Definition
Wattage is a measure of electrical power equal to one joule per second. It represents the rate of energy consumption or production. In construction and electrical systems, wattage determines circuit sizing, wire gauge requirements, and electrical load capacity. Watts are calculated by multiplying voltage (volts) by current (amps). Higher wattage devices consume more power and generate more heat, requiring appropriate electrical infrastructure.
Formula
For resistive loads (lights, heaters): Watts = Volts x Amps. For a 120V circuit drawing 15 amps: 120 x 15 = 1,800 watts maximum. For motor loads and inductive devices, power factor must be considered. For three-phase power: Watts = Volts x Amps x sqrt3 x Power Factor. Always calculate loads at 80% of circuit breaker capacity for continuous loads (3+ hours).
Example
Wattage Calculation Example
A bedroom circuit with 10 LED bulbs (10W each = 100W), a ceiling fan (75W), and a phone charger (5W) draws 180 watts total. On a 15-amp, 120-volt circuit (1,800 watts capacity), this uses only 10% of available capacity. For planning, allocate 3 watts per square foot for residential lighting and receptacles as a general rule.