I chose the number mostly for illustrative purposes, because the price I was quoted for 20kwh happens to very closely align with the price of my car. The other option I was given was 10kwh for half the price, so it's still stupid expensive.

Right, but if you want to power your home off solar (or as much as you can at least) you want a larger battery to even out your production/use.

that’s why i asked if 20kwh is necessary. depends on what you’re trying to do.

That's what I'm going for here. I haven't done the math because as soon as I saw the prices I was like "no way". But I think I would need more than 10kwh to avoid having to draw from the grid under the large majority of circumstances.

Again, it just seems strange to me that it's so much more expensive per kwh than my car, which I would think has a lot more going on than a home backup battery. If I could hook up my car battery to my house (I cannot), I would probably never need to draw from the grid.