How to Calculate Watts (Electrical Power)
The watt is the SI unit of power — the rate at which energy is transferred or converted, equal to one joule per second. In an electrical circuit, power in watts is simply the product of voltage and current: P = V × I. This calculator uses that direct definition to turn a voltage and current reading into a wattage figure, then extends it into practical terms: how much energy a device consumes over a day (in kilowatt-hours) and roughly what that costs on a typical electricity bill.
The power formula: P = V × I
Multiply voltage (in volts) by current (in amps) to get power (in watts): P = V × I. For example, a household circuit at 120 V drawing 5 A delivers 120 × 5 = 600 W. This is algebraically equivalent to the other two forms of the power law, P = I²R and P = V²/R, which are used instead when resistance is known but voltage or current is not — all three describe the same physical quantity of energy transfer per second.
From watts to kilowatt-hours and cost
Watts measure an instantaneous rate, but electricity bills are based on energy consumed over time, measured in kilowatt-hours (kWh). To convert, multiply power in watts by the number of hours used, then divide by 1000: E (kWh) = P (W) × t (h) ÷ 1000. Multiplying that energy figure by your utility's rate (dollars per kWh) gives the cost to run the device for that period — this calculator reports both the daily energy use and the daily and monthly cost so you can compare appliances or estimate a bill before you buy or run something.