Watt-Hours Calculator

About the Watt-Hours Calculator

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.

When This Page Helps

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, so you can size packs, estimate run time, and compare storage options without doing the voltage math by hand.

How to Use the Inputs

1. Select the calculation mode: energy from power × time, Ah → Wh, run time, or battery sizing.
2. Enter the known values (power, time, battery capacity, load).
3. For battery modes, set the depth of discharge (typical: 80% for lithium, 50% for lead-acid) and inverter efficiency (90-95%).
4. Enter the voltage for Ah ↔ Wh conversion.
5. Set your electricity rate for cost estimation.
6. Compare your energy value against common battery sizes in the visual.

Example Calculation

Tips & Best Practices

• Lithium-ion batteries should typically not be discharged below 20% (80% DoD). Lead-acid batteries: 50% DoD for longevity.
• Power banks often advertise mAh at cell voltage (3.7V). To charge a 5V device, actual usable capacity is lower: 20,000 mAh × 3.7V / 5V = ~14,800 mAh at 5V.
• Airline carry-on limit for lithium batteries is 100 Wh per battery (160 Wh with airline approval).
• To estimate monthly electricity cost: kWh per day × 30 × rate. A 1,500 W space heater running 8 h/day: 12 kWh/day × 30 × $0.12 = $43.20/month.
• Peukert's law: at higher discharge rates, batteries deliver less total energy. A battery rated 100 Ah at C/20 may deliver only 80 Ah at C/5.
• For off-grid solar, size batteries for 2-3 days of autonomy: daily Wh × 3 / DoD.

Battery Chemistry and Energy Density

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.

Grid-Scale Energy Storage

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.

Watt-Hours in Everyday Life

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.

Frequently Asked Questions

• What is the difference between Wh and Ah?

  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.

• How many watt-hours in a kWh?

  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.

• Why does my battery not last as long as its Wh rating suggests?

  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.

• How do I convert mAh to Wh?

  Wh = mAh × V / 1000. A phone battery rated 4,000 mAh at 3.85V = 4,000 × 3.85 / 1000 = 15.4 Wh.

• Can I compare batteries with different voltages?

  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.

• What is the airline carry-on limit for batteries?

  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.