Watt-hour Calculator

How to use the watt-hour calculator

What this calculator does

This calculator estimates electrical energy in watt-hours by either of two exact unit relationships. The battery method multiplies charge capacity in ampere-hours by nominal voltage. The power-time method multiplies average power in watts by elapsed time in hours. Each valid result is also converted to kilowatt-hours and joules. The tool is useful for consistent unit conversion and first-pass energy comparisons, but it does not predict real battery runtime, conversion losses, voltage sag, temperature effects, inverter efficiency, or a device's changing load profile.

When to use it

Use the battery method when comparing cells or battery packs that have different voltage and amp-hour ratings, checking whether a replacement pack stores roughly the same nominal energy, or translating a product label from mAh into Wh. Use the power-time method when estimating the energy consumed by a lamp, appliance, charger, motor, or electronic device during a known operating period. The distinction between power and energy matters: the U.S. Department of Energy explains that storage systems have both an energy capacity and a separate power capacity.

How to calculate

1. The calculator opens with a complete demonstration: 3.7 V and 1200 mAh for a small battery, plus 100 W for 6 hours for a device. Results and a validated example Excel workbook are immediately available.
2. Replace the demonstration values in either input pair. You may use one method or both. If you start entering a pair, complete both required values so the calculator does not have to guess a missing quantity.
3. Choose a unit beside each value. Changing a unit converts the current number so the physical quantity stays the same; for example, 1200 mAh becomes 1.2 Ah.
4. Read the primary result, the secondary kWh and joule values, and the conversion table. When both methods are active, Scenario ratio shows how many times the power-time result is relative to the battery result.
5. Select Download Excel to export the current typed inputs and canonical results as a real .xlsx workbook. Reset clears the demonstration and all calculated data; after Reset, Download Excel is disabled until at least one complete valid input pair is entered again.

Input guide

Voltage is a required nonnegative decimal for the battery method, entered in mV, V, or kV. A realistic example is 3.7 V. Higher voltage increases battery energy in direct proportion when Charge is unchanged. Do not substitute a charger's maximum output voltage when the battery label specifies a different nominal voltage. Voltage unit controls the displayed scale and is converted automatically when changed.

Charge is a required nonnegative decimal for the battery method, entered in mAh or Ah. A common example is 1200 mAh. Higher charge capacity increases nominal energy proportionally. The frequent mistake is multiplying mAh directly by volts without dividing by 1000; selecting mAh handles that conversion. Charge unit changes between milliampere-hours and ampere-hours without changing the underlying charge.

Power is a required nonnegative decimal for the power-time method, entered in mW, W, kW, or MW. A realistic example is 100 W. Higher average power increases energy use proportionally. Use an average operating value when load varies; a nameplate maximum can overstate ordinary consumption. Power unit converts the current value to the selected scale.

Time is a required nonnegative decimal for the power-time method, entered in seconds, minutes, hours, or days. The demonstration uses 6 hours. Longer time increases energy proportionally. A common error is entering minutes while leaving hours selected. Time unit converts the current value to preserve the same duration.

Numeric fields accept ordinary decimal notation with a dot, optional correctly placed thousands separators, and surrounding spaces. Scientific notation and decimal commas are rejected rather than silently reinterpreted. Values must be finite, nonnegative, and no greater than 1 quadrillion in the selected unit.

Output guide

Primary energy displays the first available result in Wh, prioritizing Battery energy when both methods are complete. Battery – kWh and Battery – joules are exact conversions of the battery result; zero means at least one battery quantity is zero. Power × time – Wh, Power × time – kWh, and Power × time – joules report the time-based result in three units. These are calculated identities, not efficiency-adjusted forecasts. Scenario ratio is the power-time Wh divided by battery Wh. A value above 1 means the time-based scenario contains more nominal energy; it is undefined when battery energy is zero. Active methods counts complete calculation pairs.

The Energy conversion table has one row per active method. Calculation source identifies the formula used; Watt-hours (Wh) is the canonical comparison unit; Kilowatt-hours (kWh) divides Wh by 1000; and Joules (J) multiplies Wh by 3600. NIST defines the watt as one joule per second, which is why one watt sustained for one hour equals 3600 joules.

Worked example

For the startup battery, convert 1200 mAh to 1.2 Ah and multiply by 3.7 V: 1.2 Ah × 3.7 V = 4.44 WhThen convert the result: 4.44 Wh ÷ 1000 = 0.00444 kWh, and 4.44 Wh × 3600 = 15,984 J. For the second startup scenario, 100 W × 6 h = 600 Wh, which equals 0.6 kWh or 2,160,000 J. Dividing 600 Wh by 4.44 Wh gives the displayed scenario ratio of about 135.135×. These values match the first screen, table, and downloadable workbook.

Learn more

For authoritative unit definitions and symbol conventions, consult the Bureau International des Poids et Mesures' current International System of Units brochure. It distinguishes the SI energy unit joule from the practical watt-hour commonly used for electrical energy and battery capacity.