Electrical Power Calculator
Power in watts, kW and horsepower — plus energy use and running cost.
Results update as you type.
About this calculator
Electrical power is the rate at which a device converts electrical energy, measured in watts. For a DC or purely resistive load it is simply voltage times current, P = V × I. For an AC load with a power factor below 1 — motors, transformers and other inductive loads — the real power that does useful work is P = V × I × PF, while the product V × I alone gives the larger apparent power in volt-amperes. This calculator reports the power in watts, kilowatts and mechanical horsepower, using 1 hp = 745.7 W.
As a worked example, a device on a 120 V circuit drawing 10 A at unity power factor uses P = 120 × 10 = 1,200 W, which is 1.2 kW or about 1.61 hp. If instead it were a motor with a 0.8 power factor, the real power would fall to 120 × 10 × 0.8 = 960 W even though it still draws the same 10 A. Divide watts by 745.7 to convert to horsepower, or by 1,000 to get kilowatts.
The advanced usage fields turn power into running cost. Daily energy in kilowatt-hours is the power in kW multiplied by the hours used per day; multiply by your price per kWh for the daily cost and by 30 for a monthly estimate. A 1.2 kW load run 4 hours a day uses 4.8 kWh daily, about 144 kWh a month, which at $0.15/kWh is roughly $21.60 a month — the kind of figure that helps compare appliances, size a generator or budget a workshop.
Frequently asked questions
What is the electrical power formula?
Real power in watts is P = V × I × PF. For DC or a resistive load the power factor is 1, so power is simply voltage times current.
How many watts in a horsepower?
One mechanical horsepower equals 745.7 watts, so a 1492 W load is about 2 hp. Divide watts by 745.7 to convert.
How is the running cost worked out?
Daily energy is power (kW) × hours per day, in kWh. Multiply by 30 days and your price per kWh to estimate the monthly cost.
What is power factor and when do I change it?
Power factor is the fraction of the apparent power (V × I) that does real work. It is 1 for DC and resistive loads like heaters and incandescent bulbs, but drops to roughly 0.7–0.9 for motors and many electronic supplies, reducing the real watts.
What is the difference between watts and volt-amperes?
Watts are real power (V × I × PF); volt-amperes are apparent power (V × I) with no power-factor correction. For resistive loads they are equal, but for reactive loads the VA rating is higher, which is why generator and UPS ratings are given in VA.
How do I convert watts to horsepower?
Divide the watts by 745.7. So 1,200 W is about 1.61 hp, and a 1 hp motor draws roughly 746 W of mechanical-equivalent power (more electrically, once efficiency losses are included).
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/electrical-power/curl
curl "https://calculator.free/api/v1/electrical-power/?voltage=120¤t=10"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/electrical-power/?" + new URLSearchParams({
"voltage": "120",
"current": "10"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.