Batteries are quite expensive. Lead-acid batteries are readily available, but don’t really work well for powering a house on a regular basis, because they don’t have a very long life cycle. LiON batteries work very well, but they’re fire hazards. Even worse, if you live in an area where you get freezing temperatures, they must be kept inside, because they can’t be allowed to freeze if you’re cycling them. LiFePO4 is the current best option. If you don’t charge them above 80, 85%, and never discharge below 20%, you should have a nearly infinite lifespan. But that means that for every 30kWh of power you use, you want 50kWh of battery. And currently LiFePO4 battery banks run approx. $1000/kWh (+/- depending on band). If you heat your home with electricity, and you live in e.g. North Dakota, you’re going to want more like 200kWh of batteries, because even high efficiency heat pumps can suck a lot of power when it’s -20F.

I’m currently working on getting a 17.7kW system approved by the local utility. It looks like I’ll need to step down what I’m feeding into the grid, because the line capacity out where I live is only 10kW, and they will only approve 75% of the line capacity for grid-tied systems.