It's interesting that you are considering solar for backup power during outages. I have heard that most people nowadays go grid-tied so as to benefit from the reduction in electric bills. How big of a battery bank do you intend to get? What is the expected life of the cells? How do those economics work out? Do you have a solution that will allow you to both maintain charge on a bank AND sell back power to the grid when possible?
This has been an evolution in my thinking. Our power is out at least once a month, several times a year it is out for extended periods (> 3 hours). The prospect of spending $5K for a generator that never recouped any of my investment and only got used 1 week out of the year, or $10K for batteries doing the same thing, vs a solar install for $20K that might have a chance of paying itself off someday, I picked the solar install. I also am concerned about energy prices, nothing seems to get cheaper. I considered grid tied with no batteries and a generator, but one of the intangible costs of installing a generator for me is putting in a gas line under an existing driveway and turnaround, and through an area with old and valuable trees that I don't want to risk losing. So although batteries are an expensive backup system, they make sense for me.
I'm still debating the size of the battery bank, primary sticking point is whether or not we need to run the central air. My wife runs business out of our home, sees clients here, so if it's summer and we can't cool the house we lose money and good will by canceling patient appointments. There is also the uncertainty when power goes off of how long it will last- do we cancel the whole days appointments, or just the next few? We also have well and septic, so no power, no water and no flushing in the lower level, yuck. The cost for materials is going to be somewhere in the $20-25K range, we can sell back to the power company at about 6c/kwh, there'll be some savings on taxes for energy credit and the business "pays" via some fraction of the installation being a deductible business expense. In the long run will we ever get our money back, who knows, probably depends more on what happens with energy prices in the next 10-15 years.
My friend has a battery on his solar install that he did in the early 90s, it's still fine. Not sure if that's normal but my understanding is quality batteries last 15-20 years. Especially with newer chargers that monitor batteries and exercise them regularly.
Yes there's a way to keep the batteries charged and sell power back. My understanding is the way it actually works is the grid charges the batteries and the array feeds back to power company when power is on. When power goes off, array charges batteries and powers house. The batteries are also exercised on some regular basis so they experience some discharge. I'm having someone design the system, I don't know enough to do it myself. I met with the guy once, he asked lots of questions that we have to make up our mind on before we can actually buy anything or get a final design.
