It is hard to say. We had major problems with our installer and didn't get to use our system for 3 years after it was 99% installed. They wouldn't finish the final hookup. By the time we had it connected, we were so grateful to have it working, I never looked closely at how much energy it saved us. Whatever the case, I do think it will be a long payoff (guessing >10 years, but haven't done math). I guess it depends on how long the system really lasts and how much efficiency it loses over time. We haven't had it running long enough to know that information.

I do need to study when we connected it and what happened to our power usage. I was in charge of getting the system working and my husband pays the bills, so we haven't connected the two.

I do really like having the knowledge that whatever happens with electricity, we will be in good shape. We have had a several hour power outage and it supplied power to all of our refrigerators and freezers and we had light, so that was nice. We have a wood/coal burning stove that will keep us warm. The only things we would deal with without power would be A/C and clothes washer/dryer that pull too much power and therefore aren't connected.

Our well has a separate DC pump and solar panel, so we have water (although at a reduced flow) if power goes out. I just have to go out and flip a switch. We keep the DC pump off when the power is connected because the two pumps working against each other can cause an early failure. The DC pump and solar panel were pretty cheap (only runs when sun is shining so the only water in the house at night is what is in our two pressure tanks). The house solar was pretty expensive.