Picking a Solar System
Picking a solar system for our home was based on three things: finding an installer, figuring out what we wanted, and deciding what we could afford. We needed an installer because, although I have some basic mechanical skills, I'm not qualified to design and install an electrical system that would cost thousands of dollars and connect to our entire home. The risk of wasting our money, or worse, burning down the house, was just not worth the savings.
We chose Gerald Whipple at Solar Unlimited, a local company with a number of recommendations and a fair reputation. (As noted earlier, none of the local solar companies are well-trusted by the local construction industry.) We'd seen Gerald's own solar home three years ago when my wife and I got lost looking for property. Gerald took the time to show us his system and explain how everything worked. He's since moved closer to town, but both his home and shop are solar-powered. Since we already knew him— and since he had the best reputation— he was a natural choice.
We told Gerald what we wanted. He told us how much it would cost. We gasped, and went back to the drawing board. The first lesson of solar is this: it's expensive. Our final choice included a 1 kilowatt photovoltaic array to be mounted on the barn roof, a set of batteries that (though I still don't understand the conversion between amps and watts) should give us about 24 kilowatt-hours of power, and a 4 kilowatt sine-wave inverter. I've read that not all inverters are true sine wave, and that's important if you plan to use electronic equipment. Since, as an accountant, I make my living from my computer, this was a no-brainer. The 4 kilowatt rating means we can run 4,000 watts of usage at any given time— lightbulbs, computers, electric heaters, etc.
Our system would be a "grid-tie" system, which means that it's backed up by the utility company. And if it generates more electricity than we can use at a given time, it "sells" the power back to the power company. Which in real life means that it runs the meter backwards, reducing our bill. We can never actually make a profit; the best we can do is reduce our electric bill to zero. It's my understanding that electric utilities are regulated by the states, and that most states have this restriction. That's too bad, because if homeowners could help recover their costs by selling excess power, especially during peak usage (which is when solar is at its best), this might reduce the need for new power plants!
As it is, even with $4,000 in tax credits, including the installation cost we figure our solar system will pay for itself over its life, roughly 20 years. (The batteries will need replacement sooner.) So solar won't save us any money, unless we do any add-ons ourselves. But it will provide us power backup, which is essential where we live, and it will reduce our reliance on fossil-fuel generated power from outside.
There are so many errors in this article. On one hand I don't want to discourage a curiosity in solar power and electromagnetic theory. On the other, I really do not have hope for the American public as the educated person who wrote the article gets so many basic things wrong. I learned these things in high school physics.
1: The 4 kWH reference is wrong. 4 kilowatt hours has a time component to it. It can provide 4000 watts of power for 1 hour... or 8000 watts of power for 1/2 hour... or 2000 watts of power for 2 hours... or 16000 watts of power for 15 minutes.
2. Solar power is not best during peak demand times of the grid. Depending on the time of year, peak demand on the grid varies. In the winter, it's between 6-9 a.m., when people are waking up and taking showers, heating up their homes from having it cool over night. Sunshine is not strong then. In the summer, it is between 5-7 p.m., when people are returning home and cranking up the AC from the heat of the day.
3. There is no direct conversion between watts and amps unless you know the voltage. 1 Amp on a 120V line is 120W. 2 Amps on a 60V line is 120W...
On the one hand, I thank the reader for his interest, comments, and corrections. As I mentioned, I'm a novice at solar power, and an accountant to boot. It's been over 30 years since I took high school physics; on a good day I can remember that weight = mass * acceleration.
On the other hand there are a couple of items I'd like to clarify:
1. I re-read the article looking for the typo he mentions, 4KWH instead of 4KW. I can't find it. The PVs have a 1KW rating. The inverter has a 4KW capacity. The batteries are supposed to have a 24 KWH capacity.
2. Peak usage may vary by region, I don't know. Here in Utah in the summer, our utility asks us not to run our appliances after noon. I presume the 100+ degree temperatures encourage a lot of air conditioning-- and a lot of people stay at home during the day here, there's not much commuting. Afternoon is surely when solar is at its best.
3. Thanks for the formula. I asked my installer, but didn't get an answer I could understand. Since a single-phase line in the U.S. is 120V, that's not really a variable. The batteries are rated at 2400 Amp-Hours, which would (according to this formula) equate to 24KWH at 120V, less whatever is lost in the inverting process.
1- The quote- "...should give us about 24 kilowatt-hours of power"
A kW-hour is a figure of energy. A kW is a figure of power. Joule is a measure of energy, as is a kW-hour. A Joule-second, or watt, is a measure of power. Your quote mixes up the two terms, like confusing horsepower with calorie.
2."I presume the 100+ degree temperatures encourage a lot of air conditioning-- and a lot of people stay at home during the day here, there's not much commuting."
A lot of areas of the United States will experience 100 degree summers. Factors such as diversity will apply in non-commuting areas, but I have yet to see non-commercial feeders behave the way you say. I will try to link to a graph of peak demand for a residential area.
3."Since a single-phase line in the U.S. is 120V, that's not really a variable."
Unless Utah is different, homes have 240V lines and 120 volt lines. Actually, 120 volt lines are the 240 line seperated by a neutral, giving you two halves, or 120 each. Your refrigerator, hot water heater, dishwasher, and other major appliances run off the 240 line. You usually are plugging into a 120 line.
You asked about the conversion from W to amps... there is no direct conversion as it depends on whether we are talking about car power, home power, PV panels, wind turbine, etc.
Thanks, Kevin, for the clarification. Clearly you know much more about this than I-- and I know more than most of the people I've talked to about it. This does suggest a frightening state of our educational system, and perhaps suggests why we now face such enormous challenges with respect to energy.
As to the voltage question, in my previous article I had mentioned that the circuits routed through the solar electric system were on a single-phase line. The rest of the circuits use the other phase. We have only one 240V appliance in the house, and that is our clothes dryer. Thus, with respect to end-use (and excepting the dryer), the voltage would be a constant in this situation.
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