Battery Size for Inverter Calculator

1. List every device you will run. Write down each device and its wattage. Include items that cycle on and off (like refrigerators) at their average wattage, not peak. Add a 10% buffer for devices you forgot.
2. Determine your runtime need. For power outage backup, 4-8 hours covers most grid outages. For off-grid cabins, 12-16 hours covers overnight loads. For critical medical equipment, plan for 24+ hours with a recharging source.
3. Choose your battery voltage. Match the inverter input. 12V systems are simplest for under 2,000W. 24V and 48V systems are better for larger loads because they halve or quarter the current, reducing cable size and losses.
4. Apply the formula. Required Ah = (Load x Runtime) / (Voltage x DoD x Efficiency). A 500W load for 4 hours on a 12V LiFePO4 bank at 80% DoD and 88% efficiency: (500 x 4) / (12.8 x 0.80 x 0.88) = 222Ah. The 12.8 is the nominal voltage Victron prints for a 12V lithium block, not the 12 on the label. Round up to the nearest available battery size — a 300Ah battery bank gives you headroom.

A homeowner wants to keep essentials running during a 6-hour power outage using a 3,000W pure sine wave inverter on a 48V system.

Essential loads: refrigerator (150W avg), internet router (15W), LED lights (60W), phone chargers (30W), sump pump (400W average, runs 20% of the time = 80W effective). Total: 335W.

Required energy: 335W x 6 hours = 2,010Wh. Factor in inverter efficiency (90%): 2,010 / 0.90 = 2,233Wh from the battery. At 48V nominal (51.2V on lithium) with 80% DoD: 2,233 / (51.2 x 0.80) = 55Ah.

A single 48V 100Ah LiFePO4 battery (5,120Wh total, 4,096Wh usable) handles this with room to spare. The homeowner could run these loads for about 11 hours, or add a space heater for part of the outage.

A battery's amp-hour rating is measured at one particular speed. Trojan rates its T-105 at 225Ah over 20 hours, and the same sheet prints 207Ah over 10 hours, 185Ah over 5 hours and 250Ah over 100 hours. The battery has not changed; the number you are allowed to claim has. Pull harder and lead-acid gives up capacity.

That is why this calculator asks which chemistry you are buying. On lead-acid it inverts a rate correction fitted to figures manufacturers print: Trojan's T-105 discharge table for flooded, Lifeline's published minutes-at-amps together with Victron's effective-capacity column for AGM, and Victron's gel column for gel. The fit reproduces those printed points within about 10% for flooded, 6% for AGM and 2% for gel. On LiFePO4 there is no rate term to apply, because Victron footnotes its own capacity rating "discharge current 1C": the label figure is already measured at the speed a small off-grid system runs at.

Take an off-grid cabin pulling 300W for 12 hours on a 24V bank at 50% depth of discharge through a 90% inverter. Flooded lead-acid needs a 342Ah bank. The same job on LiFePO4, at 80% depth of discharge because Battle Born prints 100% depth of discharge for its cells, needs 195Ah. That gap is the honest cost of lead-acid, and it is not visible in a sizing formula that only multiplies volts by amp-hours.

The tool says so when it is guessing. Push the discharge rate past the fastest point on the printed tables and it tells you the real bank is larger than the figure shown; drop below the slowest point and it tells you the figure is an extrapolation the other way. Compare the same numbers from the runtime side with the lead-acid runtime calculator, which uses the same fitted correction.