Understanding your battery bank size
The core formula behind this calculator
Required amp-hours equal daily load in watt-hours times autonomy days, divided by the product of system voltage, depth of discharge, and any cold weather derate. A system using 2,000Wh a day with 2 days of autonomy, a 12V LFP bank at 80% DoD, and a 10% cold weather derate needs about 463Ah, which rounds up to three 200Ah batteries or two 300Ah batteries. The battery lifespan estimator is the natural follow-up once this part is settled. Panels are only half an off-grid system. The battery storage size calculator in our Battery & Storage section sizes the battery bank your daily load needs to get through cloudy stretches.
Why chemistry changes the bank size so much
Lead-acid batteries are typically limited to 50% depth of discharge to protect their lifespan, meaning half of the rated capacity sits unused as a buffer. LFP batteries handle 80% to 100% DoD safely, so a lead-acid bank needs roughly 60% to 100% more rated Ah than an LFP bank to deliver the same usable energy. This is why most new off-grid builds default to LFP despite the higher upfront cost per Ah. See the voltage selection tool for how 12V, 24V, and 48V compare on wiring and current. The DOE's overview of battery and energy storage technology keeps the current program details, and it is worth a look before you decide. Panel wattage means little if the inverter cannot carry your loads. The solar inverter size calculator in our Inverters & Equipment section can help you size it for the biggest appliance you run.
Cold weather cuts real capacity in the northern U.S. and Canada
Both lead-acid and lithium batteries lose usable capacity as temperature drops. Lead-acid can lose 20% or more of rated capacity near freezing, and standard LFP batteries generally cannot charge below 32°F without a built-in heater, even though they can still discharge safely in the cold. Builds in Minnesota, the Canadian prairies, New England, or similar climates should size with the cold weather derate applied rather than the manufacturer's room-temperature rating.
How autonomy days should actually be chosen
2 days of autonomy covers a single bad-weather stretch without an oversized, expensive bank. Systems with a generator backup can often run a leaner 1.5 to 2 day target, since the generator covers anything beyond that. Remote properties with no backup source at all often size closer to 3 days, especially in cloudy regions like the Pacific Northwest or eastern Canada. The National Laboratory of the Rockies Annual Technology Baseline, which publishes current residential battery storage cost and performance data, is a useful reference on how these autonomy and cost trade-offs have shifted as battery prices have come down.
What to check once you have a battery bank size
With the amp-hour target settled, size an inverter to the peak load with the inverter sizing calculator, and protect the bank's wiring with the fuse and breaker sizing tool. The battery storage size calculator in our battery storage tools runs a related calculation if you want a second reference point, and the battery lifespan estimator in our maintenance and lifespan tools projects how many years this bank should realistically last. DOE's overview of battery and energy storage technology is worth a read if any of the terminology above is new.