Battery Size Calculator

How to Size a Battery for a Load

Battery size here means the amp-hour capacity you need so a load can run for a chosen time and still leave a reserve charge. Load current is power divided by battery voltage. Chemistry then scales that current and time into amp-hours.

$$I_L = \frac{P}{V}$$

Lead-acid vs Lithium-ion Formulas

Lead-acid capacity uses a simple Peukert-style correction that grows with discharge time:

$$B_{\text{Pb}} = \frac{100 \cdot I_L \cdot t}{(100 - Q)(0.02 t + 0.6)}$$

Lithium-ion capacity is linear in current and time:

$$B_{\text{Li}} = \frac{100 \cdot I_L \cdot t}{100 - Q}$$

$t$ is duration in hours and $Q$ is the percent charge you want left. Example: a 200 W desktop on a 12 V lead-acid UPS for 10 minutes with 50% remaining needs about 16.7 A and 9.2 Ah.

Do not drain lead-acid packs to empty; many designs stop near 50%. Lithium-ion packs often stay healthier if you avoid deep cycles below about 20%.

Frequently Asked Questions

Why is lead-acid capacity larger than lithium-ion for the same load?

The lead-acid formula includes a factor (0.02 t + 0.6) that is less than 1 for typical runtimes, which increases the Ah rating needed compared with an ideal linear cell.

How do I convert watt-hours?

Wh = Ah × V. A 9.2 Ah 12 V pack stores about 110 Wh of energy before reserve and efficiency losses.

What remaining charge should I pick?

Use about 50% for lead-acid and 20% for lithium-ion if you want a conservative reserve. Higher leftover charge means a larger pack.

Does this include inverter efficiency?

No. If an inverter wastes 10% as heat, increase the wattage input (or the result) by that loss before buying a pack.

Can I use this for EVs?

It sizes DC amp-hours from a constant load. Vehicle packs also need voltage, C-rate, temperature, and BMS limits that this calculator does not model.

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How to Size a Battery for a Load

Lead-acid vs Lithium-ion Formulas

Frequently Asked Questions

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