2 or 4 6V batteries.

I have no trouble going 4 nights on 2 6V Costco batteries without any generator, if it's reasonably warm anyway. Lots of furnace use may cut into that. Last time out we had to run the generator during the afternoon for a couple hours to power the AC so our youngest could nap and with that they never even felt like they were draining at all. I stuck with 2 because that's what the compartment would hold and I didn't really want to tap through into the basement storage area for more battery space. Occasional generator use seemed like an easier solution since we had to have one along anyway for the AC.

I think the batteries were $85 each, and they're Interstate. The performance is great and we'll see how well they hold up.