Watt Hours to Amp Hours Calculator

To convert watt hours to amp hours, divide the watt hours by the battery’s nominal voltage. A 1,200Wh battery at 12V holds 1,200 ÷ 12 = 100 amp hours. The same 1,200Wh on a 24V bank is 50Ah, so an amp hour figure means nothing until you know the voltage it was measured at.

The formula, and the term people get wrong

Three terms go into this conversion, and only one is ambiguous.

• Watt hours (Wh) are energy: the work stored in the battery, and the only figure that compares directly between two products of different design.
• Volts (V) is the nominal voltage of the battery itself. Not the 120V at the outlet, not the 5V at a USB port, not the 14.6V a charger pushes. This is the term that gets picked wrong.
• Amp hours (Ah) are charge: how much current the battery can supply, and for how long, at one voltage. Change the voltage and identical stored energy becomes a different amp hour number.

So 1,200Wh ÷ 12V = 100Ah and 1,200Wh ÷ 24V = 50Ah. Nothing about the battery changed; only the denominator did. That is why watt hours are a sound shopping unit and amp hours are not, and why the reverse conversion matters too: loose batteries are sold in Ah.

Quick reference table: Wh to Ah

Columns are the three nominal system voltages you will actually meet: 12V for a single battery, a car or an RV; 24V for a mid-size off-grid bank; 48V for a home battery or a larger array. Figures are rounded to one decimal place.

The 1,200Wh row is worth memorising: at 12V, watt hours and amp hours cross at exactly 100, and every other row is that relationship scaled. Halve the amp hours each time the voltage doubles.

One caveat before you screenshot it. A lithium iron phosphate bank is really 12.8V, 25.6V or 51.2V, so the honest figures are about 6% lower than the columns above. If your capacity is quoted in kilowatt hours, as home batteries usually are, the kWh to amp hours calculator saves you a step.

Why watt hours to amp hours matters for portable power and solar

The two halves of this market speak different languages. Power stations are sold in watt hours, because that is what a buyer comparing two sealed boxes needs. Loose batteries — RV house banks, marine cells, LiFePO4 drop-ins — are sold in amp hours, because that is what someone wiring them needs. You cross between the two every time you plan a system, and the crossing is where sizing errors happen.

Work an example. A 1,000Wh daily load on 12V is 1,000 ÷ 12 = 83.3Ah drawn. Run it through an inverter at 88% and you really pull 1,136Wh, which is 94.7Ah. Allow a 90% depth of discharge on LiFePO4 and you need 105Ah installed, so a single 100Ah battery is marginal rather than comfortable. On lead-acid at 50% usable the same day needs 190Ah — two batteries, four times the weight.

Amp hours also decide hardware that watt hours cannot describe, because controllers, fuses, breakers and cable gauge are rated in amps. A 400W array on a 12V bank pushes roughly 400 ÷ 12 = 33A, so it wants a 40A controller, not the 30A one that looked adequate. That is most of what makes an RV or van build different from plugging appliances into a finished unit. For panels and battery together, use the solar battery calculator, and our solar sizing guide handles peak sun hours properly. If you would rather buy than build, what size power station do I need turns the same load list into capacity and output figures.

The voltage on the label is not the voltage in the pack

Battery voltages are naming conventions. A “12V” lithium battery is four 3.2V cells in series, so 12.8V, and dividing by 12 instead overstates its amp hours by 6.7%. Here is what one fixed quantity of energy, 1,200Wh, becomes at each voltage you might be holding.

Use the nominal voltage, never the resting or charging voltage: a full 12V lead-acid battery reads about 12.7V and a charger drives it to 14.4V, but it only sits up there briefly. Where a manufacturer publishes both Wh and Ah for one product, divide one by the other and you have the pack voltage they used.

Power banks catch almost everyone, because their mAh rating is quoted at the 3.7V cell voltage rather than the 5V leaving the USB port. A 20,000mAh pack is 20Ah at 3.7V, which is 74Wh, and 74Wh delivered at 5V is only 14.8Ah before a single conversion loss. Nothing is faked; the figures are measured at different voltages. The Wh to mAh converter runs that case.

Usable amp hours, which is what you actually spend

Dividing watt hours by voltage tells you what the battery holds, not what you may take out or what survives the trip to an AC appliance. Two deductions sit in between, and for lead-acid the first is brutal. The table takes the same 1,200Wh, 100Ah at 12V, through both.

Two batteries with identical labels, and one gives 528Wh at the socket while the other gives 950Wh. That is the strongest argument for lithium iron phosphate, and it is invisible to anyone reading only the amp hour rating.

The inverter column disappears if the load is DC. A 12V fridge, LED lighting or a fan wired straight to the bank keeps the 12% that conversion costs, which is why DC compressor fridges are standard in vans. Cold takes more again: below freezing, expect noticeably less than rated from any lithium chemistry, and most LiFePO4 packs refuse to charge until they warm up.

Amp hours are not amps

It is tempting to read an amp hour figure as a current rating. It is not one: amp hours are amps multiplied by time. A 100Ah battery does not promise 100 amps, it promises 100 amp hours of charge, which could be 5A for 20 hours or 20A for five. What limits current is the battery management system, and its continuous discharge rating is a separate specification. Size capacity from amp hours; size the inverter, the BMS and the cable from amps.

That split is also why AC and DC figures must never be mixed in one sum. Amp hours exist only on the DC side of the inverter, where a 600W load is 50A at 12V. On the AC side the same 600W is 5A at 120V. When a wiring diagram and a load list disagree by a factor of ten, this is the reason.

Frequently asked questions