Real Power Calculator

Step-by-Step Guide

1. 1Enter Voltage: Input the supply voltage in volts (V) applied to the load.
2. 2Enter Current: Input the current drawn by the load in amps (A).
3. 3Enter Power Factor: Input the load's power factor as a decimal between 0 and 1 — 1.0 for purely resistive loads.
4. 4Read the Real Power Result: View real power in watts, calculated instantly using P = V × I × PF.
5. 5Review Apparent and Reactive Power: See apparent power (S = V × I) and reactive power (Q = √(S² - P²)) from the same inputs.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Real power (P), apparent power (S), reactive power (Q)
• ✓Automatic efficiency rating based on power factor
• ✓Full step-by-step derivation
• ✓Six built-in presets (residential, industrial, resistive, more)
• ✓Calculation history
• ✓Export calculation as a text file
• ✓No signup required

Pro Tips

• 💡Use a load's rated or nameplate power factor when available, rather than assuming 1.0 by default — most inductive loads (motors, ballasts, transformers) have a PF well below 1.0, and assuming unity power factor significantly overstates real power.
• 💡For three-phase motors, remember this calculator computes single-phase real power (P = V × I × PF) — multiply by √3 for the total three-phase real power when all three phases carry equal current.
• 💡Cross-check calculated real power against a wattmeter reading when available. If they disagree significantly, either the assumed power factor is wrong or the voltage/current readings don't reflect actual operating conditions.
• 💡Use this calculator's apparent power output for cable and breaker sizing decisions, not real power — protection devices need to handle the full current the load draws, which corresponds to apparent power, not just the useful portion.
• 💡When comparing equipment options, remember that a lower power factor at the same wattage rating means higher current draw — two 1,000W loads with different power factors will draw noticeably different currents from the supply.

Common Mistakes to Avoid

• ✕Using P = V × I without the power factor term. This calculates apparent power, not real power, and overstates actual energy consumption for any load with a power factor below 1.0.
• ✕Assuming all loads are purely resistive (PF = 1.0) by default. Motors, fluorescent lighting, and switching power supplies all have power factors well below 1.0, sometimes as low as 0.5-0.7.
• ✕Forgetting the √3 factor when scaling single-phase real power calculations to three-phase systems. This omission underestimates total three-phase real power by roughly 42%.
• ✕Confusing real power (watts, what you're billed for on most residential meters) with apparent power (volt-amps, what determines actual current draw and equipment sizing needs).
• ✕Using a generic 'typical' power factor value instead of the specific equipment's rated or measured PF for critical calculations. Power factor varies significantly between equipment types and even between units of the same equipment type.