How to Convert Watts to Amps
Quick answer
- Formula:
A = W ÷ V - 1800 W at 120 V: 15 A
- 1800 W at 240 V: 7.5 A
- You always need voltage, watts and amps measure different things, so a voltage value is required.
Want to skip the math? Use the watts-to-amps calculator.
Why you need voltage to convert
Watts measure power, the rate at which energy is used or transferred. Amps measure current, the flow of charge through a conductor. These are two different quantities, and the only way to move between them is through voltage.
The relationship comes from Watt's Law: P = V × I. Rearranging for current gives you the conversion formula: I = P / V, or in everyday notation, A = W ÷ V.
The same device behaves very differently depending on the voltage it runs on. A 1800 W space heater draws 15 A at 120 V but only 7.5 A at 240 V, same power, very different current, completely different wiring implications.
The formula
A = W ÷ V
Where:
- A = current in amps
- W = power in watts
- V = voltage in volts
Steps: (1) Find the wattage, usually on the device nameplate or in the manual. (2) Identify the voltage, 120 V for standard US outlets, 240 V for large appliances, 12 V for battery systems. (3) Divide watts by volts.
Worked examples
Example 1: Space heater at 120 V
A 1500 W space heater runs on a standard 120 V circuit.
A = 1500 ÷ 120 = 12.5 A
That is well within a 15 A breaker's capacity, but only if the heater is the only sustained load on the circuit, since the 80% continuous load guideline brings the practical limit to 12 A.
Example 2: Dryer at 240 V
An electric dryer draws 5400 W on a 240 V circuit.
A = 5400 ÷ 240 = 22.5 A
That is why dryers need a dedicated 30 A circuit, a 20 A circuit would not have enough headroom for sustained use.
Example 3: 12V battery system
A 120 W load running from a 12 V battery bank.
A = 120 ÷ 12 = 10 A
The same 120 W at 120 V would only draw 1 A. Low-voltage systems use far more current for the same power, which is why wire gauge matters more in 12 V setups.
Reference table: common wattages at 120 V and 240 V
| Device | Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|---|
| LED bulb | 10 W | 0.08 A | 0.04 A |
| Phone charger | 20 W | 0.17 A | 0.08 A |
| Microwave (1000 W rated) | ~1400 W input | ~11.7 A | ~5.8 A |
| Space heater | 1500 W | 12.5 A | 6.25 A |
| Electric dryer | 5400 W | 45 A | 22.5 A |
Note: microwave input wattage is typically 30–40% higher than rated output wattage. Always use nameplate input wattage for circuit planning.
Circuit planning: what the amp number means in practice
Once you have the amp draw, you can compare it to your circuit's breaker rating. The key rule for sustained loads: do not exceed 80% of the breaker's rated current. A 15 A breaker is designed for sustained loads up to 12 A; a 20 A breaker handles sustained loads up to 16 A.
If your calculated amp draw is close to or above the 80% threshold, and especially if other loads share the same circuit, that is a sign you either need a dedicated circuit or should reconsider whether the load is appropriate for that circuit.
The conversion here is a planning estimate based on nameplate watts. Real draw can differ slightly due to efficiency losses, startup surges, and how the device actually behaves under load.
Related tools and guides
- Watts-to-Amps Calculator, enter watts and voltage, get amps instantly
- Amps-to-Watts Calculator, reverse direction
- How to Convert Amps to Watts
- How Many Amps Does a 1500 W Space Heater Draw?
- Electricity Basics
FAQ
Can I convert watts to amps without knowing the voltage?
No. Watts and amps are different quantities. Without voltage you cannot convert between them. If you know the device is a standard US household appliance, assume 120 V as a starting point, but always verify the actual voltage if you are planning wiring or circuit loads.
Does the formula work for all types of loads?
It works well for resistive loads like heaters and incandescent bulbs. For motors and electronics with reactive components, power factor means the actual current drawn will be higher than the simple formula predicts. For most household planning purposes, the formula gives a useful estimate.
My device nameplate shows amps, not watts. Can I still use this?
If your nameplate already shows amps, you do not need to convert. Multiply amps by voltage to get watts if needed: W = A × V.
What if I have multiple devices on one circuit?
Add up the amp draws of all devices that might run at the same time, then check the total against the 80% guideline for the circuit's breaker rating. For example, on a 15 A circuit, the sustained total should stay below 12 A.
Why does the same device draw more amps on 120 V than 240 V?
Because lower voltage requires more current to deliver the same power. This is the tradeoff behind 240 V circuits for large appliances, the higher voltage means smaller current, which means less heat in the wiring for the same power delivery.
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.