Kits for doing solar electric or thermal panels for under $200 are a scam. "Running your meter backwards", will require at least a 1 to 2 KW system, charge controller, inverter, optional battery bank (depending on type of inverter), enclosures, wiring, frames, etc. Costs run from $3 to $5 per watt just for panels and double that for aux. systems and installation. If you do it your selft, you are on the low end, have it contracted out, on the high end of the range. In order to produce about 1/3 of the power consumed in an average home, you would need at least a 1 to 2 kw array. Best to do both thermal and electric panels. As for payback, we have invested approx. $20,000 in 200 sf of thermal panels and a 1 kw PV system for off grid emergency power. Payback is long to infinite, but we did not do it for payback or to run the meter backwards. Avoid DIY panels and low priced kits as they are not real.