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From the Q&A

With TJ Byers


Photovoltaics Basics

Question:

I have five solar panels (surplus supplies) that I would like to get some use out of. The panels — SX-120s from BP Solar — have an operating voltage of 33.7 volts at 3.5 amps and are rated at 120 watts. I had planned to connect the panels in parallel to charge a 24-volt battery array. The array would consist of two 12-volt batteries, in series, so that I can tap into the bank to obtain both 24 volts and 12 volts, which I will use to power outdoor lighting and a remote work shed. Inside the shed, I also want to get a DC to AC inverter for indoor lighting and possibly at least one AC outlet. Because of zoning codes, the wiring between the two buildings has to be low voltage. I thought I could do the photovoltaics myself, but have since discovered that I need a lot of help. Help!

Art
via Internet


Answer:

This is a tall order for a column with limited space, but I'll try my best to answer it. First, you have the right approach by having these particular panels charge a 24-volt battery bank.

The building blocks include the solar panels, batteries, charge controller, breaker box, and your load (lights, etc.). The voltage of the solar panels has to be matched to the charging profile of the battery, which you have in hand. That is, you can't charge a 24-volt battery with a 12-volt panel.

The second major block is the storage batteries, which provide electricity when there is no sun. Let's say the sun shines six hours per day and you want to be able to use that energy 24/7. Here's where it gets sticky. You want enough battery capacity to serve your worst-case scenario, but you don't want to spend money on capacity (extra batteries) that you'll never use. So, you need to balance the space for the batteries and their expense with your needs. A rule of thumb is to multiply your load requirements (in amp-hours) by four to arrive at the needed battery capacity. For example, if you require four amps each day for four hours, your amp-hour need is 16 AH, which means you need a 64 AH battery.

Between the photovoltaics and batteries is a charge controller — basically, a voltage regulator. It prevents the solar panels from overcharging the batteries. The charge controller can take many forms, including series linear, series switching, or shunt. Which one you use depends on your situation and what you want to do with the excess solar energy produced when it's not being channeled to the load and batteries. The last item is the load, which can take on an infinite number of configurations. A breaker box will be recommended for the distribution of the power and for overload protection (mostly for fire protection).

As for tapping into the battery array for both 12-volt and 24-volt sourcing, I'm not so sure you can do that. You're going to have a hard time balancing out the loads, even in a three-wire system. One battery will always have more usage than the other. I suggest you run the system at 24 volts and use buck switching voltage regulators at the loads that need 12 volts. Another benefit of using a 24-volt system is that the wiring resistance loss is considerably less, enabling you to run thinner wires between your batteries and your work shed.


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