Forget using solar or wind for backup, use it all the time! Problem is, a full-up photovoltaic system to take the whole house off-grid is tens of thousands of dollers. Even a system to take just the fridge off-grid is several thousand. (That includes a DC fridge to eliminate the conversion losses from a DC to AC inverter) On the upside, the fridge doesn't need to run all the time, so the system could also power TV, internet, etc when it's off. Still a long time for payback; 10+ years or so. Hard to justify when a small Honda generator is a few hundred bucks.