For jdc1, ROI is a poor way to look at an investment. You need to use Discounted Cash Flow Rate of Return (aka Internal Rate of Return). This method allows you to put a percentage return on any investment. Thus you can compare it to treasuries, stock market indices, or heck fire, drilling oil wells. Then you need to factor in risk, if it doesn't return 2 or 3 times the 10 year treasury rate, why would you invest in it?
Make sure you include the costs of remove/install the solar when you need a new roof, etc, and etc. Since I live where we have short winter days, low sol angles, and snow; the math just didn't work out. Much better to buy electric and invest the money.
I did put a 50 amp in the garage, for the welder and a possible EV, as soon as we can buy one with 400 mile cold and snow range (special trip every 2 weeks, no stops), and is comparable over its lifetime costs to a Honda CRV. Right now depreciation kills an EV over the 8 to 10 years we keep a car. They are a gitting there, but not quite yet.
And yes, I really do look at the world this way, by nature and training.
