What Is a Watt? — Understanding Electrical Power
The watt (W) is the SI unit of power, named after Scottish engineer James Watt. One watt equals one joule of energy transferred per second: 1 W = 1 J/s. In electrical circuits, one watt is the power dissipated when a current of one ampere flows through a potential difference of one volt: 1 W = 1 V × 1 A.
How to Calculate Watts — Watt's Law
Watt's law defines the relationship between electrical power (P), voltage (V), and current (I):
This is the fundamental power equation for DC circuits. Combined with Ohm's Law (V = I × R), you can derive all four variables from any two known quantities.
Ohm's Law — Volts, Amps, and Ohms
Ohm's law states that voltage equals current multiplied by resistance:
- Electric Current (I) — the rate of charge flow, measured in amperes [A].
- Resistance (R) — opposition to current flow, measured in ohms [Ω].
- Voltage (V) — electric potential difference, measured in volts [V].
All 12 Formulas — Power, Voltage, Resistance, Current
Combining Watt's and Ohm's laws gives 12 interrelated formulas:
| Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Power (P) | P = V × I | P = V² / R | P = I² × R |
| Voltage (V) | V = P / I | V = I × R | V = √(P × R) |
| Current (I) | I = P / V | I = V / R | I = √(P / R) |
| Resistance (R) | R = V / I | R = V² / P | R = P / I² |
Unit Systems
This calculator supports both metric (SI) and American (imperial) units for power:
- Metric (SI): mW, W, kW, MW
- American: HP (horsepower = 745.7 W), BTU/hr (= 0.293 W)
All electrical units (V, A, Ω) are shared across both systems with standard SI prefixes (milli, kilo, mega).
Examples
Example 1 — Calculate Amps from Watts and Volts
A 60 W light bulb runs at 120 V. What current does it draw?
I = P / V = 60 W / 120 V = 0.5 A
Example 2 — Find Watts from Voltage and Resistance
A resistor has 4 V across it and 8 Ω resistance. What power does it dissipate?
P = V² / R = (4 V)² / 8 Ω = 16 / 8 = 2 W
Example 3 — Find Voltage from Power and Resistance
An electric heater consumes 1 kW and has a resistance of 14.4 Ω. What voltage does it need?
V = √(P × R) = √(1000 × 14.4) = √14400 = 120 V
Example 4 — Convert Watts to Horsepower
A motor outputs 3,000 W. What is this in horsepower?
HP = P / 745.7 = 3000 / 745.7 ≈ 4.02 HP
FAQs
What is a watt in simple terms?
A watt is a unit of power — it measures how fast energy is being used or generated. A 100 W light bulb uses 100 joules of energy every second.
What is the difference between watts and volt-amperes (VA)?
Watts (W) measure real power in DC or resistive AC circuits. Volt-amperes (VA) measure apparent power in AC circuits, which includes reactive power from inductors and capacitors. For pure resistive loads, W = VA.
How many watts is 1 horsepower?
1 mechanical horsepower (HP) = 745.699872 watts.
How many watts is 1 BTU/hr?
1 BTU/hour ≈ 0.29307 watts (so 1 watt ≈ 3.412 BTU/hr).
What connects volts, amps, watts, and ohms?
They are connected through Watt's Law (P = V × I) and Ohm's Law (V = I × R). Know any two — this calculator finds the other two instantly.