Formulas
- Ohm’s law: V = R·I
- Power: P = V·I = R·I² = V² ÷ R
V is the voltage in volts, I the current in amperes, R the resistance in ohms (Ω) and P the power in watts.
Examples
- 12 V across 6 Ω: current of 2 A and power of 24 W.
- A 1,500 W hair dryer on 120 V draws 12.5 A, and its resistance is 9.6 Ω.
To estimate what an appliance costs to run, use the electricity cost calculator.
Frequently asked questions
Does Ohm’s law work for every device?
It works for resistors and heaters. LEDs, motors and electronics do not follow it exactly, but P = V·I is still valid.
What is the difference between watts and watt-hours?
Watts measure power at a moment. Watt-hours (or kWh) measure energy over time: 1,000 W for 1 hour = 1 kWh.
Why does a thin wire get hot?
It has more resistance. Heat is P = R·I², so more resistance with the same current means more heat.
See also
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