Watts vs Amps
Watts and amps are two of the most commonly confused electrical units, and mixing them up can lead to tripped breakers or overloaded circuits. Amps measure electrical current — the flow of electrons; watts measure electrical power — the rate at which energy is consumed or delivered. They are related through voltage using Ohm's Law.
Watts (W)
Watts measure electrical power — the rate at which a device consumes or produces energy. A watt is one joule of energy per second. Watts appear on appliance labels as the total power draw.
W = V × A (or W = A² × Ω)- •Measures electrical power (energy per second)
- •Appears on appliance labels (e.g., "60W bulb")
- •W = Volts × Amps (in DC circuits)
- •Used to calculate electricity bills (kWh)
- •Larger units: kilowatt (kW), megawatt (MW)
Amps (A)
Amps (amperes) measure electrical current — the volume of electrons flowing through a conductor per second. Amps determine how much current a wire or circuit breaker must handle safely.
A = W ÷ V- •Measures electrical current flow
- •Determines wire and breaker sizing
- •A = Watts ÷ Volts
- •Higher amps = thicker wire required
- •Circuit breakers are rated in amps (15A, 20A, etc.)
Key Differences
| Aspect | Watts (W) | Amps (A) |
|---|---|---|
| What It Measures | Power (energy rate) | Current (electron flow) |
| Unit Symbol | W | A |
| Used For | Energy consumption & cost | Wire and breaker sizing |
| Relationship | W = V × A | A = W ÷ V |
| 1,200W at 120V | 1,200 W | 10 A |
| Utility Bills | Measured in kWh (kilowatt-hours) | Not used for billing |
When to Use Each
Use watts when calculating electricity costs, sizing a generator, or comparing appliance energy use. Use amps when sizing circuit breakers, checking whether a circuit can handle a new appliance, or selecting wire gauge. The key formula: Amps = Watts ÷ Volts. In the US (120V), a 1,200W appliance draws 10 amps.
Frequently Asked Questions
How do I convert watts to amps?
Divide watts by voltage: Amps = Watts ÷ Volts. In the US on a standard 120V outlet: a 600W device draws 5A, a 1,200W device draws 10A, and a 1,800W device draws 15A (which is the limit of a standard 15-amp circuit). For 240V circuits (dryers, ovens), divide by 240.
Why does a 15-amp circuit breaker trip when I plug in a 1,500W heater?
A 1,500W heater on a 120V circuit draws 12.5 amps (1,500 ÷ 120). If other devices on the same circuit draw even 3 more amps, the total exceeds 15A and the breaker trips for safety. Circuit breakers are rated to protect the wiring, not just individual devices.
What is a kilowatt-hour (kWh)?
A kilowatt-hour is the unit used on your electricity bill. It represents 1,000 watts running for one hour. A 100W light bulb running for 10 hours uses 1 kWh. If your utility charges $0.15/kWh, that bulb costs $0.015 per hour — or $1.50 for 100 hours of use.