Home solar production falls with shorter days

As the sun has dropped in the sky and the days have gotten shorter and shorter, so too has our solar production.

Not surprising. That’s the nature of the sun as an energy source, or really of the earth as a planet which rotates as it orbits the sun.

Still, our solar production, and the monthly drop in that production, is interesting to examine – at least for those of us who are “solar-heads”.

So here’s a brief look at what’s happened to the production of our 5.59 kW home solar system as we’ve moved from a peak sun month like July to a sun’s-very-low-in-the-sky-plus-it’s-cloudier month like December.

It’s interesting, and heartening, to note that so far, in the almost six months our 5.59 kW system has been online here in Aurora, Colo. it’s exceeded the monthly projections REC Solar charted for us – by quite a bit.

Solar system “over”-performing
Of course, REC Solar told us that it tends to underestimate total annual production in order to account for variability in weather as well as a certain amount of uncertainty about how a given system will perform. It might also have to do with making customers feel better when they see their system “over”-perform.

This approach has worked in our case 😉

In the five full months for which we have data for your system (which officially went online on June 28, 2010), it’s produced 598 kWh more than REC Solar predicted it would. In September of this year, it pumped out a whopping 192 kWh than predicted.

Of course, “extra” production has dropped quite a bit as the sun has dipped in the sky in the Northern Hemisphere. In November, we produced only 74 kWh more than predicted. December’s “extra” margin is likely to be similar that.

Production in December lowest
As our monthly production has inevitably dropped, so too has our ability to bank what we call Sun Miles® — miles driven by an electric vehicle (EV) or plug-in hybrid electric vehicle (PHEV) whose batteries have been charged using solar energy and/or using electricity from kWh credits amassed via solar offset generation — or, put differently, our monthly kWh net has dropped.

It’s interesting, and heartening, to note that so far, in the almost six months our 5.59 kW system has been online here in Aurora, Colo. it’s exceeded the monthly projections REC Solar charted for us – by quite a bit.

However, we’ve still managed a plus kWh net so far in December – the lowest sun production month of the year, albeit a small one. We’ll probably make 80 kWh more than we use this December.

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Below is a table overview of our solar production so far. Below this are screen shots of our monthly solar production as charted on Google PowerMeter (but only from Sept. – Dec. – we didn’t have PowerMeter in July and August). That data is pushed to PowerMeter by our TED (The Energy Detective) system.

Unfortunately, with TED and Google PowerMeter, which measure both electricity production and home use, you cannot easily publish only your solar production data to the web. You’re forced to publish all of your data. This is a huge shortcoming, one that we hope will be addressed by TED and Google in the future so that we can easily publish our solar system’s production live to SolarChargedDriving.Com. [We don’t want to publish our home electricity use live to the web for obvious security reasons]

Month	Predicted Solar Production	Actual Solar Production	Net Electricity Production
July	832 kWh	924 kWh	+684 kWh
August	781 kWh	937 kWh	+707 kWh
September	698 kWh	890 kWh	+650 kWh
October	626 kWh	710 kWh	+440 kWh
November	466 kWh	540 kWh	+210 kWh
December	463 kWh	500 kWh (estimate)	+80 kWh