Calculate how long an inverter and battery keep your load running.
Battery runtime is energy divided by power, but two derations sit between the nameplate and reality. Amp-hours times voltage gives nameplate watt-hours — a 150 Ah 12 V battery is 1,800 Wh — and then depth of discharge cuts what you may actually use: taking a flooded lead-acid battery below 50% shortens its life dramatically, while lithium tolerates 80 to 90%. Inverter conversion losses take another 10 to 20% on top. So that 1,800 Wh battery realistically delivers about 765 Wh, which runs a 400 W load for just under two hours. The other trap is the discharge rate itself: lead-acid capacity is rated at a slow 20-hour draw and falls further under heavy load, so a high-power run gives less than this figure suggests.
Inverter Battery Backup
Usable Wh = Ah x V x depth of discharge% x inverter efficiency%; backup hours = usable Wh / load watts
About 1.9 hours on lead-acid at 50% depth of discharge with an 85% inverter. The same battery in lithium at 80% depth gives around 3 hours.
Usually Peukert's effect. Lead-acid capacity is rated at a 20-hour discharge; pull the same battery hard and effective capacity drops 20-40%. Ageing batteries also lose capacity long before they fail outright.