Amp Hours to Watt Hours Calculator

To convert amp hours to watt hours, multiply the amp hours by the battery’s voltage. A 100Ah battery at 12V holds 100 × 12 = 1,200 watt hours. The voltage is not an optional detail: the same 100Ah rating at 24V is 2,400Wh, exactly twice the energy behind an identical label.

The formula, explained

Amp hours measure charge. Watt hours measure energy. Voltage is what turns one into the other, and it is the only term in the conversion that requires any judgement.

• Amp hours (Ah) is the figure printed on the battery. A 100Ah battery supplies 100 amps for one hour, or 5 amps for 20 hours. It is a statement about current and time, nothing more.
• Volts (V) is the nominal voltage of the finished pack — not one cell inside it, and not the 14.6V a charger pushes in while charging.
• Watt hours (Wh) is energy, the unit every load list, spec sheet and solar estimate is written in. A 1,200Wh battery runs a 100W load for roughly 12 hours before losses.

Kilowatt hours are the same answer divided by 1,000, which is how utilities bill and how home batteries are sold; the Ah to kWh calculator skips that step.

Amp hours to watt hours conversion table

Voltages below are nominal: 3.7V is a single lithium-ion cell, and 12V, 24V and 48V are the standard bank configurations. Find your rating on the left and read across to your pack’s column.

The 100Ah row is highlighted because it is the industry’s default unit. A 100Ah 12V LiFePO4 block is what most van, boat and shed installations are built from, and 1,200Wh is the figure to hold in your head.

Read across and a second fact appears. 50Ah at 24V and 100Ah at 12V are both 1,200Wh: same energy, same runtime, but the 24V version moves it at half the current, which means thinner cable, smaller fuses and less voltage drop. That is the whole argument for a higher-voltage bank, and it is invisible if you only read amp hours.

Why this conversion matters for portable power and solar

The industry uses two units for one thing. Power stations are sold in watt hours, because that is what a buyer comparing two 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. Every build crosses that line, and watt hours is the side worth standing on.

Sizing a battery bank is three steps:

1. Add up what you want to run, in watt hours per day.
2. Divide by your pack voltage to get the amp hours you need on paper.
3. Divide again by the fraction of the battery you are allowed to use, because no chemistry gives you all of it.

Worked through: a 900Wh daily load on a 12V system is 900 ÷ 12 = 75Ah drawn. On lithium iron phosphate at 90% usable that is 83Ah, so a single 100Ah battery covers a day. On lead-acid at 50% usable it is 150Ah, so the identical job needs two 100Ah batteries at roughly four times the weight.

Solar meets the same figure from the other end. Panels are rated in watts, so multiply the array by realistic peak sun hours and take off system losses: 300W × 4 peak sun hours × 75% is about 900Wh a day, which replaces exactly the load above. The solar battery calculator pairs the two, and our solar sizing guide handles peak sun hours properly, because that is the assumption most estimates get wrong.

Going the other way, from a power station’s watt hour rating back to the amp hours batteries are sold in, use the watt hours to amp hours calculator. To size the box rather than the bank, what size power station do I need turns a load list into capacity and output figures, and off-grid RV power covers the van case, where the DC bank and the AC inverter are separate decisions.

A 12V battery is rarely 12 volts

Nominal voltage is a naming convention, not a measurement, and chemistries land in different places. It is the usual source of the gap between your arithmetic and the manufacturer’s printed watt hour figure.

The gap between 12.0V and 12.8V is 6.7%, which is why a “100Ah” LiFePO4 battery is advertised as 1,280Wh rather than 1,200Wh. Neither figure is wrong. Use 12.8V to match the spec sheet, or 12V for a planning number with a cushion in it. Do not use the fully charged 13.6V or 14.6V: the battery sits there only briefly, and quoting it overstates capacity by a tenth.

Power bank ratings catch people the other way. Capacities are quoted in mAh at the cell’s 3.7V, not at the 5V leaving the USB port, so a 20,000mAh bank stores 74Wh rather than 100Wh. The mAh to Wh calculator handles that case, and 100Wh is the threshold that matters for airline carry-on rules.

Rated watt hours are not usable watt hours

Amp hours times volts gives the energy stored in the chemistry, not the energy that reaches your appliance. Two deductions sit in between: how deeply you may discharge the battery, and what the inverter loses turning DC into AC.

So the honest planning figure for a 100Ah LiFePO4 battery running AC appliances is around 950Wh, not the 1,200Wh the multiplication gives or the 1,280Wh on the box. Plan on that and you will not be surprised at two in the morning.

Two things move that answer. DC loads — a 12V compressor fridge, LED lighting, a fan wired straight to the bank — skip the inverter and keep that 10 to 12%, which is why DC fridges are standard in vans. Cold costs real capacity: below freezing, expect noticeably less than rated from any lithium chemistry, and most LiFePO4 packs refuse to charge until they warm up. Size for winter if the battery lives in a garage or an unheated shed.

Frequently asked questions