What is the SI unit of power?
The watt (W), where 1 W = 1 J/s. Named after James Watt.
Power Calculator
P = W / t
Calculate power from work and time. P = W/t — rate of energy transfer. Also P = Fv.
Calculate
Enter values above to calculate
How it calculates
1
P = W / t
2
W in joules, t in seconds
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Result in watts (1 W = 1 J/s)
Formula
P = W / t
Variable Table
| Symbol | Quantity | SI Unit |
|---|---|---|
| P | Power | W = J/s |
| W | Work or energy | J |
| t | Time | s |
| F | Force | N |
| v | Velocity | m/s |
How to Use This Calculator
This power calculator is built for quick physics checks and worked-problem review. Enter values in the units shown beside each input, then compare the result with the formula and variable table before using it in a longer solution. The calculator does the arithmetic, but the physics still depends on choosing a model that matches the situation.
Start by identifying the system, the known quantities, and the quantity you want to find. If a value is given in a non-SI unit, convert it before substitution. A correct numerical answer with mixed units can still be physically wrong, especially when squared units, inverse seconds, charges, temperatures, or distances are involved.
Assumptions and Limits
The formula P = W / t is a model, not a universal description of every possible case. It assumes the quantities in the variable table are the relevant quantities for the problem and that hidden effects are either negligible or already included in the inputs. If friction, drag, relativistic speeds, changing fields, non-constant temperature, or geometry-specific effects matter, check whether a more complete model is needed.
Use the result as a magnitude and units check. Ask whether the answer has the right sign, whether it grows or shrinks when an input changes, and whether the limiting cases make sense. Setting an input to zero, doubling a quantity, or using a very large value is often enough to catch a formula choice or unit mistake before it reaches a final answer.
Worked Example
An engine does 50,000 J of work in 10 s. Find power.
Step 1: P = W / t
Step 2: P = 50000 / 10
Answer: P = 5000 W = 5 kW
Common Mistakes
- 1 horsepower ≈ 746 W
- Power is the rate of energy transfer — not the total energy
- P = Fv is useful when force and velocity are known instead of work and time
Related
Frequently Asked Questions
What is P = Fv?
Alternative form for power: P = F × v, where F is force and v is velocity. Useful for engines and motors.