How to Convert Amps to Watts
Quick answer
- Formula:
W = A × V - 15 A at 120 V: 1800 W
- 20 A at 240 V: 4800 W
- You always need voltage, amps and watts measure different things.
Want to skip the math? Use the amps-to-watts calculator.
Why voltage is required
Amps measure current, the rate of charge flow through a conductor. Watts measure power, the rate at which energy is used. These are related through voltage, which is what "pushes" current through a circuit.
The relationship is Watt's Law: P = V × I. In common notation, that is W = A × V. You need all three pieces (two known, one to solve for) to use the formula.
This is why a 15 A circuit at 120 V delivers 1800 W, but a 15 A circuit at 240 V delivers 3600 W. Same current, twice the voltage, twice the power.
The formula
W = A × V
Where:
- W = power in watts
- A = current in amps
- V = voltage in volts
Steps: (1) Find the amp rating, usually on the device nameplate or circuit breaker label. (2) Identify the voltage, 120 V for standard US outlets, 240 V for large appliances. (3) Multiply amps by volts.
Worked examples
Example 1: Standard 15 A circuit at 120 V
What is the maximum power a standard 15 A household circuit can handle?
W = 15 × 120 = 1800 W
This is the theoretical maximum. The practical safe limit for sustained loads is 80% of that: 1440 W. So if you are stacking devices on a 15 A circuit, keep the combined wattage well under 1440 W.
Example 2: 20 A circuit at 240 V (dryer outlet)
A dryer outlet rated 20 A at 240 V.
W = 20 × 240 = 4800 W
That is 4.8 kW of available power, enough for most electric dryers, which typically draw 4–6 kW.
Example 3: 12 V battery system
A load drawing 10 A from a 12 V battery bank.
W = 10 × 12 = 120 W
At 12 V, a 10 A draw is only 120 W, roughly comparable to a single light fixture. In a 12 V system, amps "spend" on power very quickly because the voltage is low.
Reference table: amps to watts at common voltages
| Amps (A) | Watts at 12 V | Watts at 120 V | Watts at 240 V |
|---|---|---|---|
| 1 A | 12 W | 120 W | 240 W |
| 5 A | 60 W | 600 W | 1200 W |
| 10 A | 120 W | 1200 W | 2400 W |
| 15 A | 180 W | 1800 W | 3600 W |
| 20 A | 240 W | 2400 W | 4800 W |
| 30 A | 360 W | 3600 W | 7200 W |
Real-world context: circuit capacity vs device draw
Converting amps to watts is useful when you know a circuit's amperage and want to understand how much wattage it can support. This comes up often when planning workshop setups, kitchen circuits, or multi-device loads where you are adding up appliances to see if they fit.
The 80% continuous load rule still applies: for any sustained load (running longer than 3 hours), keep total wattage at or below 80% of the circuit's theoretical watt maximum. For a 15 A / 120 V circuit that is 1440 W; for a 20 A / 120 V circuit that is 1920 W.
Remember this is a single-phase estimate assuming unity power factor. Motors and some electronics draw more current than the simple formula predicts because of reactive power components, but for most household resistive loads like heaters and incandescent bulbs, the formula is a reliable planning estimate.
Related tools and guides
- Amps-to-Watts Calculator, enter amps and voltage, get watts instantly
- Watts-to-Amps Calculator, reverse direction
- How to Convert Watts to Amps
- How Many Watts Is a 15 A Circuit at 120 V?
- Electricity Basics
FAQ
Can I convert amps to watts without voltage?
No. Amps and watts are different units. Voltage is required to relate them. If you know the device is running on a standard US household circuit, 120 V is a reasonable default, but always verify the actual voltage for circuit planning.
Why does voltage matter so much?
Voltage is the multiplier in the formula. Double the voltage at the same amperage and you double the watts. That is why 240 V circuits deliver so much more power than 120 V circuits at the same breaker rating, the higher voltage does more work per amp.
Does this formula work for AC and DC circuits?
Yes for DC and for resistive AC loads. For AC circuits with reactive components (motors, transformers, some electronics), the real power in watts will be lower than A × V because of power factor. The formula still gives a useful upper bound for planning purposes.
My circuit breaker is rated 15 A. How many watts can I run?
At 120 V: theoretical maximum is 1800 W (15 × 120). Practical sustained limit is 1440 W (80% of 1800). Add up the wattages of devices running at the same time and compare to 1440 W for sustained loads.
Is the result watts or kilowatts?
The formula W = A × V gives the result in watts. To convert to kilowatts, divide by 1000. For example, 1800 W = 1.8 kW.
Disclaimer: Results are informational estimates for learning and planning only. Always follow the applicable electrical code and consult a qualified licensed electrician for safety-critical work.