Amps to Watts Calculator

About the Amps to Watts Calculator

An amps to watts calculator converts electrical current (amperes/amps) to power (watts) at a specified voltage, using the fundamental power equation P = V × I. This conversion is needed whenever you know a circuit's current rating and voltage but need to know the power in watts — for example, when sizing a generator (rated in kW) to power a known set of circuits (rated in amps), when selecting a power supply for electronics (rated in watts) to power components drawing known currents, or when understanding the maximum load capacity of a circuit breaker in watts. Our calculator handles DC circuits, single-phase AC (with power factor), three-phase AC (with power factor), and converts results between watts, kilowatts, and BTU/hr for HVAC applications. It also applies the NEC 80% continuous load rule to show the safe continuous load limit for any circuit.

How It Works

DC and resistive single-phase AC: P (watts) = V (volts) × I (amps). Three-phase AC: P = V × I × √3 × PF. Examples: Standard US residential outlet 15A at 120V: P_max = 120 × 15 = 1,800W. NEC 80% continuous limit: 1,800 × 0.8 = 1,440W safe continuous load. Heavy appliance circuit 30A at 240V: P_max = 240 × 30 = 7,200W (clothes dryer, EV charger, large AC). Three-phase industrial panel 100A at 480V (PF=0.85): P = 480 × 100 × 1.732 × 0.85 = 70,681W ≈ 70.7 kW. Typical residential circuits in watts: 15A/120V = 1,800W; 20A/120V = 2,400W; 30A/240V = 7,200W; 40A/240V = 9,600W; 50A/240V = 12,000W.

Tips & Best Practices

• ✓NEC 80% rule: branch circuits must not be loaded above 80% of their rated ampacity for continuous loads (defined as loads running for 3+ hours). A 20A circuit supports only 16A = 1,920W for continuous loads.
• ✓Generator sizing: add up all loads in watts, then select a generator with at least 125% of that total (to accommodate motor starting surge). Express the generator capacity in watts or kW and compare directly to the sum.
• ✓240V vs 120V efficiency: the same power delivered at 240V requires half the current as at 120V. Half the current means one-quarter the resistive (I²R) losses in wiring. High-power loads (ovens, water heaters, AC) benefit significantly from 240V supply.
• ✓Power factor: inductive loads (motors, transformers, fluorescent lights) have power factor < 1.0. A motor rated 15A at 240V with PF = 0.8 delivers only 240 × 15 × 0.8 = 2,880W of real work, not 3,600W apparent power. Size wiring for the amperage, not the watt calculation.
• ✓Shore power for boats: boat shore power connections are typically 30A/125V (3,750W) or 50A/125/250V (12,500W). Understanding amps-to-watts helps plan which appliances can run simultaneously without tripping the shore power breaker.
• ✓UPS sizing: uninterruptible power supplies are rated in VA (volt-amperes, apparent power) and watts (real power). For resistive loads (computers, servers), VA ≈ Watts. For loads with power factor < 1, VA > Watts. Convert your amperage loads to watts, then add 25% margin when selecting UPS capacity.
• ✓Data centre power: server racks are rated in kW per rack. Facility power is sized in amps per circuit. Converting between the two using voltage and power factor is fundamental to data centre capacity planning.
• ✓Electrical service sizing: a 200A residential service at 240V = 200 × 240 = 48,000 VA = 48 kVA. At 0.9 PF: 43.2 kW available real power. This is the total capacity all circuits in the home share.

Who Uses This Calculator

Electricians sizing circuit breakers and calculating load schedules. Generator and UPS owners calculating total load capacity. Homeowners determining which appliances can run on a single circuit. Solar and battery storage designers sizing inverter outputs. Industrial engineers calculating motor and equipment power requirements. Data centre engineers sizing power distribution units and feeds. Marine electricians designing boat electrical systems.