Solar panels – eight years of data

Solar electricity, 8 years’ data

With eight full years of solar electricity data, we can start to see some trends and statistics.

The vital statistics for our rooftop installation

21 Sunpower SPR215 panels

Xantrex 5kW GT4.0 inverter (OEM by Sunpower)

South-facing at 20 degrees tilt to the horizontal

No shading

Los Altos, California 94024 (latitude ~ 38 degrees North)

This gives a nominal (STC) 4.5 kW, or 4.0 kW PTC rating.


Solar output over eight years









The graph above shows the breakdown by weekly production (expressed as kWh/day) as well as cumulative results, comparing eight years.

First, daily average production ranges from ~10kWh around the winter solstice to ~24kWh/day in mid-summer.  Summer is more significant for production, and also has fewer cloudy days as shown by the variation in readings.  The figures are weekly data, so there’s some averaging, nonetheless the range in January is 6 – 16 kWh/day or +/-45%, while in August it’s 22-25kWh/day or +/-6% (we normally travel during July so I can’t take weekly readings, hence the straight lines for that month, but the averages are correct).   More reliable sunshine during the peak months is a lucky combination for us.

Despite the significant week-to-week variation, the cumulative graphs (upward/rightward curves on right-hand scale) are remarkably similar.

We can see a few trends.  First, there’s a general reduction in production with time.  This could be because


  1. I’m getting lazy and washing the panels less frequently as time goes on.
  2. There’s some degradation in panel performance over time.  To be eligible for government rebates they must have at least 80% of rated efficiency after 25 years.
  3. The weather has been getting cloudier over the last few years.  Or hotter (high temperatures reduce panel efficiency drops as the temperature increases).
  4. A degradation in inverter performance over time.





It’s really difficult to separate these effects.  I used an insolation sensor and over/under panel thermometers for a while to try and model the effects of cloud and temperature but there was so much daily variation and sensor inaccuracy that the results were inconclusive.   But there seems to be a general loss of 1.2% per year in output.  I’ll try to get on the roof and wash the panels more frequently this year, but it’s difficult to justify the use of water in drought years.

Here’s a graph showing maximum/minimums for equivalent weeks through the year.  There’s a pretty wide spread, mostly attributable to weather conditions, I think.





In winter months, we see about +-40% variation, while even in mid-summer it’s about +-25% so that’s a large variation.  And remember, this is from weekly data – daily readings would show even more variation.


Financial considerations



Since PG&E replaced our electricity meter and changed the way it presents its monthly bill, it’s difficult to get separate peak/off-peak readings which means I can’t easily calculate the true value of electricity savings.  Also, since early 2013 we have an electric car which charges overnight, so that makes cost savings before/after inconsistent. Therefore the $ figures above are based on the US Energy Information Administration figures for average California residential rates.  Prices are not inflation-adjusted.

The total savings to date are about $8176, so we are on our way to a 15-year payback (ignoring inflation).  This is about what was expected initially – with the declines in panel and installation costs (albeit offset by declines in rebates) since 2007, payback for most California residents should be less than 10 years as of 2016.

And, as a footnote, since we acquired the Tesla our electricity consumption has increased, some annual figures:

–         Household electricity consumption (net of car)                 8500 kWh

–         Electricity generated by solar panels                                   6500 kWh

–         Electricity used by electric car                                             2500 kWh

Which has the effect of increasing our consumption of course, but the interesting thing is that this makes the solar more valuable because it offsets the most expensive electricity first, due to the net-metering tariff.  But there’s a counter-offset in that our off-peak electricity use in certain months rises considerably, taking us over the 200+%-of-baseline, which makes even off-peak electricity quite expensive.  But that’s a more involved calculation, for another time.


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