Watt Hours Calculator - Wh, kWh, and Joules from Charge or Power

A watt hours calculator converts electric charge in milliamp hours or amp hours and voltage into energy in watt hours, or multiplies power in watts by time in hours. It returns watt hours, kilowatt hours, and joules so you can compare battery energy, estimate device consumption, or convert between billing and physics units.

• • Comparing battery energy across chemistries: Convert the mAh rating printed on a Li-ion cell and a NiMH cell into watt hours at their respective nominal voltages so the comparison is energy-based, not charge-based.
• • Estimating device energy consumption: Multiply the wattage of a light bulb, charger, or heater by the hours it runs to get the watt hours consumed in a day, month, or billing cycle.
• • Converting between energy billing units: Turn watt hours into kilowatt hours for billing or into joules for physics calculations.
• • Sizing a power bank or backup battery: Add up device watt hours over the target runtime, then pick a battery with enough Wh capacity.

Charge mode takes the mAh or Ah rating and nominal voltage to compute stored energy. Power-time mode takes the wattage and operating hours to compute consumed energy. Both modes return Wh, kWh, and joules.

When the next step is to find the watts from voltage and current before converting to watt hours, Electrical Power Calculator turns V and A into P on the same screen.

The calculator applies two formulas depending on the selected mode. Charge mode multiplies the charge in amp hours by the voltage in volts to get watt hours. Power-time mode multiplies the power in watts by the time in hours to get watt hours. Both results then convert to kilowatt hours by dividing by 1000 and to joules by multiplying by 3600.

• Ah: Charge in amp hours. If the input is in mAh, divide by 1000 first.
• V: Nominal voltage in volts. For a battery, this is the voltage printed on the label (3.7 V for Li-ion, 1.5 V for alkaline, 12 V for car batteries).
• W: Power in watts. If the input is in kW, multiply by 1000 first.
• t: Time in hours. The duration the device runs or the energy flows.
• Wh: Energy in watt hours. One watt hour equals one watt of power flowing for one hour.
• kWh: Energy in kilowatt hours. The unit utility companies use for billing.
• J: Energy in joules. The SI unit of energy, equal to one watt-second.

Both formulas compute Wh first, then divide by 1000 for kWh and multiply by 3600 for joules. The charge mode formula Wh = Ah × V comes from the definition of the watt as one joule per second and the amp-hour as 3600 coulombs.

According to Wikipedia - Kilowatt hour, one watt hour is the amount of energy transferred when one watt of power is applied for one hour, and one kilowatt hour equals 1000 watt hours

According to Omni Calculator - Watt-hour Calculator, the energy in watt hours equals the charge in amp hours multiplied by the voltage in volts, or the power in watts multiplied by the time in hours

When the watt-hour result needs to feed into BTU, calories, or therm conversions for a broader energy audit, Energy Converter handles the cross-unit step without leaving the energy cluster.

Four ideas cover most watt-hour problems in physics, battery comparison, and energy billing.

These four ideas carry over to any watt-hour problem, from comparing AA batteries to sizing a solar storage bank.

When the physics problem moves from electrical energy to mechanical work or kinetic energy, Work Energy Power Calculator carries the same joule values through the work-energy-power formulas.

Five steps from a battery label or device nameplate to an energy answer in Wh, kWh, and joules.

1. 1 Pick the calculation mode: Choose charge mode when you have a battery mAh or Ah rating and the nominal voltage. Choose power-time mode when you have a device wattage and operating hours.
2. 2 Enter the voltage or power: In charge mode, type the nominal voltage from the battery label. In power-time mode, type the device wattage from the nameplate and pick watts or kilowatts.
3. 3 Enter the charge or time: In charge mode, type the mAh or Ah rating and pick the matching unit. In power-time mode, type the operating hours.
4. 4 Read the energy results: The result panel shows Wh, kWh, and joules. In charge mode, the panel also shows Ah and mAh.
5. 5 Use the results: Compare Wh values across batteries to rank energy capacity. Use kWh to estimate utility cost. Use joules for physics homework or engineering calculations.

Practical example: a 5000 mAh power bank at 3.7 V returns 18.5 Wh. A phone drawing 5 W for 4 hours uses 20 Wh, so this power bank holds slightly less than one full charge.

Once the watt hours are known, Battery Life Calculator estimates how long a device will run on that energy at a given power draw so the Wh figure becomes a runtime prediction.

A watt-hours workflow catches the cases where comparing mAh ratings across different battery chemistries would give a misleading answer.

• • Two input modes in one tool: Switch between charge-to-energy and power-time-to-energy without leaving the page or opening a second calculator.
• • Three energy output units on one screen: Wh, kWh, and joules appear side by side for battery comparison, billing, and physics homework.
• • Fair battery comparison across chemistries: Converting mAh to Wh at each battery's nominal voltage puts Li-ion, NiMH, alkaline, and lead-acid cells on the same footing.
• • Input unit flexibility: Charge accepts mAh or Ah, power accepts W or kW, matching the label on the battery or device.
• • Cross-validation-friendly defaults: The default 1200 mAh at 3.7 V returns 4.44 Wh, matching the Omni Calculator worked example for a Li-ion polymer cell.

The same Wh figure airlines use to rate spare lithium batteries (100 Wh carry-on limit), utility companies use for billing, and physics textbooks use for energy problems.

When the voltage is unknown but the current and resistance are available, Ohm's Law Calculator returns V, I, R, and P for any two of the four so the voltage input here can be filled in.

Four factors shape the watt-hour result, plus two limitations worth knowing.

• • Wh = Ah × V assumes constant voltage throughout discharge. Real batteries have a discharge curve, so computed Wh is an estimate based on nominal voltage.
• • Wh = W × t assumes constant power draw. Devices with motors or compressors draw varying power, so nameplate wattage is a typical or maximum value.

For same-chemistry comparison, the nominal-voltage approximation is close enough. For precise energy accounting, measure the actual voltage-current profile over a full discharge cycle.

According to NIST - International System of Units (SI), the watt is the SI unit of power equal to one joule per second, and one watt hour equals 3600 joules since one hour contains 3600 seconds

When the battery sizing question goes deeper into C-rate, discharge current, and runtime tradeoffs, Battery Capacity Calculator adds the discharge-profile layer on top of the Wh number computed here.