Understanding This Calculator
Ohm’s Law is the backbone of basic electrical calculations: power (watts) equals voltage (volts) multiplied by current (amps). Whether you’re sizing a circuit breaker, choosing an extension cord, figuring out how many amps an appliance draws, or estimating a generator’s capacity, this three-way relationship lets you solve for whichever value you don’t already know.
Pick which quantity you need above, enter the two values you do know, and the calculator rearranges the formula automatically. Keep in mind real-world circuits also involve power factor for AC loads with reactive components (motors, some electronics) — this calculator assumes a simple resistive/DC-style relationship, which is accurate for most household appliances and DC systems.
- P power in watts (W)
- V voltage in volts (V)
- I current in amps (A)
| Circuit | Voltage | Typical Max Amps |
|---|---|---|
| Standard US outlet | 120V | 15-20A |
| US large appliance | 240V | 30-50A |
| Standard EU outlet | 230V | 13-16A |
Frequently Asked Questions
Why does my appliance’s amp draw seem lower than expected?
Many appliances list “rated” or nameplate wattage that reflects peak draw, not steady-state running draw. Motors and compressors also briefly draw far more current at startup than during normal operation, which this simple formula doesn’t capture.
Does this work for AC circuits with a power factor below 1?
Not exactly — for AC loads with inductive or capacitive components, real power (watts) equals volts × amps × power factor. This calculator assumes power factor of 1, which is accurate for resistive loads like heaters and incandescent bulbs.