Calculate energy in watt-hours from power and time, Ah and voltage, or estimate battery run time and required capacity. Includes DoD, efficiency, cost estimation, and battery comparison visual.
A watt-hour (Wh) is a unit of energy equal to one watt sustained for one hour, or 3,600 joules. It is the standard billing unit for electricity (as kWh) and the rating unit for batteries. Understanding watt-hours is essential for sizing batteries, estimating run times, calculating electricity costs, and comparing energy storage devices.
The basic relationship is Energy = Power × Time. A 100 W light bulb running for 10 hours consumes 1,000 Wh = 1 kWh. Battery capacity is often given in milliamp-hours (mAh) or amp-hours (Ah), which must be multiplied by voltage to get watt-hours: Wh = Ah × V. A 20,000 mAh power bank at 3.7 V stores 74 Wh.
This calculator handles four modes: compute Wh from power and time, convert between Ah and Wh, estimate battery run time (accounting for depth of discharge and inverter efficiency), and calculate required battery size for a given load and duration. A battery comparison visual puts your result in context from AA cells to home Powerwalls.
Battery datasheets mix mAh, Ah, Wh, and kWh. Converting between them requires knowing the voltage. Run time estimates must account for depth of discharge (you shouldn't drain LiFePO4 below 20%) and inverter losses. This calculator handles all these conversions and adjustments in one place. Keep these notes focused on your operational context.
Energy (Wh) = Power (W) × Time (h) Energy (Wh) = Capacity (Ah) × Voltage (V) Run Time (h) = (Wh × DoD × η) / Load (W) Required Battery (Wh) = (Load × Hours) / (DoD × η) Where DoD = depth of discharge (0–1), η = inverter efficiency (0–1)
Result: Run time = 3.6 hours
A 500 Wh battery with 80% DoD provides 400 Wh of usable energy. At 90% inverter efficiency, 360 Wh reaches the load. Dividing by 100 W gives 3.6 hours of run time. The actual capacity needed to run 100 W for 3.6 hours would be exactly 500 Wh.
Not all watt-hours are created equal. Lead-acid batteries store about 30-50 Wh/kg. Lithium-ion (NMC) achieves 150-260 Wh/kg. LiFePO4 provides 90-160 Wh/kg but with superior cycle life. Solid-state batteries promise 400+ Wh/kg. Energy density determines how compact and light a battery can be for a given Wh rating — critical for EVs, drones, and portable electronics.
Utility companies measure energy in megawatt-hours (MWh) and gigawatt-hours (GWh). A Tesla Megapack stores 3.9 MWh. The Hornsdale Power Reserve in Australia stores 194 MWh. Pumped hydro, the most common grid storage, can store GWh by pumping water uphill during surplus and generating during demand — converting between kinetic and electrical energy measured in watt-hours at enormous scale.
A smartphone battery (15 Wh) stores enough energy to lift a 55 kg person about 10 meters — surprisingly little. A Tesla Model 3 battery (60 kWh) stores the energy equivalent of about 2 gallons of gasoline (though the EV uses it 3-4× more efficiently). Understanding watt-hours helps compare energy sources: 1 gallon of gasoline ≈ 33.7 kWh, 1 kg of lithium battery ≈ 0.2 kWh, 1 kg of TNT ≈ 1.2 kWh.
Wh (watt-hours) measures energy. Ah (amp-hours) measures charge. Wh = Ah × V. A 100 Ah battery at 12V stores 1,200 Wh. At 48V, the same 100 Ah stores 4,800 Wh. Always use Wh for energy comparisons across different voltage systems.
1 kWh = 1,000 Wh. This is the standard electricity billing unit. A typical US household uses about 30 kWh per day or 900 kWh per month.
Several factors: depth of discharge (never 100%), inverter/converter losses (5-15%), temperature effects (cold reduces capacity), self-discharge, and Peukert effect (high loads reduce effective capacity). Account for 70-80% usable energy in practice.
Wh = mAh × V / 1000. A phone battery rated 4,000 mAh at 3.85V = 4,000 × 3.85 / 1000 = 15.4 Wh.
Yes, use Wh. A 12V 100 Ah battery (1,200 Wh) stores the same energy as a 48V 25 Ah battery (1,200 Wh). Watt-hours normalize for voltage differences.
Most airlines allow lithium batteries up to 100 Wh per battery in carry-on. Batteries between 100-160 Wh require airline approval (limit 2). Above 160 Wh is prohibited. Spare batteries must be in carry-on, not checked luggage.