Energy converter

Watts to Amp-hours Calculator

Convert power in watts and run time into battery capacity in amp-hours.

How Watts to Ah works

Amp-hours describe how much charge a battery can deliver, while watts describe the power a load draws. To convert, first find the energy in watt-hours (power × time), then divide by the battery voltage.

This calculation is essential for sizing batteries in solar, RV, and off-grid systems. Because it depends on voltage, the same load needs far fewer amp-hours at 48 V than at 12 V.

Formula

1Ah = (W × h) ÷ V

Watt-hours = watts × hours; amp-hours = watt-hours ÷ volts.

Watts to Ah conversion table

Reference Table5 rows

Amp-hours needed to run a load for 5 hours at 12 V.
Load Power↕	Energy (Wh)↕	At 12 V↕	At 24 V↕
50 W	250 Wh	20.8 Ah	10.4 Ah
100 W	500 Wh	41.7 Ah	20.8 Ah
200 W	1,000 Wh	83.3 Ah	41.7 Ah
500 W	2,500 Wh	208 Ah	104 Ah
1,000 W	5,000 Wh	417 Ah	208 Ah

💡 Click any row to auto-fill the calculator aboveClick column headers to sort

FAQ: Watts to Ah

How do you convert watts to amp-hours?

Multiply watts by hours to get watt-hours, then divide by the battery voltage. Ah = (W × h) ÷ V. For example, 100 W for 5 h at 12 V = 500 Wh ÷ 12 = 41.7 Ah.

Why does voltage matter for amp-hours?

Amp-hours measure charge, not energy. The same watt-hours require fewer amp-hours at higher voltage, so a 48 V battery needs a quarter of the Ah a 12 V battery would for the same load.

Should I add margin to the amp-hour result?

Yes. Real batteries should not be fully discharged, so add headroom — for lead-acid, size for about twice the calculated Ah to stay within a 50% depth of discharge.

Tools related to Watts to Ah

Watts to Joules ConverterThe full power × time energy calculator with three modes.kWh to JoulesTurn the kWh figure on your electricity bill into joules — 1 kWh = 3,600,000 J.Joules to kWhConvert a joule or megajoule figure into billable kilowatt-hours — divide by 3,600,000.Watts to AmpsCalculate amps from watts and volts for DC and AC circuits — I = P ÷ V.Joules to CaloriesConvert joules into calories and food Calories (kcal) — 1 cal = 4.184 J.Amps to WattsCalculate watts from amps and volts for DC and AC circuits — P = I × V.Volts to WattsConvert potential (volts) to power (watts) given the electric current — P = V × I.Wh to JoulesConvert watt-hours to joules — 1 Wh = 3,600 J.BTU to JoulesConvert heating or cooling thermal units (BTU) to joules — 1 BTU = 1,055.06 J.BTU/hr to WattsConvert heating/cooling power (BTU/hr) to electrical watts — 1 BTU/hr = 0.293071 W.mAh to JoulesConvert battery milliamp-hours (mAh) to joules given nominal voltage — E = mAh × V × 3.6.Battery Run TimeEstimate battery run time under a given load — active hours = capacity × volts × DoD ÷ load.Appliance Energy CostCalculate electricity cost for any appliance — cost = power × hours ÷ 1,000 × rate.Ohm's LawSolve Voltage, Current, Resistance, and Power from any two inputs using Ohm's Law.Voltage DropCalculate voltage drop over long electrical wire runs — V_drop = 2 × K × I × L ÷ Area.AC BTUCalculate heating/cooling BTUs needed for a room — base BTU = area × 20.Watts to HPTurn watts into horsepower — 1 mechanical hp = 745.7 W.HP to WattsTurn horsepower into watts — 1 mechanical hp = 745.7 W.Joules to WattsTurn joules of energy into watts of power — W = J ÷ seconds.Watts to OhmsFind resistance in ohms from power and voltage — R = V² ÷ P.Joules to eVTurn joules into electronvolts — 1 eV = 1.602176634×10⁻¹⁹ J.Electricity CostFind the running cost of any device — cost = (W ÷ 1000) × hours × rate.Ah to WattsFind the load a battery supports — W = (Ah × volts) ÷ hours.Watts to VoltsFind voltage from power and current — V = P ÷ I.Joules to WhTurn joules into watt-hours — 1 Wh = 3,600 J.Joules to BTUTurn joules into BTU — 1 BTU = 1,055.06 J.Watts to Joules per SecondWatts and joules per second are the same unit — 1 W = 1 J/s.