This is hugely variable, the "payoff time" is almost entirely driven by local rates for purchasing and selling electricity. And whether you need to pay $20-$50/

This is hugely variable, the "payoff time" is almost entirely driven by local rates for purchasing and selling electricity. And whether you need to pay $20-$50/month no matter how much electricity you consume. As such, the payback time is almost entirely a decision based on local regulatory policy, unconnected to the actual cost of the systems. The US also uses fragmented regulatory policy to make our residential solar cost many multiples more than other developed countries' costs.