Physics Calculator
Watt Hour Calculator
Calculate electrical energy in watt-hours (Wh) and kilowatt-hours (kWh) from a power rating and a duration, or directly from voltage and current. Useful for sizing batteries and solar banks, estimating appliance consumption, and converting between energy units with a full step-by-step breakdown.
Watt Hour Calculator
Calculate energy in Watt-hours (Wh) and kilowatt-hours (kWh)
Appliance power rating in Watts
Results
Enter power (or voltage and current) plus a duration, then click Calculate Watt-Hours
Understanding Watt-Hours
A watt-hour is a unit of energy — the total amount of electricity consumed or stored — while a watt is a unit of power, the rate at which that energy flows. One watt-hour is the energy used by a one-watt device running for one hour.
This distinction matters in practice. A 1500 W space heater and a 10 W LED bulb draw wildly different power, but if the heater runs for 6 minutes and the bulb runs for 15 hours, both consume exactly 150 Wh. Energy is always power multiplied by duration, which is why battery capacities, solar output, and utility bills are all expressed in watt-hours or kilowatt-hours rather than watts.
Watt-Hour Formulas & Calculations
Every watt-hour calculation reduces to power multiplied by time in hours. The only variation is how you arrive at the power figure and what unit your duration is in:
1. Energy from Power and Time
The main formula, used when you already know the wattage of the load:
2. Energy from Voltage and Current
Used for DC circuits and battery systems, where power is derived from P = V × I:
3. Kilowatt-Hour Conversion
Utilities bill in kilowatt-hours, so larger results are easier to read after dividing by 1000:
4. Time Unit Conversion
Because the formula requires hours, minutes and seconds must be normalised first:
Practical Impact of Watt-Hour Calculations
Getting energy figures right determines whether a battery bank lasts the night and whether a bill estimate is trustworthy:
- Battery and Solar Sizing: Watt-hours let you compare storage across different voltages directly. A 12 V 100 Ah battery (1200 Wh) and a 24 V 50 Ah battery (1200 Wh) hold identical energy, a fact that amp-hour ratings alone would hide.
- Accurate Cost Estimation: Converting to kilowatt-hours maps your result straight onto the unit your utility bills in, so multiplying by your tariff gives a direct cost per appliance and reveals which devices dominate your bill.
- Nameplate Rating Trap: Using a maximum nameplate wattage as if it ran continuously badly overstates energy use. Cycling loads such as refrigerators and air conditioners only draw full power a fraction of the time, so always use real running hours rather than clock hours.
Energy Unit Conversion Reference
Watt-hours sit alongside several related energy units. These equivalences let you move between them:
| Unit | Equal To | Typical Use |
|---|---|---|
| 1 Watt-hour (Wh) | 3,600 joules | Small devices, power banks, laptop batteries |
| 1 Kilowatt-hour (kWh) | 1,000 Wh | Utility billing unit, home solar and EV charging |
| 1 Megawatt-hour (MWh) | 1,000 kWh | Grid generation and industrial consumption |
| 1 Amp-hour (Ah) | Voltage × 1 Wh/V | Battery capacity at a stated nominal voltage |
| 1 BTU | 0.293 Wh | Heating and cooling capacity ratings |
Common Appliance Energy Matrix
Typical power ratings and the resulting energy for common run times. Use these as sanity checks on your own results:
| Appliance | Power | 30 min | 2 hours | 8 hours |
|---|---|---|---|---|
| LED Bulb | 10 W | 5 Wh | 20 Wh | 80 Wh |
| Laptop | 50 W | 25 Wh | 100 Wh | 400 Wh |
| Refrigerator (running) | 150 W | 75 Wh | 300 Wh | 1,200 Wh |
| Microwave | 1,000 W | 500 Wh | 2,000 Wh | 8,000 Wh |
| Space Heater | 1,500 W | 750 Wh | 3,000 Wh | 12,000 Wh |
Benefits of Using the Watt Hour Calculator
Example Calculations
Here are three scenarios worked out step-by-step:
Example Scenario 1 — Power × Time
Power: 100 W, Time: 5 hours.
Formula: Energy (Wh) = Power (W) × Time (hours)
Power (P) = 100 Watts
Time (t) = 5 hours
E = 100 × 5 = 500 Wh
Convert to kilowatt-hours = 500 / 1000 = 0.5 kWh
Result: Energy is 500 Wh (0.5 kWh)
Example Scenario 2 — Voltage × Current × Time
Voltage: 12 V, Current: 5 A, Time: 4 hours.
Formula: Energy (Wh) = Voltage (V) × Current (A) × Time (hours)
Power (P) = V × I = 12 × 5 = 60 Watts
Time (t) = 4 hours
E = 60 × 4 = 240 Wh
Convert to kilowatt-hours = 240 / 1000 = 0.24 kWh
Result: Energy is 240 Wh (0.24 kWh)
Example Scenario 3 — Short Duration in Minutes
Power: 1500 W, Time: 30 minutes.
Formula: Energy (Wh) = Power (W) × Time (hours)
Convert time to hours = 30 / 60 = 0.5 hours
E = 1500 × 0.5 = 750 Wh
Convert to kilowatt-hours = 750 / 1000 = 0.75 kWh
Result: Energy is 750 Wh (0.75 kWh)
Power Factor Tip
On DC circuits, volts × amps gives real power exactly. On AC circuits it gives apparent power in volt-amps, which only equals real watts when the power factor is 1. For motors, compressors, and older fluorescent ballasts, multiply the result by the power factor — usually 0.8 to 0.95 — or measure wattage directly with a plug-in meter for the most accurate energy figure.
Frequently Asked Questions
- How do you calculate watt-hours?
- Multiply power in watts by time in hours: Energy (Wh) = Power (W) × Time (h). A 100 W device running for 5 hours uses 100 × 5 = 500 Wh. If your duration is in minutes or seconds, divide by 60 or 3600 first to convert it to hours.
- What is the difference between watts and watt-hours?
- Watts (W) measure power — the instantaneous rate at which energy is used. Watt-hours (Wh) measure energy — the total amount consumed over a period of time. A 60 W bulb always draws 60 W, but it consumes 60 Wh after one hour and 600 Wh after ten hours.
- How do I convert watt-hours to kilowatt-hours?
- Divide watt-hours by 1000: kWh = Wh / 1000. So 2500 Wh equals 2.5 kWh. Utility companies bill in kilowatt-hours, which is why converting is useful for estimating electricity costs.
- How do I calculate watt-hours from volts and amps?
- First find power using P = V × I, then multiply by time in hours: Energy (Wh) = Voltage (V) × Current (A) × Time (h). A 12 V battery supplying 5 A for 4 hours delivers 12 × 5 × 4 = 240 Wh.
- How do I convert battery amp-hours (Ah) to watt-hours?
- Multiply the amp-hour rating by the nominal battery voltage: Wh = Ah × V. A 100 Ah battery at 12 V holds roughly 1200 Wh. This is why watt-hours let you compare batteries of different voltages fairly, while amp-hours alone do not.
- How many watt-hours are in a joule?
- One watt-hour equals 3600 joules, because one watt is one joule per second and an hour contains 3600 seconds. To convert the other way, divide joules by 3600 — so 18,000 J equals 5 Wh.
- How do I work out the cost of the energy I calculated?
- Convert your result to kilowatt-hours, then multiply by your electricity rate. If a calculation returns 750 Wh (0.75 kWh) and your utility charges $0.15 per kWh, that usage costs 0.75 × 0.15 = about $0.11.
- Does this calculator account for power factor on AC circuits?
- No. The voltage × current mode computes apparent power (volt-amps), which equals real power only when the power factor is 1. For inductive AC loads such as motors and compressors, multiply the result by the power factor — typically 0.8 to 0.95 — for a real-energy figure.
- Why does my appliance use fewer watt-hours than its rating suggests?
- Nameplate ratings describe maximum draw, but most appliances cycle on and off or run at part load. A refrigerator rated 150 W may only run its compressor a third of the time, so its real daily energy is far lower than 150 W × 24 h would imply.
- How many watt-hours does a typical household appliance use per day?
- A 10 W LED bulb on for 5 hours uses 50 Wh. A 1500 W space heater for 3 hours uses 4500 Wh (4.5 kWh). A 50 W laptop for 8 hours uses 400 Wh. Multiply each device by its real hours of use to build up your daily total.