About the Electrical Power Calculator
The Electrical Power Calculator combines five common electrical math problems into one tabbed reference: Power (P = V·I or P = V·I·PF·√3 three-phase), kW↔kVA conversion via power factor, kW↔HP conversion (1 HP ≈ 0.746 kW mechanical), motor-starting sizing with inrush multipliers, and amperage from watts. A wire-gauge cross-reference (AWG, NEC ampacity at 75°C) sits alongside the amperage output.
It is built for electricians sizing generators against motor inrush, HVAC techs sanity-checking nameplate ratings, RV and tiny-home owners planning their power budget, generator-shopping homeowners who keep getting conflicting watt numbers, and electrical-engineering students checking textbook problems against a calculator that shows the formula it used.
All calculations run locally in JavaScript. Voltage, current, power-factor, and load inputs never leave your device. The page makes no network call after first load. AWG ampacity tables (NEC 310.16 75°C column) are bundled into the JavaScript on first load.
This tool is for sizing and planning, not for design of code-compliant installations. The National Electrical Code (NEC, NFPA 70) governs U.S. residential and commercial electrical work, and most jurisdictions require a permit and licensed electrician for any new branch circuit or service change. Wire-gauge selection has additional considerations — ambient temperature derating, voltage drop over distance, parallel-conductor rules — that this calculator does not model.
Add multiple loads to build a complete electrical load schedule for panel design.
Enter wire gauge, run length, and load to check voltage drop against NEC 3%/5% limits.
Recommends breaker size based on load with 80% continuous load derating per NEC.
How to Use the Electrical Power Calculator
This calculator offers five distinct modes accessible from the tabs at the top. In Power mode, enter voltage, current, and power factor to find real power in watts and kilowatts. Toggle between single-phase and three-phase to see how the formula changes. The kW/kVA and kW/HP converters handle bidirectional conversions with a single click. Use Motor Starting to size a generator for motor inrush current, and Amperage to find amps from kVA and voltage. Each mode displays the formula being used so you can verify or learn the math behind the result.
Understanding Power Factor
Power factor is the ratio of real power (watts) to apparent power (volt-amps). A purely resistive load like a heater has a power factor of 1.0 — all the power drawn is converted to useful work. Motors, transformers, and fluorescent lighting are inductive loads with power factors typically between 0.7 and 0.9, meaning some of the current drawn does no useful work but still flows through wiring and transformers. This matters because your wiring, breakers, and generator must be sized for apparent power (kVA), not just real power (kW). A 10 kW load at 0.8 power factor actually demands 12.5 kVA from your supply. Utilities may penalize commercial customers with low power factor, so capacitor banks are often installed to bring PF closer to unity.
Single Phase vs Three Phase
Single-phase power uses two wires (one hot, one neutral) and is standard in residential settings. In the United States, most homes receive split-phase 240V service, which is technically two 120V phases 180 degrees apart. Three-phase power uses three hot conductors with voltages 120 degrees apart and is the backbone of commercial and industrial electrical systems. The key advantage of three-phase is that it delivers more power with less copper — a three-phase system delivers 1.732 times the power of a single phase at the same voltage and current. That is where the square root of three (1.732) factor appears in three-phase formulas. Common three-phase voltages include 208V, 480V, and 600V. If you are connecting a motor or large piece of equipment, check the nameplate to determine whether it requires single-phase or three-phase supply.
Generator Sizing for Motors
Electric motors draw 5 to 7 times their rated running current during startup. This inrush current lasts only a few seconds but can stall or trip a generator that is too small. The starting method dramatically affects how much inrush the generator must handle. Direct On-Line (DOL) starting applies full voltage instantly and creates the highest inrush — roughly 7 times the motor kW in generator kVA. Star-Delta starters reduce inrush to about 2.5 times by starting the motor at reduced voltage. Soft starters ramp up voltage gradually and typically need about 1.8 times. VFDs (Variable Frequency Drives) control both voltage and frequency and only require about 1.5 times, making them the most generator-friendly option. Always confirm sizing with the generator manufacturer and account for other simultaneous loads.
Voltage Drop Basics
Every wire has resistance, and as current flows through it, some voltage is lost as heat. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits combined. Excessive voltage drop causes motors to run hot and inefficiently, lights to dim, and sensitive electronics to malfunction. Voltage drop increases with wire length and load current, and decreases with larger wire gauge. For long runs — such as feeding a detached garage or barn — you may need to upsize your wire significantly. The premium voltage drop calculator lets you verify your wire selection before you pull a single foot of cable.
Electrical Safety Considerations
Working with electricity is inherently dangerous and should only be performed by qualified individuals. Always de-energize circuits before working on them and verify with a meter that power is off. The NEC requires overcurrent protection (breakers or fuses) sized to protect the conductor, not the load. A 20-amp breaker protects 12 AWG wire from overheating, regardless of what is plugged into the circuit. For continuous loads — those expected to run for three hours or more — the NEC requires the breaker to be rated at 125% of the load, which effectively means the load cannot exceed 80% of the breaker rating. This derating is one of the most commonly overlooked requirements in residential work. When in doubt, consult a licensed electrician who understands your local codes and conditions.
Looking for related tools? Try our Electricity Cost Calculator to see how much power costs you, or our Unit Converter for general measurement conversions. Explore all Everyday Calculator tools.
Frequently Asked Questions
What is the difference between watts (kW) and volt-amps (kVA)?
Watts measure real power, the energy actually doing useful work. Volt-amps measure apparent power, which includes reactive power that flows back and forth without performing work. Real power equals apparent power multiplied by the power factor, so a 10 kVA load at 0.8 PF is only 8 kW of real power.
How do I convert horsepower to kilowatts?
One mechanical horsepower equals about 0.746 kilowatts, so a 5 HP motor draws roughly 3.73 kW of real power at full load. The reverse conversion is 1 kW is about 1.341 HP. This calculator handles the conversion in either direction with one click.
Why does motor starting need extra generator capacity?
Induction motors pull 3 to 7 times their running current for a fraction of a second when they first spin up. A generator must be sized to handle that inrush plus everything already running, or the voltage will sag and drop other loads offline.
How do I calculate amps from watts and volts?
For single-phase, amps equal watts divided by (volts times power factor). For three-phase, divide by (volts times 1.732 times power factor). At 240V single-phase with PF 1.0, a 2,400W load draws 10 amps.
What is a typical power factor for a home?
Residential power factor usually averages between 0.85 and 0.95. Homes with lots of motor-driven appliances, older fluorescent fixtures, or large HVAC systems trend toward the lower end. Utilities bill residential customers by kWh, so power factor rarely affects the bill directly, but it does matter for generator and panel sizing.