Single-Phase Power Calculator
Single-phase real power = Voltage × Current × Power Factor (P = V × I × PF)
Enter voltage, current, and power factor to calculate real power in watts and apparent power in VA. For pure resistive loads, heaters, incandescent bulbs, toasters, set power factor to 1.0. For the reverse conversion, use the watts to amps calculator.
Real power: 1800 W
Apparent power: 1800 VA
What is single-phase power?
Single-phase power is the most common electrical supply type in homes and light commercial buildings. One AC voltage waveform drives current through the circuit. The power delivered depends on voltage, current, and how closely the current follows the voltage waveform, a ratio called power factor.
Real power (watts) is the actual work done; it runs motors, heats elements, and lights LEDs. Apparent power (volt-amperes, VA) is what the utility must supply, which is always equal to or greater than real power. The difference matters when sizing wiring, transformers, and generators.
Power factor ranges from 0 to 1.0. Resistive loads (electric heaters, toasters, incandescent bulbs) have PF ≈ 1.0 so real and apparent power are equal. Inductive loads (motors, fluorescent ballasts, variable-speed drives) have PF below 1.0, meaning more current flows than the watts figure alone would suggest.
- Voltage (V): standard household voltages are 120V and 240V in North America
- Current (A): amps drawn by the load under normal operating conditions
- Power factor (PF): 1.0 for resistive loads, lower (typically 0.80–0.95) for motors and electronics
For most home planning use PF = 1.0. For motors, air conditioners, and fluorescent lighting, use the nameplate power factor or 0.85 as a conservative default.
How to convert
Real power (watts)
Real power is what the appliance actually uses, the watts you pay for on your electric bill.
P (W) = V × I × PF
watts = volts × amps × power factor
Apparent power (VA)
Apparent power is what the source must supply. Use it for sizing transformers, generators, and UPS units.
S (VA) = V × I
volt-amperes = volts × amps
Power factor relationship
Power factor is the ratio of real to apparent power. A low PF means more current for the same real power output.
PF = P / S
power factor = watts / volt-amperes
Worked examples
Question: What is the real power of a 15A circuit at 120V with PF = 1.0 (resistive heater)?
Solution: P = 120 × 15 × 1.0 = 1,800 W
Question: A 2-ton central A/C draws 16A at 240V with PF = 0.92. What is the real power?
Solution: P = 240 × 16 × 0.92 = 3,532.8 W ≈ 3.5 kW; Apparent = 240 × 16 = 3,840 VA
Question: A motor draws 8A at 120V with PF = 0.80. What is real vs apparent power?
Solution: Apparent = 120 × 8 = 960 VA; Real = 120 × 8 × 0.80 = 768 W
Common mistakes and notes
- Always use operating current (from a clamp meter or nameplate), not peak or startup surge current.
- For a generator or UPS, size by apparent power (VA), they must supply the full current regardless of power factor.
- NEC continuous load rule: a circuit breaker must be sized at 125% of continuous load current.
- Nameplate watts on appliances already account for power factor; do not multiply by PF again when using nameplate watts.
Assumptions
- Single-phase AC or DC circuit.
- Power factor applies to AC circuits; use PF = 1.0 for DC and pure resistive AC loads.
- Results are planning estimates only, verify against nameplate ratings and applicable code.
Worked example
Example: 120 V × 15 A × 1.0 PF = 1,800 W real power.
FAQ
What is the formula for single-phase power?
Real power in watts = Voltage × Current × Power Factor (P = V × I × PF). For resistive loads with PF = 1.0, this simplifies to P = V × I. Apparent power in VA = Voltage × Current (S = V × I).
What is the difference between watts and VA?
Watts measure real (usable) power, what the appliance converts to heat or motion. VA (volt-amperes) measure apparent power, what the source must supply including the reactive component. They are equal only when PF = 1.0. For motors and electronics, VA is always higher than watts.
What power factor should I use for a space heater?
Use PF = 1.0 for pure resistive loads like electric space heaters, baseboard heaters, toasters, and incandescent light bulbs. Their current and voltage waveforms are perfectly in phase.
Does power factor matter for residential circuits?
For most residential planning, PF = 1.0 is a safe conservative assumption. Only adjust PF when sizing for known inductive loads, motors, HVAC compressors, or electronic ballasts, where the nameplate provides a PF value.
Is this calculator for AC or DC circuits?
It works for both. For DC circuits, set PF = 1.0 since power factor is an AC concept. The real power formula P = V × I applies directly to DC systems.
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⚠️ Sanity Check Only
This tool provides informational estimates only. It is not professional engineering advice. Electrical work is dangerous and governed by strict local codes.
Before you start:
- Verify these results with a licensed electrician.
- Cross-reference with the latest Electrical Code (NEC/CEC).
- Never work on live circuits.