Amps to Watts Calculator
Convert amps to watts using P = I × V. Enter amps and voltage to get watts. Includes conversion table for 120V (US) and 240V (UK) systems.
To convert amps to watts, you need the voltage of the circuit. The relationship between current, voltage, and power is one of the most fundamental in electrical engineering — and one of the most practically useful.
The Formula: Amps to Watts
Power (W) = Current (A) × Voltage (V)
P = I × V
Examples:
- 10A at 120V (US standard outlet): 10 × 120 = 1,200 watts
- 10A at 240V (UK standard outlet): 10 × 240 = 2,400 watts
- 30A at 240V (US dryer circuit): 30 × 240 = 7,200 watts
Watts to Amps (Reverse Calculation)
Current (A) = Power (W) ÷ Voltage (V)
I = P ÷ V
Examples:
- 2,500W tumble dryer on a 240V UK circuit: 2,500 ÷ 240 = 10.4 amps
- 1,800W microwave on a 120V US circuit: 1,800 ÷ 120 = 15 amps
Conversion Table — Common Amperages to Watts
At 120V (US standard residential outlets)
| Amps | Watts | Typical use |
|---|---|---|
| 1A | 120W | LED lighting circuit |
| 5A | 600W | Small appliances, phone chargers |
| 10A | 1,200W | Toaster, hair dryer (low) |
| 15A | 1,800W | Standard US circuit breaker limit (80% of 15A = 12A safe max) |
| 20A | 2,400W | Kitchen circuits, air conditioner |
| 30A | 3,600W | Electric dryer (240V circuit uses 30A for 7,200W) |
| 50A | 6,000W | EV charging, electric range |
At 240V (UK / European standard)
| Amps | Watts | Typical use |
|---|---|---|
| 1A | 240W | LED lighting, phone charger |
| 3A | 720W | Fused plug limit for most small UK appliances |
| 5A | 1,200W | Lamp, stereo, TV |
| 10A | 2,400W | Dishwasher, washing machine |
| 13A | 3,120W | UK standard fused plug maximum |
| 16A | 3,840W | EV home charger (3.6kW), immersion heater |
| 32A | 7,680W | 7kW EV wallbox, cooker circuit |
AC vs DC: Does It Matter?
For DC circuits (batteries, solar panels, low-voltage electronics), the formula P = I × V applies directly.
For AC circuits (household mains power), real power also depends on the power factor — a measure of how efficiently the current is being used:
Power (W) = Current (A) × Voltage (V) × Power Factor
For purely resistive loads (heaters, incandescent bulbs, kettles), the power factor is 1.0 and you can ignore it. For motors, compressors, and electronics with switching power supplies, the power factor is typically 0.8–0.95. This is why a "1,000W" motor may draw more than 1,000 VA from the supply.
For home calculations, the simpler P = I × V formula is accurate enough for resistive appliances — kettles, heaters, toasters, ovens.
Three-Phase Power (Commercial/Industrial)
For three-phase AC systems used in commercial and industrial settings:
Power (W) = √3 × Current (A) × Line Voltage (V) × Power Factor
= 1.732 × I × V × PF
Example: 20A three-phase at 415V (UK industrial) with 0.9 power factor:
1.732 × 20 × 415 × 0.9 = 12,952W ≈ 13kW
Circuit Breaker Sizing — The 80% Rule
Don't run a circuit at its full rated amperage continuously. The NEC (US) and IEE Wiring Regulations (UK) both require continuous loads to draw no more than 80% of the breaker's rating:
| Breaker rating | Maximum safe continuous load |
|---|---|
| 15A breaker | 12A (1,440W at 120V) |
| 20A breaker | 16A (1,920W at 120V or 3,840W at 240V) |
| 30A breaker | 24A (2,880W at 120V or 5,760W at 240V) |
| 32A breaker | 25.6A (6,144W at 240V) |
If you're adding appliances to an existing circuit, calculate the total amperage and ensure it stays below 80% of the breaker rating.
Practical Example: Generator Sizing
"My generator produces 30A at 240V — how many watts is that?"
30A × 240V = 7,200W = 7.2kW
At 80% safe loading: 7,200 × 0.8 = 5,760W maximum continuous load.
That generator can run:
- A 2,500W tumble dryer + 2,200W oven + 1,000W of other loads simultaneously
- Or two 2,500W space heaters with headroom for lighting
Frequently Asked Questions
Do I always need to know the voltage to convert amps to watts?
Yes — amps measure current flow, not power. The same current at a higher voltage carries more power. A 10A current at 12V is only 120W; at 240V, it's 2,400W. Always confirm the voltage of the circuit before converting.
What's the difference between watts, volt-amps (VA), and kilowatts (kW)?
Watts (W) measure real power — what actually does work and appears on your electricity bill. Volt-amps (VA) measure apparent power — the product of volts and amps without accounting for power factor. For resistive loads, W = VA. Kilowatts (kW) = watts ÷ 1,000. Your electricity meter counts kilowatt-hours (kWh).
How many watts can a standard UK plug handle?
A standard UK plug uses a 13A fuse, giving a maximum of 13 × 240 = 3,120W. Most UK appliances draw well below this. Appliances over ~2,500W (such as high-powered hair dryers, fan heaters, and some electric showers) approach the limit and should not share a socket with other devices.
How many watts can a standard US outlet handle?
Most US household outlets are on 15A circuits at 120V, giving 1,800W maximum — but the 80% rule limits safe continuous use to 1,440W. Kitchen and bathroom circuits are often 20A (2,400W max, 1,920W continuous). Never plug a 15A appliance into a 20A circuit without checking compatibility.
What is the formula if I have resistance instead of voltage?
Use Ohm's Law extensions: P = I² × R (where R is resistance in ohms). Or P = V² ÷ R if you know voltage and resistance but not current.
Related Calculators
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