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Power Calculator
Calculate power three ways: work done per unit of time, force times velocity, or electrical power from volts, amps and ohms. See the answer in watts, kilowatts, horsepower and BTU per hour, plus how much energy that power uses over time.
Formula
Known values
V
A
Ω
Enter your values to see the result.
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How to use the Power Calculator
- Choose the formula: work ÷ time, force × velocity or electrical.
- Enter the known values and pick their units. For electrical power, choose which two values you know.
- Pick the unit to show the answer in, such as kW or horsepower.
- Read the power in several units and the energy used over different durations.
- Copy or download the results to use elsewhere.
Power formulas
- Mechanical: P = W ÷ t (work divided by time)
- Force and velocity: P = F × v
- Electrical: P = V × I = I² × R = V² ÷ R
Example: lifting a 100 kg load 3 m takes W = m g h = 100 × 9.807 × 3 = 2,942 J. Doing it in 10 seconds needs 2,942 ÷ 10 = 294 W, about 0.39 hp.
Power unit conversions
| Unit | In watts | 1 kW equals |
|---|---|---|
| Kilowatt (kW) | 1,000 W | 1 kW |
| Mechanical horsepower (hp) | 745.7 W | 1.341 hp |
| Metric horsepower (PS) | 735.5 W | 1.360 PS |
| BTU per hour | 0.29307 W | 3,412 BTU/h |
| Foot-pound-force per second | 1.3558 W | 737.6 ft·lbf/s |
Typical power levels
| Example | Approximate power |
|---|---|
| LED light bulb | 8–12 W |
| Laptop while working | 30–60 W |
| Person cycling steadily | 100–200 W |
| Microwave oven | 800–1,200 W |
| Electric kettle | 1,500–3,000 W |
| Small car engine (peak) | 50–100 kW |
Frequently asked questions
What is the formula for power?
Power is the rate of doing work: P = W ÷ t. With work in joules and time in seconds, power is in watts (1 W = 1 J/s). For a constant force moving at constant velocity, P = F × v. For electricity, P = V × I, which by Ohm's law also equals I² × R and V² ÷ R.
How many watts is one horsepower?
One mechanical (imperial) horsepower is 745.7 W. One metric horsepower (PS) is 735.5 W. Electrical motor ratings sometimes use 746 W per hp.
What is the difference between power and energy?
Energy is the total amount of work done, measured in joules or kilowatt-hours. Power is how fast that energy is used or delivered, measured in watts. A 1,000 W heater running for 2 hours uses 2 kWh of energy.
How do I convert watts to BTU per hour?
Multiply watts by about 3.412. One BTU/h equals 0.29307 W, so a 1,500 W space heater gives off about 5,118 BTU/h.
Why does P = F × v work?
Work is force times distance, and power is work per unit time. Dividing distance by time gives velocity, so power is force times velocity when the force acts in the direction of motion.
Is this calculator suitable for AC circuits?
For DC circuits and purely resistive AC loads such as heaters and kettles, P = V × I is accurate using RMS values. Motors and electronics have a power factor below 1, so their real power is V × I × power factor.
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