Amps Calculator

Convert watts, amps, and volts instantly. Enter any two values — the third is calculated. Essential for sizing solar wires, fuses, and breakers.

Enter any two values — the third is calculated

Power?

Current?

calculated

Voltage?

Current

10 A

Power1,200 W

Current10 A

Voltage120 V

Resistance12 Ω

P = 120 V × 10 A = 1,200 W

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How to Use This Calculator

Choose what you're solving for

Click the Solve for Amps, Solve for Watts, or Solve for Volts button to select which value you want to calculate. The selected value's field will turn green and display the calculated result. The other two fields are your inputs.

Solve for amps (most common)

Enter watts and volts to find amps. This is the most common use: you know your device's wattage and your system voltage, and need the current to size fuses, breakers, and wire. Example: a 1,200W microwave on a 120V circuit draws 1,200 ÷ 120 = 10A.

Solve for watts

Enter amps and volts to find watts. Useful when you know a circuit's current (from a clamp meter or breaker rating) and want to understand the power consumption. A 20A circuit at 240V delivers up to 20 × 240 = 4,800W.

Solve for volts

Enter watts and amps to find voltage. Useful for identifying unknown power supplies or checking battery bank voltage given known power and current. 2,400W at 20A = 120V.

The Formula

P = V × I (Power = Voltage × Current) I = P ÷ V (Amps = Watts ÷ Volts) V = P ÷ I (Volts = Watts ÷ Amps) R = V ÷ I (Resistance = Volts ÷ Amps) P = I² × R (Power from current and resistance) P = V² ÷ R (Power from voltage and resistance)

The base relationship P = V × I is called the electric power formula. Combined with Ohm's Law (V = I × R), you can calculate any electrical quantity from any two known values. This calculator solves the P = V × I triangle — for full four-variable calculations including resistance, use the Ohm's Law Calculator.

Common Conversions

100W @ 120V AC0.83 A

1,500W space heater @ 120V12.5 A

3,000W inverter @ 24V DC125 A

3,000W inverter @ 48V DC62.5 A

400W solar panel @ 33.6V (Vmp)11.9 A

5kW solar array @ 48V104 A

20A circuit @ 240V4,800 W max

30A circuit @ 120V (RV shore power)3,600 W max

50A circuit @ 240V (RV/EV)12,000 W max

FAQ

How many amps does a 1,000W device draw at 120V vs 240V? ▼

At 120V: 1,000 ÷ 120 = 8.33A. At 240V: 1,000 ÷ 240 = 4.17A. Doubling the voltage halves the current for the same power. This is why high-power appliances (electric ranges, dryers, EV chargers) run on 240V — lower current means smaller wires, smaller breakers, and lower resistive losses.

What is the difference between watts and amps? ▼

Watts measure power — the rate of energy flow. Amps measure current — the rate of electron flow. They're related through voltage: Watts = Volts × Amps. A 12V battery delivering 10A to a load is supplying 120W of power. The same 10A at 120V is 1,200W. Current (amps) determines wire size requirements; watts determine energy consumption and battery capacity requirements.

Why does my inverter draw more amps than the AC load suggests? ▼

Inverters are not 100% efficient — typically 90-95%. To deliver 1,000W of AC power, a 92% efficient inverter must draw 1,000 ÷ 0.92 = 1,087W from the battery. At 24V, that's 1,087 ÷ 24 = 45.3A DC, not the 8.3A you'd calculate from the 120V AC output alone. Always calculate DC battery current based on inverter input power (AC watts ÷ efficiency), not AC output.

How do I convert kW to amps? ▼

Convert kW to watts first: 1 kW = 1,000W. Then divide by voltage: Amps = Watts ÷ Volts. Example: a 5 kW solar system at 240V = 5,000 ÷ 240 = 20.8A AC output. A 5 kW inverter at 48V DC = 5,000 ÷ 48 = 104A DC input. The conversion is simply: Amps = (kW × 1,000) ÷ Volts.

What size breaker do I need for a given wattage? ▼

Calculate amps first: Amps = Watts ÷ Volts. Then multiply by 1.25 for the NEC 80% continuous load rule (breakers shouldn't run above 80% of rating continuously). Example: 1,500W at 120V = 12.5A × 1.25 = 15.6A → use a 20A breaker. For motor loads with startup surge, size the breaker to handle the surge current (typically 2-3× running current) but use wire sized for running current × 1.25.