very clear day from April through August. In our location, with very low humidity (no haze) and very clean air, we meet or exceed that for several hours a day through much of the summer.

We have a Fronius IG 3000 inverter with data logging. The stated efficiency is 95%, which should turn 2.96 kW DC into 2.8 kW AC at the grid. We never quite achieve this. Maximum AC power in summer sometimes reaches 2.7 kW, but 2.5 kW is a more typical AC power at solar noon. However, we did hit 2.8 kW in early April 2010. I think it's because the panels are much cooler in the spring – daily temperature max ≈60°F rather than ≈95°F – and the efficiency of the panels deteriorates with increasing temperature.

Visit the
technical page for more details of our solar electric generation.

Ten Years of PV Data
Ten year average solar panel generation
This chart shows ten-year averages for:
● Monthly generated kWh (blue)
● Monthly used kWh (red)
● Net production kWh (green)
Net production is the difference between what we generate and what we use. A positive net production (upward green bar) means we generated more than we used. A negative net production (downward green bar) means we used more than we generated.

Not surprisingly, we have positive net production in the summer (lots of sun, minimal electric use) and negative net production in winter (much less sun, more lights, some need for supplemental electric heat). Overall, we generated 97% of the electricity that we used during these ten years.

How Much Energy Does a PV Panel Generate over the Course of One Year?

This is a question I asked when trying to decide what size system we needed, but I had a hard time finding an answer. Information on peak power is easy to come by, but – allowing for weather, changing day lengths, etc. – how much total energy is produced in one year? The answer is clearly location sensitive – better in locations that get more total hours of sunshine – so our results shouldn't be applied blindly to other locations. But in the coastal mountains of central California, where winters tend to be fairly cloudy but summers are virtually full sunshine every day, the energy production of our 2.96 kW system averages 5400 kWh/year, which is 1800 kWh per year per installed kW. That should be a reasonably good estimate for much of the American Southwest, excepting areas close enough to the coast to experience summer fog.

Payback Time

Even though not quite electrically neutral, our electric bill is zero! This is because in California solar customers have time-of-day pricing. We generate excess electricity midday in summer and sell it to PG&E at a high price. Then we mostly buy it back at night in winter when prices are lowest. Strictly speaking, PG&E should send us a check at the end of the year. But guess what?

With ten year's data, we can estimate the payback time of the system. Because of time-of-day pricing, I can't compute the exact amount we save, but I estimate the PV system saves us $1000/year in electric bills. The system cost $22,600 installed. (It would be less now; the price of panels has dropped significantly.) A California energy rebate gave a $6200 credit upfront. Then there's a 30% federal tax credit. Altogether, our net cost was $11,600. A straight-line payback calculation at $1000/year gives a 11.6 year payback time. Rising energy costs will likely lower this to, perhaps, 10 years. So the system has probably paid for itself already, and it should last another ≈15 years with little or no maintenance.

Natural Gas Energy Use

Electricity is not our only energy use. We use natural gas year-round for cooking and also for on-demand hot water. In addition, we have a natural gas fireplace downstairs for supplemental winter heat. (See the Passive Solar Design page.) Our monthly bill from the gas company shows out energy use in the truly obscure unit of therms (1 therm = 100,000 BTU, for those of you who really wanted to know), but they can be converted to kWh for comparison to electrical energy.

Ten year average of natural gas energy use
This chart shows ten-year averages for monthly natural gas energy use. Cooking and hot water, which vary little throughout the year, can be determined from the summer months when the fireplace is off.

Our natural gas energy is significantly higher than anticipated: 84% of electric energy. And from December through March, the main heating months, natural gas energy use is larger than electric energy use.

As noted elsewhere, the gas fireplace is not especially efficient at delivering fuel energy into the house as heat: 65% is the rated efficiency, and it may not be that high. Knowing what we know now, it would have been worth searching for a more efficient fireplace. Or, even more sensible, replacing the fireplace with a couple of downstairs electric baseboard heaters and adding a solar panel. A well designed wood pellet burning stove can be 90% efficient at delivering fuel energy as heat, not to mention that it uses a renewable fuel rather than fossil fuel. But a hot stove is not compatible with a house full of cats.


© 2021 Randy Knight