Generator Size Calculator
Estimate generator capacity from appliance loads or a known industrial load, then convert between kW, kVA, amps, and horsepower.
Calculator
Quick: build a load list in watts. Advanced: enter a known load in kW and its power factor.
Appliance loads
Enter running watts and starting watts per unit when known. Startup demand adds the extra surge above running watts for every listed load.
Use a known facility load or demand-study total.
Use equipment or design data when available; 0.8 is a planning assumption, not a universal value.
Enter the additional surge above running load, not the total starting watts.
Calculation Results
Planning estimate only. Actual sizing depends on motor starting characteristics, load sequencing, altitude, ambient temperature, duty cycle, and manufacturer ratings. Confirm critical loads with a qualified electrician or generator supplier.
Live Power Conversions
Results update as inputs change. Three-phase calculations use √3 (approximately 1.732).
kVA to Amps
A = kVA × 1000 ÷ (V × √3)Amps to kW
kW = √3 × V × A × PF ÷ 1000kW to Amps
A = kW × 1000 ÷ (√3 × V × PF)HP to kW
kW = HP × 0.746 ÷ efficiencykW to kVA
kVA = kW ÷ PFkVA to kW
kW = kVA × PFkW to HP
HP = kW × 1000 × efficiency ÷ 746Formula notes: kVA is apparent power; kW is real power. Three-phase voltage is line-to-line. 1 HP ≈ 746 W. Use actual equipment data for engineering design.
How to use the calculator
- Choose Quick to list appliances, or Advanced if you already know the facility load in kW.
- In Quick mode, enter each load's quantity and running watts. For motor-driven equipment, enter starting watts from its label or manufacturer data when available.
- Set the safety margin, system voltage, phase, and generator power factor. In Advanced mode, also enter the load power factor and any additional starting surge above running load.
- Review the estimated running load, peak demand, capacity with margin, recommended kW/kVA, and current. Use the converters below for separate electrical conversions.
Assumptions and limitations
Quick mode estimates starting demand by adding the extra surge (starting watts minus running watts) for every listed unit, which assumes those loads may start together. This can overestimate demand when motors start sequentially, but it may still miss complex motor-starting behavior or manufacturer-specific transient requirements. Advanced mode treats the entered surge as additional kW above the stated running load. The selected safety margin is a planning allowance, not a substitute for starting-current data or load analysis. kVA is calculated from kW using the entered generator power factor; current is estimated from kVA, voltage, and phase. Three-phase voltage is interpreted as line-to-line voltage. Altitude, ambient temperature, duty cycle, load type, harmonics, and generator derating can affect the actual unit selection. Confirm the final size and ratings with the generator manufacturer or a qualified electrical professional. This calculator provides a planning estimate and does not replace professional electrical design.
