The Solar System and Some Limitations
Our solar electric system began with six photovoltaic (PV) panels on the barn roof. That's all it takes to generate (approximately) one kilowatt of power. But it takes a great deal more equipment to regulate it, store it, and make it usable and available. The system includes a battery charger, a bank of batteries, an inverter/controller, and two other boxes that I don't know what they do. The inverter is actually a small computer that accepts and monitors inputs and provides output. Inputs include the 120 volt line from the utility, the 24 volt battery backup, and the 24 volt input from the solar panels. In its "grid-tie" mode, it's set to draw from the solar first, the utility second, and the batteries only if the utility fails. Outputs include the six circuits that are on the solar system, and the main utility line. In other words, any solar power not used within the six curcuits goes into the main household system and, if not used there, through the meter back to the utility. This explanation is no doubt simplistic, as my knowledge of these matters is severely limited, but I'm learning. Our learning process included the realization of some of the limitations of the system. First, 1,000 watts or 1 kilowatt of solar generating capability does not equal a kilowatt of usable power. Depending on equipment, conditions, and who you read, the actual usable output is in the 66% to 77% range— 660 to 770 watts. Ironically, cold weather increases the yield of the solar cells. This morning, with a temperature of negative 10 degrees and the sun halfway to its apogee, the cells were generating 820 watts— well above expectations. Cold weather also reduces the efficiency of the batteries, meaning it's harder to store that extra generated power. Though the batteries are rated to -40, we found that at below-zero temperatures the batteries are not reliable. I spent the weekend building an enclosure for the batteries that, with a portable electric heater, will keep them above freezing. (The heater uses 750 watts. The solar panels generate about 750 watts... In cold weather, it's a wash.)
There were other limitations we didn't realize. Our installer told us (during the planning phase) that we could basically put whatever curcuits we wanted on the solar system. That wasn't exactly correct: we could switch over about half our circuits. If the utility fails, we'll have power to the pumphouse, chicken room, fridge & deep freeze, bedroom (where we keep our computers), and living room plug outlets. The second and third bedrooms, kitchen appliances & lights, living room lights, washer & dryer, and the rest of the barn are not supported, so if we lose power they would not be operational. But (believe it or not) we'll survive without a microwave if we have to. And we already had a collection of flashlights, candles, and hurricane lamps because power failures are pretty frequent.
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