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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.