Electrical power — measured in watts — is the rate at which electrical energy is consumed or produced, governed by the fundamental relationship P = V × I (power equals voltage times current). Understanding these relationships is essential for sizing circuits, selecting wire gauges, choosing breakers, and estimating energy costs. A 1,500-watt space heater on a 120V circuit draws 12.5 amps — close to the 15A limit of a standard household circuit, which is why running a heater with other appliances on the same circuit often trips breakers. Our electrical power calculator converts between watts, volts, amps, and ohms using Ohm's law and the power formula, handles both AC and DC calculations (including power factor for AC circuits), and computes energy consumption in kilowatt-hours for cost estimation. Whether you are sizing a generator, checking circuit capacity, or calculating electricity costs, this tool provides instant answers with the underlying formulas displayed.
The power triangle: watts, volts, and amps
Four key relationships connect power (P in watts), voltage (V in volts), current (I in amps), and resistance (R in ohms): P = V × I, P = I²R, P = V²/R, and V = IR (Ohm's law). Knowing any two values lets you calculate the other two. A 60-watt light bulb on 120V draws I = 60/120 = 0.5 amps with resistance R = 120/0.5 = 240 ohms. For DC circuits, these formulas are straightforward. For AC circuits, the power factor (PF) affects the relationship: real power (watts) = V × I × PF. A motor drawing 10 amps at 240V with 0.85 power factor consumes 240 × 10 × 0.85 = 2,040 watts of real power, though it appears to draw 2,400 VA (volt-amps) from the utility.
Circuit capacity and breaker sizing
Standard US residential circuits: 15A at 120V provides 1,800 watts maximum capacity, 20A at 120V provides 2,400 watts, and 30A at 240V provides 7,200 watts. The NEC recommends loading circuits to no more than 80% of breaker rating for continuous loads (running more than 3 hours): a 15A circuit should not continuously draw more than 12A (1,440 watts). Common appliance loads: microwave 1,000-1,500W, hair dryer 1,200-1,800W, toaster 800-1,500W, refrigerator 100-400W (running), window AC 500-1,500W, electric dryer 4,000-5,500W (requires 30A 240V circuit), and electric range 8,000-12,000W (requires 40-50A 240V circuit). Exceeding circuit capacity causes breaker trips — a safety feature preventing wire overheating and fire.
Energy consumption and cost calculation
Energy (kilowatt-hours) = power (watts) × time (hours) / 1,000. Cost = kWh × electricity rate. A 1,500W space heater running 8 hours daily consumes 12 kWh/day, costing $1.44/day at $0.12/kWh ($43.20/month). Standby power (phantom loads) from electronics in standby mode consumes 5-10% of household electricity — a TV on standby draws 5-15W, a game console 10-25W, and a cable box 15-30W. Collectively, phantom loads cost the average household $100-200 annually. LED bulbs have revolutionized lighting costs: a 10W LED produces the same light as a 60W incandescent, saving $8-10/year per bulb at average usage. Replacing 30 incandescent bulbs saves $250-300 annually in electricity costs.