How to decide ROI?

Larger home, four air conditioners. Usually kept at 75 degrees. A number of poor builder mistakes that will never let the house be very efficient. June electricity bill $850. Winter bills $500 - $650. Plenty of clear land space for solar array. Before rebate went away, a prominent company quoted $250,000 for installation but said it would only cover 80 - 85% of electricity needs in this did not include batteries. Seemed silly to make such an investment when that amount of money could go into regular investments and we could just keep paying the electric bill. But I hate paying the electric bill and I’m always tempted to try and become energy. Independent. Without the rebate nowadays, are companies providing better pricing and, with the advent of sodium batteries, should we wait a year or two and do a re-look? Just wondering how those of you made the leap determined return on investment on larger projects?