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Power and Battery Calculators

Convert between watts, amps, volts, watt-hours and amp-hours, and find out what those numbers actually mean for a battery you are about to buy.

The power and battery calculators here all revolve around four quantities that people routinely mix up: watts, watt-hours, amps and amp-hours. Two of them describe a rate. Two describe an amount. Getting them the wrong way round is the most expensive error in battery sizing, and it happens constantly.

A watt is a rate. It is how fast something consumes energy right now, the way miles per hour describes a car in motion. A watt-hour is a quantity: one watt drawn for one hour. A 60W fan and a 1,200Wh battery are not comparable figures, but divide the second by the first and you get twenty hours of fan. That division is most of what battery sizing is.

Amps work the same way one layer down, with a complication. Current depends on voltage as well as power, so converting watts to amps is impossible until you say what voltage the circuit runs at: the same 1,200W load draws 10 amps at 120V and 100 amps at 12V. Amp-hours are then the quantity version, a stored charge rather than stored energy.

That is why an amp-hour rating on its own tells you nothing. A 100Ah battery holds 1,200Wh at 12V and 2,400Wh at 24V — identical labels, double the energy. Any honest comparison between two batteries has to happen in watt-hours, which is what the amp hours to watt hours and watt hours to amp hours converters are for.

A portable power station puts two of these numbers on the box, and they answer different questions. Capacity, in watt-hours, answers how long. Output, in watts, answers whether it will run the thing at all. A 2,000Wh unit with a 1,000W inverter holds a fridge for about a day and refuses a 1,500W kettle outright. Both numbers have to clear your requirement independently. Our guide to what size power station you need works through both, and the finder filters on them together.

The usual mistake is buying the wrong number. Most oversizing is capacity bought when the real constraint was output — a bigger battery that still trips on the same appliance — or the reverse, plenty of output and ninety minutes of runtime. Settle both before you shop: the peak watts you need running at once, and the watt-hours you need across the whole outage.

Power and current

Watts to AmpsEnter the wattage and the supply voltage to get the current draw.Amps to WattsEnter the current and the supply voltage to get the power draw.Volts to WattsVoltage alone does not give you watts — you need the current as well.Watts to VoltsEnter the power and the current to get the voltage.kW to AmpsKilowatts to current, for single-phase or three-phase supplies.VA to WattsApparent power to real power. The difference is the power factor.

Battery capacity

Amp Hours to Watt HoursBattery capacity in Ah is meaningless without the voltage. This adds it.Watt Hours to Amp HoursConvert a power station’s Wh rating into the Ah figure batteries are sold in.Ah to kWhAmp hours to kilowatt hours, the unit your electricity bill uses.kWh to AhKilowatt hours back to amp hours at your system voltage.mAh to WhPower bank capacity in the unit airlines actually limit.Wh to mAhWatt hours to milliamp hours at a given cell voltage.Watts to kWhA wattage held for a number of hours, in billable kilowatt hours.

Heating and cooling

BTU to WattsBTU per hour to watts, for sizing a battery to an air conditioner or heater.Watts to BTUWatts to BTU per hour, the unit air conditioners are sold in.

Solar

Solar Panel AngleYour latitude sets the tilt. Pick a season to bias it.Solar Battery SizeDaily consumption, days of autonomy and depth of discharge, in amp hours.