Physics Power Calculator
Power is the rate at which energy is transferred or work is done. In physics it is one of the most useful quantities for comparing engines, motors, athletes, and machines of all kinds. The SI unit of power is the watt (W), equal to one joule per second. This calculator supports two methods of computing power. Mode 1 uses the basic definition: P = W / t, where work (W) is in joules and time (t) is in seconds. Mode 2 uses the equivalent form for a constant force: P = F * v, where force (F) is in newtons and velocity (v) is in metres per second. Both methods give power in watts. Results are also shown in kilowatts (1 kW = 1,000 W) and mechanical horsepower (1 hp = 745.7 W) for practical reference. Horsepower is commonly used to rate vehicle engines and electric motors in the United States, while watts and kilowatts are the standard SI units. Use the tabs below to switch between calculation modes.
Power: -- W (-- kW, -- hp)
How power is calculated
Power measures how quickly energy is delivered. A high-powered engine does the same work as a low-powered one, but in less time. Both formulas below are equivalent: the second is derived by substituting W = F * d into P = W / t and noting that d / t = v.
Mode 1: Power from work and time
P = W / t
where P = power (W), W = work (J), t = time (s)
Mode 2: Power from force and velocity
P = F × v
where P = power (W), F = force (N), v = velocity (m/s)
Unit conversions
1 kW = 1,000 W
1 hp (mechanical) = 745.7 W
Worked example (Mode 1)
1,000 J of work done in 5 s:
- P = 1,000 / 5 = 200.00 W
- In kilowatts: 200 / 1,000 = 0.20 kW
- In horsepower: 200 / 745.7 = 0.27 hp
Worked example (Mode 2)
A force of 200 N applied to an object moving at 1 m/s:
- P = 200 × 1 = 200.00 W
Power calculator: frequently asked questions
What is power in physics?
Power is the rate at which energy is transferred or work is done. The formula is P = W / t, where W is work in joules and t is time in seconds. The SI unit of power is the watt (W), defined as one joule per second (J/s). Power can also be expressed as P = F * v, the product of force and velocity.
What is a watt?
The watt (W) is the SI unit of power, named after Scottish inventor James Watt. It equals one joule per second (1 W = 1 J/s = 1 kg m² s⁻³). Common multiples include the kilowatt (kW, 1,000 W) and megawatt (MW, 1,000,000 W). Everyday appliances are typically rated in watts or kilowatts.
How does mechanical horsepower relate to watts?
Mechanical horsepower (hp) was defined by James Watt in the 18th century as approximately 550 foot-pounds per second. The exact conversion used internationally today is 1 mechanical horsepower = 745.7 watts. Electric motors are often rated in horsepower, while the watt is the standard SI unit.
What is the difference between power and energy?
Energy is the capacity to do work, measured in joules (J). Power is the rate at which energy is used or work is done, measured in watts (W = J/s). A 100 W light bulb uses 100 joules of energy every second. Over one hour it uses 360,000 J (= 0.1 kWh). Energy = Power x time.
What is efficiency in the context of power?
Efficiency is the ratio of useful output power to total input power, expressed as a percentage: efficiency = (P_out / P_in) * 100%. A motor that inputs 1,000 W and delivers 850 W of mechanical output is 85% efficient. The remaining 150 W is lost as heat. Real machines always have efficiency below 100% due to friction and other losses.
Official sources
- NIST SP 330 (2019) "The International System of Units (SI)": NIST SP 330 PDF.
- NIST SI Unit definitions: nist.gov SI units.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only.