I'm looking at putting solar on our house, and I'm trying to figure out if it makes sense to go with something that would be more useful in an extended blackout. There seem to be more or less three options:

• Default: no off-grid capability.

• Secure power supply (SPS): an outlet in the basement that gives you power when the grid is down and the sun is shining.

• Batteries: storage and the ability to switch the whole house over to run off them.

Talking to solar people and looking online, it doesn't sound like it's possible to have an intermediate option where the whole house has power, but only when the sun is shining. I don't really understand why this can't be done with an interlock, but that's what they say.

An SPS is only a bit more expensive [1], while a system with batteries looks like it would be about $10k. We could also do something in between, like charging a UPS or other batteries from an SPS and using it to power things when the sun was down. In general it seems like a potentially useful tool for combining with other things. Have others thought much about it?

A big question here is how likely we would be to need something like this. Guessing, maybe over the 25 year life of the system there's a ~5% chance we have a blackout lasting more than a week and a ~2% chance that we have one lasting more than a month? I think at these odds it's worth it to get the SPS, some long extension cords, and maybe a UPS, but not worth it to get a full off-grid-capable-with-batteries system?

[1] Most inverters don't have one, but SMA's do. Which means it's restricting inverter choice rather than directly costing more. Call it $250?