Piecing together a spanking-new $600 solar-electric system

by Rex A. Ewing

Got 600 bucks hiding in an old book somewhere? Maybe it’s time to bring electricity into that little homestead you’ve got tucked away in the woods. But wait a minute, you say, with justifiable hesitancy. Solar-electric systems all cost thousands, don’t they? No, just the expensive ones.

Would you like to run a few lights, maybe a tv/DVD combo or a small computer? How about some moderately-sized power tools? An investment of $600 will get you there with all new components, as long as you don’t crank your expectations up too high. How? Read on.

First off, for the sake of simplicity we’ll assume the system is for either a generator-run workshop, or a weekend cabin, since that’s really where a $600 solar system belongs. In this way the PV module can spend more time collecting energy than you’ll spend using it, so you can invest less money in energy production (PV modules) and more money in energy storage (batteries).

But what components do you need to buy, and how much are they going to set you back? Fifteen minutes of surfing on the Internet turned up the following items. I’m sure a more thorough search would produce even more favorable results:

  • One Uni-Solar 32-watt amorphous-silicon PV module, 12 volts: $180.00
  • One Morningstar 6-amp charge controller, 12 volts: $40.00
  • Two Deka 92 amp-hour sealed batteries, 12 volts: ($130.00 each) $260.00
  • One Aims 800-watt modified sine wave inverter, 12 volts: $65.00

TOTAL: $545.00

This leaves you with $55 for wire, battery cables, mounting hardware, fuses between components, and the miscellaneous odds and ends that are always needed for any project of moderate complexity. What can you expect from this bargain-basement system? First I’ll explain the components, then we’ll take it out for a theoretical test drive.

Summing-up the parts

First, the 32-watt amorphous silicon PV module. I chose amorphous silicon, as opposed to crystalline silicon, for its superior performance in low light conditions, since you’ll want to capture every ray of sunlight you can. It’s nominally rated at 32 watts, but for reasons too complicated to explain here, the most power you’ll ever see it produce with a standard charge controller is around 25 watts. This peak production will be for the two or three hours that straddle midday, when the sun is highest in the sky. Though output varies with the seasons, this small module will produce between 0.15 and 0.20 kWh of power each sunny day; considerably less during cloudy periods. So, in a reasonably sunny climate, you should be able to count on about one kWh of energy per week, give or take. What can you do with that much-or that little-energy? Keep reading.

Next in line is the 6-amp charge controller. The positive and negative leads from the PV module go into it, and the + and – leads to the battery come out of it. A simple, inexpensive charge controller like this one does exactly two things: it charges the batteries without overcharging them, and it prevents electrical current from running backwards from the batteries into the PV module during the evening hours. While the latter function could be easily performed by an inexpensive blocking diode, if you want to be able to leave your system for days or weeks at a time, you’ll absolutely need the charge-conditioning capabilities of a charge controller.

Now for the batteries. You’ll want sealed gel-type batteries, even though they’re pricier than flooded lead-acid batteries. Why? Two reasons: first, you won’t have to build a sealed box to keep them in; a box which would have to be vented to the outside to prevent the buildup of flammable hydrogen gas. Secondly, you won’t have to worry about all your water cooking out if you have to leave the system alone for a few months while you’re off exploring the headwaters of the Amazon.

With a rating of 92 amp hours each, the two batteries wired in parallel (+ to +, and – to – ) will store a total 184 amp hours, or 2.2 kWh of power (12 volts x 184 AH = 2,208 watt-hours, or 2.2 kWh). You’ll never be able to use all that power, however. In fact, the most you’ll want to discharge the batteries on a regular basis will be about 50 percent, though if you occasionally have to dip a little deeper into the wattage reserves it certainly won’t hurt anything. To be certain, you should test the batteries from time to time with an inexpensive multi-tester. If they’re below 12.25 volts after sitting idle for a few minutes with no load, they’ll need rest and recharging before being asked to do much more work.

That leaves the inverter. If you only wanted to run a few lights and a TV, or a laptop computer with a car adapter, or even an RV-type water pump, you could get by without the inverter, but the extra cost for DC lights or a 12-volt DC television would probably pay for the inverter, anyway. Besides, you’ll inevitably want to run something that needs 120-volt AC (especially if the system is going in a workshop), so bite the bullet now and buy the thing.

A $65, 800-watt inverter lacks many features common on more expensive inverters, such as battery-charging capabilities, so you won’t be able to plug a gas generator into the inverter to give your batteries a little pick-me-up. Also, the modified sine wave it produces is but a crude approximation of the graceful, undulating waveform the power company (or a fancier inverter) sends through the lines, and some sensitive electronic devices may not work well, at all. But for most of the things most of us use electricity for, the modified-sine-wave inverter will be perfectly satisfactory.

Okay…what can it do?

As you may have noticed, the usable power stored in the two batteries is roughly equal to a week’s output from the single 32-watt PV module, so each week you’ll have around one kilowatt-hour of stored sunlight at your disposal. What can you do with it? One kWh will run a 20-inch tv for 20 hours, a portable stereo for 100 hours, a laptop computer for 40 hours, or a 12-watt compact-fluorescent light bulb for 80 hours.

The 800-watt inverter (with a 2,000-watt surge capacity) will run a small vacuum cleaner, a drill or a small drill press, a sander, a jigsaw or small band saw, but not a large circular saw. It will handle many toasters and coffee makers, but not all. A blender would be child’s play for this inverter, a microwave a impossibility. A hair dryer on low, yes; on high, forget it.

And when I want more?

With the exception of the inverter, this system can be easily expanded. Any number of similar modules can be wired together in parallel, so long as the modules are of the same wattage. The 6-amp charge controller can manage up to three 32-watt modules, and extra charge controllers can be wired into the system, in parallel, as your lust for power begins to swell.

Batteries, of course, are always happy to see their numbers multiply.

But alas, the inverter is what it is. It cannot be connected to another inverter to provide more power (though more expensive models can be), nor can it be configured to operate at a higher input voltage, should you ever get ambitious and change the system voltage to 24 or 48 volts. On the other hand, at $65, does it really matter? A slightly-used 800-watt AC power source that can draw power right off the battery is a handy accessory any vehicle would be proud to have tucked away next to the spare tire.

So, while you’re saving up to buy the deluxe 4000-watt pure sine-wave inverter with battery charging capabilities, enjoy the little $600 starter system that got your foot in the solar-energy door, and try to imagine where it all might lead.