Watt Hours to Amp Hours Calculator

To convert watt hours to amp hours, divide the energy in watt-hours by the battery voltage: Ah = Wh ÷ V. For example, 1,200 watt-hours at 12 volts equals 1,200 ÷ 12 = 100 amp-hours. The same 1,200 Wh at 24 volts is only 50 Ah, and at 48 volts it is 25 Ah — the energy is identical, but higher voltage means fewer amp-hours are needed to store it.

This matters because battery labels mix two different units. Amp-hours (Ah) describe charge, while watt-hours (Wh) describe energy. Only energy tells you how much work a battery can actually do, and only energy lets you compare a 3.7V phone cell against a 48V solar bank. The calculator above converts instantly and also shows milliamp-hours and kilowatt-hours so you can cross-check any spec sheet.

The conversion rests on the definition of a watt-hour: one watt-hour is one volt pushing one amp for one hour. Rearranged for amp-hours, the formula is:

The key is choosing the right voltage. Battery capacity is always quoted at nominal voltage — the average voltage across a discharge — not the fully-charged voltage. A "12V" lead-acid or LiFePO4 battery is nominal 12V even though it reads 12.8–13.6V when full. Using the higher charged voltage understates amp-hours; always divide by the nominal figure shown in the table below.

This is simply the inverse of the amp hours to watt hours conversion (Wh = Ah × V). If you know two of the three quantities — energy, charge, or voltage — you can always solve for the third, the same way Ohm's law links volts, amps, and resistance.

These examples show the same math across different voltages and battery scales.

Example 1 — Solar generator label: A portable power station is rated 1,200 Wh. Its internal pack is 12V. Amp-hours = 1,200 ÷ 12 = 100 Ah. The same station built on a 24V pack would be a 50 Ah battery storing the identical energy — useful to know when comparing two units that quote different units.

Example 2 — Phone battery: A phone battery stores 14.8 Wh on a single 3.7V Li-ion cell. Amp-hours = 14.8 ÷ 3.7 = 4 Ah, which is 4,000 mAh — exactly the number printed on the phone spec sheet. This is why phones quote mAh: at a fixed 3.7V, mAh is a convenient stand-in for energy.

Example 3 — Off-grid bank: An installer specifies 10 kWh of storage at 48V. Convert kWh to Wh first (10,000 Wh), then Ah = 10,000 ÷ 48 = 208 Ah. You would round up to a 200–210 Ah 48V battery. Notice how 48V keeps the amp-hours — and therefore the cable and fuse sizes — small for a large amount of energy.

Example 4 — Reverse check: If a 12V battery is labeled 100 Ah, its energy is 100 × 12 = 1,200 Wh, confirming Example 1. Running that check both directions is the fastest way to catch a mislabeled or exaggerated capacity claim.

Amp-hours alone are meaningless without a voltage, yet marketing frequently quotes only Ah. A 100 Ah 12V battery and a 100 Ah 48V battery are wildly different products: the first stores 1,200 Wh, the second 4,800 Wh — four times the energy. Always convert to watt-hours before comparing.

Higher voltage is desirable in larger systems because power equals volts times amps. To deliver 2,000 watts, a 12V system must move about 167 amps, demanding thick, expensive cable and large fuses; a 48V system moves only about 42 amps for the same power. That is why RVs and small setups stay at 12V for simplicity, while whole-home and commercial storage move to 48V. When you size a bank, decide the voltage first, then use this calculator to find the amp-hours you need. Pair it with the battery bank size calculator and series and parallel battery calculator to plan the wiring.

Four units appear on battery labels, and they answer two different questions. Charge — amp-hours (Ah) and milliamp-hours (mAh) — measures how much current a battery can supply over time. Energy — watt-hours (Wh) and kilowatt-hours (kWh) — measures how much work it can do. Voltage is the bridge between them.

Milliamp-hours and amp-hours differ only by a factor of 1,000: a 4,000 mAh cell is a 4 Ah cell. Watt-hours and kilowatt-hours also differ by 1,000: 1,200 Wh is 1.2 kWh. The only conversion that requires more than moving a decimal point is between charge and energy, and that is where voltage enters — energy equals charge times voltage. This is why you can never compare a mAh figure to a Wh figure directly; you must know the voltage first. Manufacturers exploit this ambiguity: a "20,000 mAh" power bank sounds larger than a "74 Wh" one, yet at 3.7V they are identical. Convert everything to watt-hours with this tool and the marketing noise disappears.

Keep these anchor conversions handy: at 12V, 1 Ah equals 12 Wh; at 3.7V, 1 Ah equals 3.7 Wh; and 1,000 mAh always equals 1 Ah. From those, every other figure follows. For the energy-first direction, the Ah to kWh calculator takes charge straight to kilowatt-hours.

The amp-hours this calculator returns are rated — the capacity assuming a full discharge to empty. In practice, no battery should be drained completely, so the amp-hours you can actually use are fewer. The gap depends entirely on chemistry.

Lithium iron phosphate (LiFePO4) tolerates deep cycling, so 80–100% of the rated amp-hours are usable with little effect on lifespan. Lead-acid batteries — flooded, AGM, and gel — should stop near 50% to avoid rapid capacity loss; routine deep discharges can cut their cycle life from 500 cycles to under 200. That means a "100 Ah" lead-acid battery realistically delivers about 50 usable amp-hours, while a "100 Ah" lithium battery delivers 80 or more. When you convert watt-hours to amp-hours to size a purchase, remember that the rated figure is a ceiling, not a working number. If you need 100 usable amp-hours from lead-acid, buy roughly 200 rated amp-hours; from lithium, buy about 120. To fold depth of discharge into a runtime estimate directly, use the battery runtime calculator, and to size a full bank around usable energy, use the battery bank size calculator.

The amp-hours from a Wh-to-Ah conversion assume laboratory conditions — about 25°C (77°F) and a slow, standard discharge. Two field factors shift the real number, and both are worth building into a plan.

Temperature. Manufacturers rate capacity at room temperature. In the cold, chemistry slows and effective amp-hours fall: lead-acid can shed 20–35% of capacity near freezing, and lithium loses some too and must not be charged below 0°C without a heater. A battery that converts to 100 Ah on paper may behave like 70–80 Ah on a winter morning, so cold-climate builds add 20–30% headroom or insulate the enclosure.

Discharge rate. Draw current faster than the rating assumes and lead-acid delivers fewer amp-hours — the Peukert effect. A battery rated 100 Ah over 20 hours might give only 75–80 Ah when emptied in two. LiFePO4 is largely immune, holding its rated amp-hours across most practical loads. When your converted figure feeds a high-current inverter, check the datasheet's C-rating before trusting it.

1. Enter energy in watt-hours. If your figure is in kilowatt-hours, multiply by 1,000 first (10 kWh = 10,000 Wh). If it is in milliwatt-hours, divide by 1,000.
2. Select the nominal system voltage. Choose 3.7V for a single Li-ion cell, or 12/24/48V for a battery bank. Use the nominal value, not the charged voltage.
3. Read the amp-hours result. This is the charge capacity you would see on a battery label rated at that voltage.
4. Check the secondary outputs. Milliamp-hours (mAh) is handy for small cells; kilowatt-hours (kWh) is handy for whole-home and utility comparisons.

To go the other way — from amp-hours back to energy — use the amp hours to watt hours calculator. To estimate how long that energy lasts under a load, use the battery runtime calculator.

• Dividing by charged voltage instead of nominal. Using 14.4V instead of 12V understates amp-hours by 20%.
• Comparing amp-hours across different voltages. 100 Ah at 12V is not the same as 100 Ah at 24V — convert both to watt-hours first.
• Confusing mAh and Ah. 5,000 mAh is 5 Ah; a factor-of-1,000 slip is common with phone and power-bank ratings.
• Ignoring depth of discharge. The Ah figure is rated capacity; usable capacity is lower (about 50% for lead-acid, 80–100% for lithium).
• Forgetting inverter losses. Amp-hours describe stored energy; delivered AC energy is 5–15% less after inverter conversion.

For a fuller picture of usable energy, combine this conversion with the runtime calculator, which layers depth of discharge and efficiency on top of the raw Wh-to-Ah math.

Solar and off-grid design. System sizing usually starts in kilowatt-hours of daily consumption, but batteries are sold in amp-hours at a chosen bank voltage. Converting Wh to Ah is the bridge between an energy audit and a shopping list.

RV, van, and marine builds. DIY builders spec lithium packs by amp-hours but plan loads in watts. Converting a target energy budget into amp-hours tells you how many 100 Ah or 200 Ah batteries to buy and how to wire them in series or parallel.

Power stations and UPS units. Manufacturers advertise watt-hours because it is the honest comparison across chemistries and voltages. Converting back to amp-hours lets you verify the internal pack and estimate charge current requirements.

Drone, e-bike, and RC batteries. These are quoted in mAh at a cell-stack voltage. Converting to watt-hours (and back) reveals the true energy, which is what actually governs range and flight time. For a full storage plan, continue with the solar battery bank calculator.