Unified Battery Bank Sizing Calculator
Sizes required battery capacity (Ah and kWh) from daily electrical consumption and autonomy days. To calculate how long an existing battery bank will last under load, use the Battery Runtime Estimator. To determine recharge time, use the Battery Charge Time Estimator.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Lithium vs Lead-Acid: The Usable Capacity Trap
Calculation Assumptions
- Considers usable depth of discharge: 80%–90% for LiFePO4 vs 50% for AGM/Gel lead-acid.
- Applies inverter conversion efficiency and autonomy reserve multiplier.
Engineering Limitations
- Cold temperature capacity derating must be factored separately for unconditioned battery compartments.
A daily load of 1,800 Watt-hours with 1.5 days of autonomy requires 2,700 Wh usable; on a 12V LiFePO4 bank (90% DoD), this calculates to a 250Ah (3,200 Wh nominal) battery bank.
The biggest marketing pitfall in battery sizing is comparing nameplate Amp-hours instead of usable energy:
- Lead-Acid (AGM/Gel/Flooded): Discharging a lead-acid battery beyond 50% state-of-charge causes rapid sulfation of lead plates, collapsing cycle life from 800+ cycles down to under 200 cycles. A 200Ah AGM battery only delivers 100Ah of usable power.
- Lithium Iron Phosphate (LiFePO4): LiFePO4 cells easily deliver 80% to 90% usable depth-of-discharge for 3,000 to 5,000 cycles. A 100Ah lithium battery delivers 80Ah to 90Ah of real power.
- Weight & Physical Footprint: Because lead-acid requires twice the nameplate capacity and has four times the physical density of lithium cells, an AGM bank is roughly 3.5 to 4 times heavier than an equivalent usable LiFePO4 bank.
Frequently Asked Questions
How do you calculate required battery bank size in Amp-Hours?
Calculate daily total energy consumption in Watt-hours (Wh) by multiplying each appliance's wattage by its daily operating hours. Divide total Wh by nominal battery voltage (12V, 24V, or 48V) to determine basic Amp-hours, then divide by the usable Depth of Discharge (DoD — 80% for LiFePO4, 50% for AGM) and system inverter efficiency (typically 85-90%).
Why can LiFePO4 batteries be discharged deeper than AGM or Lead-Acid?
Lithium Iron Phosphate (LiFePO4) chemistry exhibits exceptional cycle life (3,000 to 5,000+ cycles) even when regularly discharged to 80% or 90% Depth of Discharge (DoD). In contrast, lead-acid and AGM batteries experience rapid sulfation and plate degradation if discharged below 50% DoD, effectively halving their usable nameplate capacity.
How many days of autonomy should an off-grid battery bank have?
Days of autonomy represents how long your battery bank can power essential loads without any solar, alternator, or generator charging input. Standard design practices recommend 1 to 2 days for mobile RV/campervan systems with multiple recharge sources, and 3 to 5 days for remote residential off-grid solar installations subject to extended inclement weather.
Is a 24V or 48V battery bank better than 12V?
For systems with inverters over 2000W or total daily consumption exceeding 2000 Wh, higher system voltages offer immense advantages. Doubling voltage from 12V to 24V cuts operating current in half, reducing conductor cable thickness, decreasing copper cost, lowering voltage drop, and minimizing thermal losses by 75% (P_loss = I^2 * R).