Power Calculator
Power from work and time (P = W/t), force and speed (P = Fv) or voltage and current (P = VI) in watts, kW, horsepower and BTU/h, with energy use per hour and day.
Power Calculator: with the default inputs, power is 300 W.
Energy transferred.
Force along the direction of motion.
- Horsepower (mechanical)
- 0.40231 hp = 745.7 W = 550 ft·lbf/s.
- Energy per hour
- 0.3 kWh
- Energy per day
- 7.2 kWh
- Formula used
- P = W ÷ t = 3,000 J ÷ 10 s
Assumptions
- Steady (constant) power over the interval.
- Force is along the direction of motion in P = Fv.
- P = VI gives real power for resistive loads (power factor 1).
| Unit | Value | One of these is |
|---|---|---|
| Watts (W) | 300 | 1 joule per second — the SI unit |
| Kilowatts (kW) | 0.3 | 1,000 W |
| Mechanical horsepower (hp) | 0.4023 | 745.700 W = 550 ft·lbf/s |
| Metric horsepower (PS, ch, CV) | 0.4079 | 735.499 W — European car figures |
| BTU per hour | 1,023.6425 | 0.293071 W — heating and cooling equipment |
| Tons of refrigeration | 0.0853 | 12,000 BTU/h — air-conditioning capacity |
| Foot-pounds per second | 221.2686 | 1.355818 W |
| Kilocalories per hour | 257.9536 | 1.163 W — dietary and lab work |
Conversion factors from NIST Special Publication 811. Mechanical horsepower is the 550 ft·lbf/s figure used in the US; metric horsepower is about 1.4% smaller, which is why the same engine is quoted at different numbers on either side of the Atlantic.
How this is worked out
The formula
P = W ÷ t (work per unit time) P = F × v (force along the motion × speed) P = V × I (electrical) P = power (W = J/s) W = work or energy (J), t = time (s) F = force (N), v = speed (m/s) V = voltage (V), I = current (A) 1 hp = 745.7 W 1 kW = 1.341 hp 1 W = 3.412 BTU/h
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Formula
- Choose one of 3 options.P = W ÷ t — from work (energy) & time · P = F × v — from force & speed · P = V × I — from voltage & current
- Work done (energy)
- Energy transferred.in J · defaults to 3000
- Time
- A number.in s · 0 or more · defaults to 10
- Force
- Force along the direction of motion.defaults to 500
- Speed
- A number.in m/s · defaults to 2
- Voltage
- A number.defaults to 120
- Current
- A number.defaults to 10
What you get back
- Powermain answer
- Horsepower (mechanical)
- 1 hp = 745.7 W = 550 ft·lbf/s.
- Energy per hour
- Energy per day
- Formula used
What this assumes
- Steady (constant) power over the interval.
- Force is along the direction of motion in P = Fv.
- P = VI gives real power for resistive loads (power factor 1).
About this calculator
Power is how fast energy is transferred: one watt is one joule per second. The same quantity shows up in three costumes. Lifting a load is work over time, P = W ÷ t. A motor or engine pushing at constant speed delivers P = F × v. A heater or battery delivers P = V × I. Pick the form that matches what you know, and the calculator converts the answer to watts, kilowatts, horsepower and BTU/h, plus the energy that power would consume in an hour and a day — the bridge to your electricity bill.
Horsepower, explained
James Watt needed a number his customers understood, so he rated steam engines against horses and settled on 550 foot-pounds per second, which is 745.7 W. That is the mechanical horsepower used for engines and motors. Metric horsepower (PS, cv) is 735.5 W, about 1.4% less, and electric-motor nameplates in the US often quote output in hp while the input is in kW. A fit cyclist sustains about 0.3 hp; a kettle draws 2–3 hp worth of electricity; a family car makes 100–200 hp at full throttle but cruises on 20.
Reading the results
- Power can be shown in W, kW, hp or BTU/h (the last is how air conditioners and furnaces are rated: 12,000 BTU/h = 3.517 kW).
- Energy per hour / day is power × time in kWh, the unit utilities bill. A 1,200 W device running all day uses 28.8 kWh.
- In the force × speed mode, F is the component of force along the motion; a force at right angles does no work and delivers no power.
Common mistakes
- Confusing power with energy. A 100 W bulb for 10 hours and a 1,000 W heater for 1 hour use the same energy (1 kWh); they differ in power.
- Using km/h in P = Fv. The watt is defined with m/s; the calculator converts.
- Reading P = VI for AC as exact. For resistive loads it is; for motors and electronics multiply by the power factor to get real power.
- Forgetting efficiency. Electrical input power to a motor is larger than its mechanical output; a 1 hp motor at 85% efficiency draws about 880 W.
Frequently asked questions
▸What is the formula for power?
P = W ÷ t: energy transferred divided by the time it took, in watts (joules per second). Equivalent forms are P = F × v for mechanical motion and P = V × I for electricity.
▸How many watts are in a horsepower?
745.7 W in a mechanical horsepower (550 ft·lbf/s). Metric horsepower is 735.5 W. To convert kW to hp, multiply by 1.341.
▸What is the difference between power and energy?
Energy is the amount (joules, kWh); power is the rate (watts). Power × time = energy. Utilities bill energy in kilowatt-hours, which is why the calculator shows energy per hour and per day.
▸How do I calculate the power of a motor from force and speed?
Multiply the force it applies along the direction of motion by the speed: P = F × v. A winch pulling with 2,000 N at 0.5 m/s delivers 1,000 W, or 1.34 hp, before losses.
▸Does P = VI work for AC circuits?
For purely resistive loads such as heaters and incandescent bulbs, yes (using RMS values). For motors, transformers and electronics, real power is V × I × power factor; V × I alone is apparent power in volt-amperes.
Put this calculator on your own site
A working power, free for any site, with no ads and no sign-up. It resizes to fit wherever you paste it and updates itself as this page improves.
Paste this anywhere. It works on any site, carries no ads, never expires, and always shows the current version.
Power Calculator by CalculateItNow
The page's own title. The clearest description of what the link leads to.
The credit line sits outside the widget on purpose, so it is a real link on your page rather than one buried in a frame. Please keep it — it is what pays for CalculateItNow staying free and ad-free. The script only resizes the widget to fit its contents; drop it and the widget still works.
Browse every calculator widget·How to add it to WordPress, Squarespace or Wix
Related calculators
The questions people ask next to a power.
Solve V = IR and P = VI for voltage, current, resistance or power from two known values, with the rearranged formula shown and all 12 Ohm's-law equations.
Find what any appliance costs to run per day, month and year from its wattage, hours of use and your price per kWh — plus a table of common appliances at your rate.
Calculate kinetic energy from mass and velocity (KE = ½mv²) in joules, kJ or kcal, plus momentum — or solve for the velocity or mass that gives a known energy.
Newton's second law solved for force, mass or acceleration, plus weight on any planet from NASA surface gravities — in newtons, pounds-force and kgf.
Output voltage of a two-resistor divider (Vout = Vin·R2/(R1+R2)), the current and power in each resistor, and how a load resistance pulls the output down.
Convert between wavelength, frequency and photon energy (λ = v/f, E = hf) with exact CODATA constants, auto-scaled units from km to pm, and the spectrum band named.