Single-Phase Power Calculator

About the Single-Phase Power Calculator

Single-phase power is the standard electrical supply for most residential homes and light commercial applications. It powers everything from lights and outlets to kitchen appliances and small HVAC systems. In North America, single-phase power is delivered at 120V (standard outlets) and 240V (dryers, ranges, water heaters) from a split-phase transformer.

The relationship between watts, volts, and amps is straightforward for resistive loads (like heaters), where Watts = Volts × Amps. For inductive loads (like motors and compressors), power factor must be included: Watts = Volts × Amps × Power Factor. Power factor accounts for the phase difference between voltage and current.

This calculator computes single-phase power in watts and kilowatts from voltage, current, and power factor. It's essential for determining circuit requirements, selecting breaker sizes, and verifying that circuits aren't overloaded.

This analytical approach supports both immediate cost reduction and long-term sustainability goals, helping organizations balance economic and environmental priorities in their energy management.

When This Page Helps

Single-phase power calculations are fundamental for residential electrical work, circuit planning, and load verification. This calculator handles power factor so you can estimate both resistive and inductive loads correctly.

How to Use the Inputs

1. Enter the voltage (120V or 240V for most US applications).
2. Enter the current in amps.
3. Enter the power factor (1.0 for resistive loads, 0.80–0.95 for motors).
4. View the real power in watts and kW.
5. Verify against the circuit breaker and wire size ratings.
6. Use results for load calculations and panel planning.

Example Calculation

Tips & Best Practices

• Use PF = 1.0 for resistive loads (heaters, incandescent lights, toasters).
• Use PF = 0.80–0.95 for motor-driven loads (AC compressors, pumps, fans).
• Circuit breakers should only be loaded to 80% of their rating for continuous loads.
• A 15A/120V circuit provides 1,800W maximum (1,440W continuous).
• A 20A/240V circuit provides 4,800W maximum (3,840W continuous).
• Measure actual current with a clamp meter to verify calculations.

Single-Phase vs Three-Phase

Single-phase uses two conductors (hot + neutral) and delivers pulsating power. Three-phase uses three hot conductors and delivers constant power. Most residential service is single-phase (split-phase). Commercial buildings typically have three-phase service for heavier loads.

Understanding 120V and 240V Circuits

In a US split-phase system, the transformer center tap provides the neutral. Each hot leg is 120V to neutral. Between the two hot legs is 240V. This allows the same service entrance to power both 120V outlets and 240V appliances.

Common Circuit Sizes

15A/120V (1,800W): General lighting and outlets. 20A/120V (2,400W): Kitchen, bathroom, and garage outlets. 30A/240V (7,200W): Dryers, small ranges. 40A/240V (9,600W): Ranges and cooktops. 50A/240V (12,000W): Large ranges and EV chargers.

Frequently Asked Questions

• What is the difference between watts and VA?

  Watts (W) is real power doing useful work. Volt-amps (VA) is apparent power, which is the total power the circuit supplies. VA = V × I. Watts = VA × power factor. For resistive loads, W = VA. For inductive loads, W < VA.

• Why does power factor matter?

  Power factor determines how much of the current actually does useful work. A motor with PF = 0.80 draws 25% more current than a resistive load of the same wattage. This extra current must be considered for wire and breaker sizing.

• What is 120V vs 240V?

  US homes receive 240V from the utility, split into two 120V legs. Standard outlets provide 120V (one leg + neutral). Large appliances like dryers and ranges use 240V (both legs). Each leg provides 120V to neutral.

• How do I calculate amps from watts?

  I = W / (V × PF). For example, a 1,500W heater on 120V at PF 1.0: I = 1,500 / (120 × 1.0) = 12.5 amps. This would require a 15A circuit minimum (20A recommended for continuous use).

• What is a continuous load?

  A continuous load runs for 3 hours or more (like a space heater or pool pump). The NEC requires that continuous loads not exceed 80% of the circuit breaker rating. A 20A breaker can only carry 16A continuously.

• How do I calculate wire size from amps?

  14 AWG copper wire handles 15A. 12 AWG handles 20A. 10 AWG handles 30A. 8 AWG handles 40A. Always size wire for the breaker rating, not just the expected load. Long runs require upsizing for voltage drop.