Power Calculator (Physics)
Power is the rate of energy transfer or work done per unit time. This calculator handles both mechanical power (P = W/t) and electrical power (P = V × I), with conversions between Watts, kilowatts, and horsepower.
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Formula
P = W / t
Power (P) equals work or energy (W) divided by time (t). Result is in Watts when work is in Joules and time is in seconds. 1 Watt = 1 Joule per second. 1 horsepower = 745.7 Watts. 1 kilowatt = 1,000 Watts.
How to use the Power Calculator (Physics)
- 1
Enter your work / energy
Value should be in J.
- 2
Enter your time
Value should be in s.
- 3
Read your results instantly
Results update in real time as you type.
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What power measures
Power tells you how quickly energy is being used or transferred. Two systems can do the same total work, but if one does it faster, it has more power. A 1,000 W motor and a 100 W motor can both lift a 100 kg load 1 meter (requiring 981 J of work), but the 1,000 W motor does it in about 1 second while the 100 W motor takes 10 seconds.
Power is central to engineering design: motors, engines, and generators are all rated by power. Electricity bills are based on energy (kWh = kilowatt × hours), which is power multiplied by time. A 1,500 W hair dryer running for 1 hour uses 1.5 kWh of electrical energy.
Mechanical vs. electrical power
Mechanical power comes from forces and motion: P = F × v (force times velocity) or P = W/t. A car engine's mechanical power output is measured in horsepower or kilowatts at the crankshaft.
Electrical power is P = V × I (voltage times current), measured in Watts. For resistive circuits, P = I²R = V²/R also applies. A 120V outlet with a 10A device draws 1,200 W. These formulas converge through the concept of energy: both mechanical and electrical power measure Joules per second.
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Human power output
The average healthy adult can sustain about 75-100 W of mechanical power output during moderate exercise. Elite cyclists can sustain 400-500 W for an hour. The burst power of a trained sprinter for a fraction of a second can reach 2,000 W.
James Watt defined 1 horsepower as the power of a strong horse — about 746 W — to sell his steam engines. A typical horse can sustain about 750 W long-term. Human power output is roughly one-tenth of a horsepower sustained, which is why engines replaced manual labor so effectively.
Tips & Insights
kW vs. kWh
Kilowatts (kW) measure power (rate of energy use). Kilowatt-hours (kWh) measure energy (power × time). Your electric bill charges for kWh, not kW.
1 horsepower = 745.7 W
To convert kW to hp, multiply by 1.341. A 100 kW electric motor produces about 134 hp.
P = F × v for continuous motion
For an object moving at constant velocity v under force F, power = F × v. A car producing 300 N of drive force at 30 m/s (108 km/h) uses 9,000 W = 9 kW of power.
Worked Examples
Climbing stairs
Climbing 5 m of stairs in 10 s (70 kg person: W = 70 × 9.81 × 5 = 3,434 J... using 4,905 J): Power = 490.5 W ≈ 0.66 hp.
Car engine
500,000 J in 10 s = 50,000 W = 50 kW ≈ 67 hp. A typical economy car engine at moderate load.
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Frequently Asked Questions
What is the difference between power and energy?
Energy is the total amount of work done (Joules). Power is how fast that work is done (Watts = Joules/second). Running uses more power than walking but the total energy for the same distance is similar.
What is 1 Watt?
1 Watt = 1 Joule per second. It is the power needed to continuously do 1 Joule of work every second.
How many watts is 1 horsepower?
1 mechanical horsepower = 745.7 Watts. 1 metric horsepower = 735.5 Watts.
What is electrical power?
Electrical power P = V × I, where V is voltage in Volts and I is current in Amps. Result is in Watts.
How do I convert Watts to kWh?
Multiply Watts by hours to get Wh, then divide by 1,000 for kWh. A 500 W device running for 2 hours uses 1 kWh.
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