Gas station without pumps

2015 August 10

I’ve got solar power

Filed under: Uncategorized — gasstationwithoutpumps @ 20:14
Tags: , ,

This summer I finally bowed to my wife’s wishes and had solar panels installed on our garage.

They make no sense economically—our electricity bill is only $420/year and the panels cost us about $20k—for a payback period of about 45 years (it will be a little less after tax rebates).  Of course, that ignores the maintenance costs—the inverters aren’t designed to last 45 years and will need replacing well before then, and the panels need to be cleaned once or twice a year.

We don’t use a lot of electricity (usage before solar installed):

Pattern of electricity use by month.  Far more electricity is used locally in the winter for lighting than in the summer—air-conditioning is rare here.  My peak usage was 300kWh in January and my minimum was 163kWh in May.

Pattern of electricity use by month. Far more electricity is used locally in the winter for lighting than in the summer—air-conditioning is rare here. My peak usage was 300kWh in January and my minimum was 163kWh in May.

Our solar installation has 8 Sunviva Optimus 270-60-4-1B0 60-cell panels, each nominally 270 W, but the highest output I’ve seen from one is 237W.  The observed peak is less than the rating, because the panels are not getting 1000 W/m2 at 25°C, which is what the panels are tested at. The panels are not optimally oriented to the sun, so they get somewhat less light per square meter (multiply by the cosine of the angle from the optimum). Even if they were getting 1000 W/m2, with that much sun they’d get pretty warm, and the maximum power drops by about 1/8th to below 240W at 50°C.

One interesting observation is that peak power occurs right after dips due to cloud cover—I think that the panels warm up quickly in full sun, and the solar cells are less efficient when they are warm.

The Enphase M250 microinverters are rated for 240W continuous (250W peak), so are a good match for the panels.

The panels are not oriented optimally for solar power, because it is cheaper for installation and maintenance to mount them flat on the garage roof and buy more panels than to make complicated roof racks to get them to the optimal orientation. The garage is oriented so that one half of the roof faces southeast and the other half northwest. The highest peak power is on the southeast half, but the maximum energy generation is on the northwest half, because morning fog limits the sunlight from the east, but afternoons are usually clear, so the northwest panels generate power for longer.  Around 1:20pm, both sets of panels are illuminated, and I get a peak power of about 1.57kW (~210W from each southeast panel and ~185W from each northwest panel).

The west-facing panels generate more power and more valuable power, because I have time-of-use service, and peak power costs 30¢–32¢ a kWh (90¢–92¢ on SmartDays™), while morning part-peak costs only 18¢–21¢.

The panels are generating about 10kWh per day (at the beginning of August—I expect about 30% of that in winter on good days, and far less on rainy days). Note: total insolation for Santa Cruz is about 7.41 kWh/m2/day in June and only 1.99 kWh/m2/day in December [], so minimum winter production at 27% of peak summer production seems reasonable. The average insolation over the year is 4.764 kWh/m2/day, so I expect to average 7 kWh/day over the year (maybe a bit less for bad weather),

Based on these estimates, I expect to generate about 2.5MWh a year, and I use about 2.6MWh a year, so there will be only small net usage or surplus. PG&E will buy any surplus electricity for about 4¢ a kWh.  An astute follower of the solar market may wonder why our system cost so much. Well—it’s complicated.  We were putting the solar panels on our garage roof, but the shingles there were within 5 years of their expected lifespan, so we ended up reroofing the garage. And the city wouldn’t give us a permit for the solar installation unless we replaced the 60-year-old fuse box, added bigger grounding rods, put the electric feed to the house above the roof line, … .  So we ended up with a much bigger project than just adding solar, coming to about $10/W, rather than the more expected $5/W.

I’m not certain about the details of PG&E’s net metering scheme, but I think that if we use electricity at peak times on one day, we get charged full price for it, even if we generate excess electricity at peak times on other days. It would be fairer to have net computations take time-of-use and time-of-generation into account, so that net peak use is computed, with any surplus credited to part-peak, then net part-peak computed, with any surplus to off-peak, then final net computed. But I don’t think that there is any day-to-day carryover of peak surplus power. I’ll have to check the net energy metering contract very carefully to figure out the exact rules.

Of course, total energy use for the house is much larger, since we heat the house, heat water, cook, and dry clothes with natural gas. My natural gas use is about 433 therms/year, which translates to 12.7MWh, about 4.9× my electricity use. So the solar panels don’t make the house energy neutral. Even if we do generate some surplus electricity, switching some usage from natural gas to electricity doesn’t make economic sense, as natural gas costs about $1.28/therm or 4.4¢/kWh, only slightly more than PG&E would pay for the surplus electricity.


  1. I don’t have time to look at your numbers, but, there’s something very wrong there… I’m hoping you got some competitive bids, but, my electric bill was nearly double yours, I got panels that have completely replaced my bills (got $4 back this year), and the bids I got ranged from $13K to $20K, before the federal rebate.

    Comment by Ron Goodman — 2015 August 10 @ 21:43 | Reply

    • My initial quotes were indeed more in that range, but that was before the decision to reroof the garage and the discovery that all the electrical stuff outside the house had to be replaced to bring things up to code. Your house may also be better aligned to the sun than my garage—my 2.16kW system is producing at most 11.2kWh a day.

      I also requested US-made panels, which raised the price of them somewhat, and microinverters rather than a single large inverter, which is also a somewhat more expensive design. I’d originally planned a much smaller system, but when I saw how much of the price was fixed (the reroofing and redoing the electric service to the house), I added extra panels.

      So I can well believe that someone else may be able to get a system with about the same power output as mine for half the price, even here in Santa Cruz.

      Comment by gasstationwithoutpumps — 2015 August 10 @ 22:07 | Reply

  2. You are all wrong.
    I am in solar energy for 25 years .
    Lately amortisation period is 5 years, at most 6 years . Your 45 years is a rediculuse one .

    Comment by mgozaydin — 2015 August 10 @ 22:00 | Reply

    • I don’t believe that I am wrong about how much my system cost nor about how long it will take to pay off. For heavy users of electricity, and a large solar setup, the amortization period is indeed much shorter. Indeed, the advertising for solar locally talks about people saving $200 a month on their electric bills—five times what my biggest monthly bill is.

      PG&E has very low prices for small users of electricity, and if you use your electricity mainly in winter and off-peak in summer, you can make the costs even lower with their time-of-use plans. The $420/year I estimated in the post is actually MORE than I spent on electricity in the past year.

      The first couple of solar contractors I talked with were not interested in doing the system once they found out how little electricity we used—there is no way to recoup the costs unless you imagine enormous increases in power costs over the next few years (possible, but unlikely).

      As I told Ron, my costs were higher than most, even locally, for a number of reasons, and it is probably possible for others in town to get a similarly sized system for about half the cost per watt.

      Someone who uses a lot of peak-time electricity (at 30¢–60¢ a kWh) can amortize the panels a lot quicker than I can (my average electricity cost year-round is about 16¢ a kWh).

      So I agree that the solar panels make no economic sense for me—but that doesn’t mean that I am “all wrong”.

      Comment by gasstationwithoutpumps — 2015 August 10 @ 22:20 | Reply

      • You are right.
        You are not all wrong.
        Sorry I was too critical . I apologise.

        Comment by mgozaydin — 2015 August 10 @ 22:53 | Reply

  3. […] got my first Pacific Gas and Electric bill since getting the solar panels put in, and I think I finally understand how the Net Energy Metering (NEM) works—their explanations of […]

    Pingback by First post-solar electric bill | Gas station without pumps — 2015 September 3 @ 00:04 | Reply

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