The simplest possible battery sizing would be "load times days," but real batteries can't be run down to empty, and no system is perfectly efficient. Two corrections turn a naive estimate into a usable design number:
Putting both corrections together: Required Wh = (Load × Days) / (DoD × Efficiency), and dividing by your chosen bank voltage converts that into amp-hours (Ah), the unit battery capacity is usually rated in.
| Quantity | Formula / Typical Value |
|---|---|
| Required energy | Wh = (Load×Days) / (DoD×Efficiency) |
| Required amp-hours | Ah = Wh / Bank Voltage |
| Lead-acid safe DoD | ∼50% (flooded/AGM), some deep-cycle up to 60-70% |
| Lithium (LiFePO4) safe DoD | ∼80–90%, sometimes rated to nearly 100% |
| Typical system efficiency | 85–95% |
Once you know your required Ah, the next design steps are the Solar Panel Sizing calculator (panels to recharge this bank daily) and the Charge Controller Sizing calculator (matching a controller to your array and this battery voltage).
Use Wh = (Daily Load × Autonomy Days) / (Depth of Discharge × System Efficiency), then divide by your battery bank's voltage to get amp-hours (Ah).
DoD is the fraction of a battery's rated capacity that is safe to regularly use without significantly shortening its lifespan. A battery limited to 50% DoD needs roughly twice the rated capacity of one rated for 100% DoD, for the same usable energy.
50% is a common conservative figure for flooded and AGM lead-acid batteries used in regular cycling service; going deeper (60-80%) regularly can noticeably shorten cycle life, though occasional deeper discharges in an emergency are generally tolerated.
80-90% is typical and conservative for long cycle life; many LiFePO4 cells are rated for even deeper discharge, but leaving a small margin (rather than fully depleting to 0%) is still good practice for longevity.
1-2 days is common for grid-backup or sunny-climate off-grid systems; 3+ days is often used in cloudy climates or for critical loads where reliability matters more than upfront cost, since more autonomy directly means a larger, more expensive battery bank.
Energy (Wh) = Voltage × Amp-hours, so for a fixed energy requirement, a higher system voltage needs proportionally fewer amp-hours. This is why larger systems often move to 24V or 48V battery banks — it reduces both the Ah rating needed and the current (and wiring size) at a given power level.
85-95% is typical, covering inverter conversion losses and the battery's own round-trip charge/discharge efficiency. Lithium batteries generally have higher round-trip efficiency than lead-acid, so the higher end of this range is more realistic for lithium systems.
Size battery capacity (Ah/kWh) for your average daily energy consumption over the autonomy period, but separately verify your inverter and wiring can handle your peak instantaneous power draw (see the Solar Inverter Sizing calculator) — these are two different sizing questions.
Generally not — extra capacity beyond the minimum increases your safety margin, reduces average DoD per cycle (extending battery life), and provides headroom for load growth, at the cost of higher upfront price. Significant oversizing mainly matters if it goes unused for long periods, which can affect lead-acid batteries' health if left at low states of charge.
This calculator tells you how much energy you need to store; the Solar Panel Sizing calculator (using the same daily load figure) tells you how much panel wattage is needed to actually recharge that battery bank each day — both are necessary parts of a complete off-grid design.
Yes — most battery chemistries (especially lead-acid) deliver noticeably less usable capacity in cold temperatures and can have reduced lifespan in very hot conditions, so systems in extreme climates often add extra margin beyond this calculator's room-temperature-based estimate.
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