Enter your load, voltage, and runtime, and this battery size calculator provides the capacity, current draw, and discharge rate across lead-acid, lithium-ion, or LiFePO4 batteries.
Battery Size Calculation Results
Battery Bank Size Calculator: Amp-Hour Capacity for Off-Grid and Backup Power Loads
This calculator converts a DC load, system voltage, discharge duration, and reserve charge into the minimum battery bank capacity needed to run it, expressed in amp-hours. Off-grid solar installers, RV and marine electricians, and backup or UPS system designers use it to turn a known load and autonomy period into a battery bank size before picking individual cells or batteries.
How to Use the Battery Size Calculator
Enter the load in watts, system voltage in Vdc, the battery chemistry, how many hours the load must run, and the percentage of charge you want left in reserve. The hero figure shows required capacity in amp-hours; the four cards below break out load current, discharge rate (C-rate), safe current limits for the selected chemistry, and total energy draw.
The Amp-Hour Sizing Formula
The capacity math follows the same load-to-current, duration-to-runtime approach set out in IEEE 1013, the IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone Photovoltaic Systems: convert the load to current, then extend the runtime to account for the portion of capacity you don’t intend to use.
$$I = \frac{P}{V}$$
where $P$ is the load in watts and $V$ is system voltage. Runtime is then extended to a full-depletion time:
$$t_{full} = \frac{t}{1 – r}$$
where $t$ is the duration you entered and $r$ is the reserve fraction (Remaining charge % ÷ 100). Capacity follows directly:
$$Ah = I \times t_{full}$$
The C-rate figures use the standard relationship current = C-rate × rated capacity, as characterized in IEC 61960 battery capacity testing methodology.
Common mistake: the Remaining charge % field is the reserve you want to keep, not the depth of discharge. Entering 80 here sizes the bank so 80% stays untouched — the opposite of an 80%-depth-of-discharge lithium setup, which calls for entering 20.
How Load, Duration, and Reserve Combine
The battery type dropdown does not change the headline Ah figure. Only load, voltage, duration, and reserve percentage feed the capacity formula — chemistry only changes the safe-current and stress-percentage figures in the results grid. Two setups with identical load, voltage, duration, and reserve but different chemistries will always return the same required Ah bank size, with different “max safe current” numbers underneath.
Verified Reference Values Used in This Calculator
| Parameter | Value | Source |
|---|---|---|
| Lead-acid capacity discharge-rate convention | C/20 (0.05C) | Standard stationary lead-acid capacity testing convention (IEEE/IEC 20-hour rate) |
| Continuous-load safety margin for breaker/conductor sizing | 125% of calculated current | NEC 210.19(A)(1) / 210.20(A) |
The lead-acid 0.05C default lines up with this 20-hour rating convention. The 1.0C (lithium-ion) and 0.5C (LiFePO4) “chemistry spec rate” defaults are common placeholder values, not figures pulled from one universal standard — continuous discharge ratings for lithium cells vary by manufacturer and can run from about 0.5C to 3C or higher. Check your battery’s datasheet before relying on these defaults for safety-critical sizing.
Battery Size Calculator: Frequently Asked Questions
What does the “Remaining charge %” field control?
It’s the percentage of capacity you want to keep in reserve, not the depth of discharge. A value of 50 sizes the bank to use only the other 50% of capacity before hitting your reserve.
Does changing the battery type change the required Ah size?
No. Capacity math uses only load, voltage, duration, and reserve. Chemistry only recalculates the safe-current and stress-percentage figures in the results grid, not the headline Ah number.
How is the Load Current figure calculated?
Current (A) equals load in watts divided by system voltage — Ohm’s law applied to a DC circuit: $I = P/V$.
Should I trust the calculator’s default C-rate for my lithium battery?
Treat it as a rough placeholder only. Lead-acid’s 0.05C (C/20) rating is a widely used convention; lithium continuous-discharge ratings vary by manufacturer, so check your battery’s datasheet.
Does this calculator account for inverter losses on AC loads?
No. Enter the DC-side wattage. For an inverter-fed AC load, divide the AC watts by the inverter’s efficiency (commonly 85–95%) before entering it here.