Watt-Hours ↔ Amp-Hours Converter

Convert between Wh, kWh, Ah and mAh at any voltage. Edit any field — the others update instantly.

How much you can safely use without harming battery life.

Conversions are exact for the voltage you enter. Real batteries lose a little to heat and conversion (round-trip) inefficiency, so energy actually delivered to an AC load through an inverter is typically 5–15% below the watt-hours shown.

Building an off-grid or van/RV system? Use the Battery Runtime Calculator to see how long a load will last, or the Off-Grid Solar Calculator to size panels, battery bank and inverter together.

Watt-hours and amp-hours: what's the difference?

These two units describe a battery from different angles, and mixing them up is the single most common mistake in off-grid and battery sizing.

Amp-hours (Ah) measure electric charge — how much current a battery can supply for how long, regardless of voltage. A 100 Ah battery can deliver 100 amps for one hour, or 5 amps for twenty hours.

Watt-hours (Wh) measure energy — and energy is what actually runs your appliances. Because energy depends on voltage, watt-hours are the only fair way to compare two batteries at different voltages.

The conversion formula

Wh = Ah × V    and    Ah = Wh ÷ V

That's the whole thing. The voltage is the bridge between the two. For milliamp-hours, divide by 1000 first: Wh = (mAh ÷ 1000) × V

Kilowatt-hours are the same energy on a bigger scale — just watt-hours ÷ 1000. So to convert amp hours to kWh you go kWh = (Ah × V) ÷ 1000 and to go from kWh to Ah you reverse it: Ah = (kWh × 1000) ÷ V The kWh field above stays in step with everything else, so it doubles as an Ah–kWh calculator in both directions.

Why voltage matters so much

This is where intuition fails people. Look at two batteries both rated 100 Ah:

Same amp-hours, but the 24 V battery stores twice the energy. This is exactly why larger off-grid systems run at 24 V or 48 V — at higher voltage you move the same energy with less current, which means thinner wires, smaller fuses and lower losses. If someone quotes you a battery in amp-hours without a voltage, the number is meaningless on its own.

A worked example

You have a 12 V LiFePO4 battery rated 100 Ah and want to know its energy and how much you can actually use.

So a "100 Ah" battery realistically gives you about 960 Wh of work — enough to run a 40 W laptop charger and lights for the evening, or a 12 V fridge averaging 40 W for roughly a day.

Common voltage reference

Battery / deviceNominal voltage
Single lithium-ion / Li-po cell3.6–3.7 V
USB power bank (cell rating)3.7 V
Car / lead-acid "12V"12 V
LiFePO4 "12V" (4 cells)12.8 V
Larger off-grid bank24 V or 48 V

Power banks are usually advertised at the 3.7 V cell voltage. That's why a "20,000 mAh" bank only charges a phone (whose battery is also ~3.7 V) two to three times rather than the four you might expect — the 5 V USB output and conversion losses eat into the rated 74 Wh.

Frequently asked questions

How do you convert watt-hours to amp-hours?

Divide watt-hours by the battery voltage: Ah = Wh ÷ V. For example, 1200 Wh on a 12 V battery is 1200 ÷ 12 = 100 Ah. To go the other way, multiply: Wh = Ah × V.

How do I convert amp-hours to kWh (and kWh to Ah)?

Kilowatt-hours are just watt-hours ÷ 1000, so first turn Ah into Wh and then scale: kWh = (Ah × V) ÷ 1000. A 100 Ah battery at 12 V is 1,200 Wh, which is 1.2 kWh. To go from kWh to Ah, reverse it: Ah = (kWh × 1000) ÷ V — so 5 kWh on a 48 V bank is 5,000 ÷ 48 ≈ 104 Ah. The kWh field makes this Ah–kWh calculator work in both directions as you type.

What voltage should I use — 12V, 24V or 48V?

Use the nominal voltage of the battery or bank you're describing. A single LiFePO4 "12V" battery is nominally 12.8 V, a lead-acid 12V battery about 12 V, and a single lithium cell 3.6–3.7 V. The amp-hour figure only means something at a stated voltage — which is why two batteries with the same Wh can show very different Ah ratings.

How do I convert mAh to Wh for a power bank?

Convert mAh to Ah by dividing by 1000, then multiply by the cell voltage: Wh = (mAh ÷ 1000) × V. A 20,000 mAh bank rated at its 3.7 V cell voltage holds 20 × 3.7 = 74 Wh. The 5 V USB output delivers fewer usable Wh after conversion losses.

Why is the usable capacity lower than the rated capacity?

Fully draining a battery shortens its life, so you only use part of the rated capacity. The safe fraction is the depth of discharge (DoD). Lead-acid and AGM are usually limited to about 50% DoD; LiFePO4 lithium tolerates 80–100%. Usable Wh = rated Wh × DoD.

Is amp-hours the same as watt-hours?

No. Amp-hours measure charge and ignore voltage; watt-hours measure energy and account for voltage. Watt-hours are the fairer comparison, because 100 Ah at 12 V (1,200 Wh) stores half the energy of 100 Ah at 24 V (2,400 Wh).

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