How big a battery do you actually need?

It depends mainly on which loads you want covered and for how long. The battery storage size calculator converts your loads and desired backup hours into a kWh figure directly, and the critical loads calculator narrows that down if you only need to cover essentials. The hybrid inverter sizing calculator checks whether your inverter can deliver what that battery stores.

Critical loads only (fridge, some lighting, internet, medical equipment) needs a much smaller and cheaper battery than whole home backup, which includes HVAC and other high-draw appliances. The whole home sizing tool and critical loads tool compare both directly. The oversizing for future battery calculator works out how much extra capacity to plan for if your loads grow.

No. Depth of discharge and round trip efficiency both reduce usable capacity below the nameplate figure, typically by 10 to 20% combined. The usable vs nameplate calculator shows the real number for your battery. NREL's Annual Technology Baseline for residential battery storage publishes the cost and performance figures behind these derates.

If your solar array stays connected to your battery through a backup gateway, yes, it recharges the battery each day the sun is out, which can stretch a single battery across a multi-day outage. See the solar recharge sizing tool for the difference this makes. The Department of Energy's inverter basics guide covers how the inverter handles that recharge.