The more households that buy them, the less peak power generation is needed and less large scale battery deployments. If the ROI of a household battery was just 4%, you are better off economically paying higher power bills and sticking that money in an index fund. But if subsidies increase that ROI, more people buy batteries. The money the government contributes hopefully ends up less than they would need to spend on large scale battery deployments or on legacy power generation to power peak usage times. It also has the side effect of getting more citizens (literally) invested in sustainable power usage, and people get more interested in insulating their homes, buying more efficient appliances, moving away from gas etc.

> which is why there’s a subsidy required for people to buy them?

the gov't also offers interest free (but inflation indexed) loans to tertiary education.

Just because there's a subsidy, doesn't mean the tax payer is paying a price for inefficiency. The policy itself needs to be individually examined to determine whether it's an efficient use of funds, not simply that it's a subsidy (time frame needs to be taken into account too).

Is it so out of the ordinary that a government tries to help people save money or what's the question? Sounds like you've only had the American experience in life unfortunately.

I know people who would purchase solar panels and batteries, but they do not have enough capital to do so.

The government loan changes the calculus. Allows for short term thinking and a long term benefit.

Yes. People can't always afford super economical things when the initial cost is high and the pay-off takes a while, but is easily worth it in the end.