PV Gap

Kyle Said:

Solar Micro Home Power Usage?

We Answered:

Hey Senior, you have a nice project there. Let me tell you about ours briefly, then give you my suggestions. We had a 600 sq. foot log cabin in Northern Michigan that had utility power, but the power was constantly going out. We started with a small system, just to run some lights and electronics in the great room during outages. Using that, I learned a great deal about solar and wind. Now it's 10 years later, we have a 1.4 kw solar array on the garage roof, and a 900 watt wind turbine in the field behind our home. In the summer, the solar array produces all our power, just barely, and in the winter, the wind turbine does about half the job. We intentionally undershot to save on cost, and because we still had the utility to help out.

If I were in your shoes, this is what I would do. Design a good quality system, with a slightly oversized battery bank, undersize the array, and add a generator. A good system will have a good quality sine wave inverter and Trojan or Surette batteries, a digital counting solar charge controller and UL listed disconnects. The disconnects are the only thing preventing your wiring from catching fire if something shorts out. The counting charge controller will help you keep track of your solar output, so you will know if something is not working properly, and the sine wave inverter will run everything in your house. Cheaper inverters not only have things that they can not power, like electronic battery chargers and furnace cards, but they will actually damage a few items you might plug into them. I found this out the hard way, a new charger for my Dewalt drill cost $55.

The reason to oversize the batteries and undersize the array is two fold. Look in the library for a book by Richard Perez called, "The Complete Battery Book." Just read the chapters on lead acid batteries. Once you buy your batteries, you can't add more to them later, the old and new ones fight each other. Solar panels however, are the most expensive part of your system, you can add any amount later, even different brands, they all get along fine. So if you start with 1000 watts of solar, and decide a year later you need 400 more, no problem, and you've spread out the expense over a couple years, and more importantly, not over bought in the beginning. The generator is necessary because no matter how powerful and well designed your system is, you wil always have a day or two at the end of the month where you come up short, and it isn't good for battery longevity to run them down really deep trying to get through the rainy week. If you design your system well, you should only have to start the generator one or two times a month, for just a few hours each.

You have listed a fairly conservative list of power usage here, totalling about 2.5 kwh each day. I would expect a bit more than that, an extra light or two, and so on. Think about LED lighting too, my suggestion is looking for LED Christmas lights after the holiday, they are twice as efficient as CF lights, and fun. We have a 130 foot string lining our deck roof, very nice, only uses 12 watts. What I would do first is subscribe to Home Power Magazine, it's the only periodical devoted to this, and it's inexpensive. I will list it below. Also, if there is a renewable energy fair near you next spring, go to it, that's how we got started. Home Power will have info on this. Incidentally, our home was featured in that magazine twice, once for our small system, and again when we upgraded. You can go to their website after you subscribe, use their search engine, and look for an article called, "Starting Small."

You're also picking a good time to get into this. Solar panels have suddenly come down in price, although batteries are heading up. There are some good deals to be made on telecommunications batteries right now, all the cell phone companies overbought for their towers, now they are consolidating. I will also list a couple other places to look for info on your future system.

How much power you need is a little nebulous to determine, if I had to guess with the usage info you have provided, I would think 600 - 800 watts of solar might just do it, so you might think of going with something closer to 1000, but you can start at 600 or so and move up. If you are going to stand alone and not have the utility, your batteries should hold about 5 days of power without any solar gain. So at 2.5kwh per day, 12.5 kwh of battery storage would be adequate, a little more would be better. If you only discharge your batteries 10 to 20 percent each day, they should last about 10 years. Our bank is 11 years old now, it is in need of replacement next spring, we have several cells that have failed. You'll have to learn about watering and rotating batteries, check out the sources below. As an example, a Trojan T-105, which is their golf cart battery, holds 220 amp hours at 6 volts. AH X Volts = watt hours, so 220 X 6 = 1320 watt hours