Wednesday, May 1, 2013

Ol' Backwoods' Solar Battery System

Ol' Backwoods has a small solar/battery system that I built, used to power my ham radios, lights, a fan and miscellaneous loads in my electronics lab.

I'm storing energy in two Group 27 12V marine-type, AGM-deep-cycle, lead-acid battery, like this one, except mine is a different brand.  The two batteries in parallel have a nominal capacity of 184 Amp-Hours.  For the uninitiated, this means I could run a 1-amp load (like my LED bench light plus my ham radios in receive mode) for approximately 184 hours.

If there is no sun for several days, and the battery voltage drops, I use a 120-volt-to-12VDC power supply to charge the battery back up.

The battery is normally charged via two 20-Watt '12-Volt' solar panels, in series.

Yes, in series.  No, it doesn't 'kill' my 12-Volt battery.  Hang on, Ol' Backwoods will explain.

I have a solar charge controller than can accept up to 40V.  At full sun, my two panels in series produce 37 Volts open circuit; that is, with no load on the panels except my digital voltmeter (< 0.01 ╬╝Amps).
Data plate on solar panel #1

At full sun, with a load in the range of 1.2 Amps to 0.1 Amps, the panels in series produce between 13.6 and 14.6 Volts.   Why does the voltage drop so much?  Because the current from a silicon solar cell is a nonlinear function of the terminal voltage.  Solar cells have what the industry calls a Maximum Power Point (MPP), which is a combination of voltage and current at which the produce maximum power.  Usually, the MPP is marked on the solar panel's data plate, like mine, at the right.

At my latitude, and without adjusting
Data plate on solar panel #2
the solar panels' tilt (this is explained later in this article), I am getting about 80% of the one of the panels' maximum power, about 16 Watts.

If you are getting 80% of the rated power on only one panel, why did you connect two of them in series, Backwoods?  If you are getting 1.1 Amps at 14.6 volts, why not put the two panels in parallel, and get 2.2 Amps at 14.6 volts, or 32 Watts instead of 16?

I could do that, but it would only work efficiently when the sun is high and bright.  At lower angles, or when it is cloudy or hazy, having the 2nd panel in series boosts the voltage, so that I can still charge my battery even in non-optimum conditions.  The 2nd panel is my backup for lower-light conditions.

For calculating the optimum tilt of the panels for various times of the year, or even a fixed tilt, here is an excellent guide.

Calculating my roof angle
Ol' Backwoods happens to have a large roof surface that faces south, and is turned ever-so-slightly to the west.

As a first approximation, I wanted to know how bad it would be if I simply put the solar panels at the same tilt as the south-facing roof, what kind of efficiency figures should I expect.  Well, that let me to the need to calculate my roof angle.  Sure, I could have measured it, but I had a photo of the side of my house, and I used the inverse tangent of the rise and run of the roof (in pixels) to calculate the roof angle from the horizontal, in degrees.

The easiest installation is just to lay the solar panels at the roof angle, and leave them in the same tilt all year.  There is a calculation on the website listed above for that.  To orient you, zero tilt angle means the panel points overhead, straight up.  Positive angles points more toward the equator (south).  Negative angles point more northerly; in the northern hemisphere, where I live, all the angles are positive.

Optimum Angle, Fixed Angle All Year

My latitude: 35.9 °
lat * 0.76 + 3.1 = 30°

My south-facing roof is pitched at 32.1°; therefore, it angle is nearly optimum.

There is also an equation for optimum angles if you are adjusting the solar panel angles twice a year, on March 30 and September 12th.  In the Northern hemisphere, you go the "summer" angle on March 30 and the "winter" angle on Sept 12; in the Southern hemisphere, it's reversed.

Adjusting Twice a Year (Mar 30 & Sept 12); Optimum Angles:

Best summer angle: lat * 0.93 - 21    = 12 °
Best winter angle: lat * 0.875 + 19.2 = 51 °

Similarly, there is a small efficiency gain for adjusting the panels 4 times per year.

Adjusting Angle 4 Times a Year (April 18, Aug 24, Oct 7, March 5)
Best tilt for summer:     lat * 0.92 - 24.3 =

Best tilt for spring/fall: lat * 0.98 - 2.3 = 32.8°(nearly perfect for my roof angle!)

Best tilt for winter:       lat * 0.89 + 24 = 56°

(This post will be modified to add more information.)


  1. Well, awesome. Now build me one! Lol. I need some solar power in my life especially in the winter. My electric bills are outta control!

    1. If your highest electric bills are in the winter, that means you're heating with electricity, and that is the worst use for solar-generated electricity. To heat a home with solar, it's best not to convert to electricity and then convert back, because that's inefficient and wasteful. If you want to learn more about solar heating, go to my friend Steve Harris's site,, and buy his book on solar heating. There's a lot you can do with it for very little money.

      Solar electricity is mostly for running radios, lighting, entertainment electronics, freezers, refrigerators, ice makers, and other low-to-medium loads.

  2. This comment has been removed by the author.

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