Watt-Hour Calculator
Enter the power of a device and the time it runs, or its voltage and amp-hour rating, to find energy in watt-hours (Wh) and kilowatt-hours (kWh). You also get the electricity cost at your local rate, the reverse amp-hour equivalent, and an optional battery runtime estimate. All values update as you type.
What is a watt-hour?
A watt-hour (Wh) is a unit of energy equal to the energy transferred by a one-watt load running for exactly one hour. It is the everyday unit for measuring electricity consumption and battery capacity. One kilowatt-hour (kWh) equals 1000 Wh, and that is what your utility company counts on your electricity bill. Smaller devices use milliwatt-hours (mWh) - for example a smartphone battery rated at 3500 mAh at 3.85 V holds about 13.5 Wh, which is why you can charge it dozens of times from a 100 Wh power bank.
Three ways to calculate watt-hours
The most common calculation is Wh = Watts x Hours: multiply the rated power of a device by how long it runs. A 60 W bulb left on for 10 hours uses 600 Wh (0.6 kWh). Battery capacity is calculated differently: Wh = Volts x Amp-hours. A 12 V, 100 Ah lead-acid battery stores 1200 Wh. If the capacity is in milliamp-hours (common for phone and drone batteries), divide by 1000 first: Wh = V x mAh / 1000. The third mode is reverse-solving for runtime: Hours = Wh / Watts. Factor in system efficiency (losses in wiring, inverters, battery chemistry) by multiplying the stored Wh by the efficiency percentage before dividing by the load.
Reading your electricity bill: kWh and cost
Your utility meter counts kilowatt-hours consumed. The average US residential rate is about $0.16 per kWh, though it ranges from roughly $0.10 (Louisiana) to over $0.35 (Hawaii and California). To find the annual cost of any device, multiply its daily kWh by 365 and by your rate. For example, a refrigerator drawing 150 W constantly uses 150 x 24 / 1000 = 3.6 kWh per day. At $0.16 that is $0.58 per day, about $210 per year. Identifying high-wattage devices and shifting them off-peak or upgrading to efficient models is the fastest way to cut your electricity bill.
Battery sizing for solar, RV and off-grid systems
When sizing a battery bank, start with your total daily load in watt-hours. Add a 20-30% margin for days with low solar input and never discharge a lead-acid battery below 50% of capacity (lithium batteries tolerate deeper discharge, down to about 20%). For a system that needs 2000 Wh per day with a 12 V lead-acid bank at 50% depth of discharge, you need at least 2000 / (12 x 0.5) = 333 Ah of rated capacity. Lithium iron phosphate (LiFePO4) packs are more expensive per Ah but tolerate 80-90% discharge, making them more cost-effective per usable watt-hour over thousands of charge cycles.
Typical device wattages for energy estimates
| Device | Typical wattage (W) | Wh per 8 h |
|---|---|---|
| LED light bulb | 9 | 72 |
| Laptop computer | 45-65 | 360-520 |
| Desktop PC + monitor | 150 | 1200 |
| Refrigerator (modern) | 100-150 | 800-1200 |
| Ceiling fan | 50 | 400 |
| Phone charger | 5-20 | 40-160 |
| Television (55 in, LED) | 80-100 | 640-800 |
| Microwave oven | 1000-1500 | not continuous |
| Washing machine | 500-1000 | not continuous |
| Electric vehicle charger (Level 2) | 7200 | 57600 |
Representative values - always check the label or spec sheet for your specific appliance.
Frequently asked questions
What is the difference between watts and watt-hours?
Watts measure power, which is the rate of energy transfer at any instant. Watt-hours measure energy, which is power accumulated over time. A 100 W bulb draws 100 W at every moment it is on; run it for 3 hours and it has consumed 300 Wh of energy. Think of watts as speed and watt-hours as distance.
How do I convert milliamp-hours (mAh) to watt-hours?
Multiply the mAh rating by the battery voltage, then divide by 1000: Wh = mAh x V / 1000. A 3000 mAh battery at 3.7 V holds 3000 x 3.7 / 1000 = 11.1 Wh. The calculator does this automatically when you select the mAh unit in the Volts x Amp-hours mode.
How do I convert watt-hours to kilowatt-hours?
Simply divide watt-hours by 1000. If your device uses 750 Wh per session, that is 0.75 kWh. Most electricity bills are priced per kWh, so this conversion is the link between the power of a device and the cost to run it.
What does system efficiency mean for battery runtime?
No energy transfer is perfectly efficient. Inverters that convert DC battery power to AC mains power typically lose 5-15% as heat. Battery internal resistance wastes another 2-10% depending on chemistry and age. Wiring losses add a small fraction. An 85% efficiency figure means only 85% of the rated watt-hours actually reach your device as useful work. Always factor this in when estimating runtime or you will overestimate how long a battery lasts.
How many watt-hours does my phone charger use?
A typical smartphone charger delivers 5-20 W depending on whether fast charging is active. At 10 W average for a one-hour charge, that is 10 Wh - less than a penny at standard rates. Even charging a phone every day for a year uses under 4 kWh total, which costs about $0.60. Phone chargers are not a meaningful contributor to household electricity bills.
Can I use this calculator for solar panel output?
Yes. A 400 W solar panel producing power for 5 peak sun hours per day generates 400 x 5 = 2000 Wh (2 kWh) per day. Use the Watts x time mode, enter 400 W and 5 hours. Note that peak sun hours vary by location and season - your installer or a solar insolation tool can give you the figure for your area.