Generator Sizing Calculator
Estimate the minimum generator size needed to power your electrical loads, accounting for power factor and a recommended safety margin.
Sum of all appliance/equipment wattages running simultaneously.
Multiplier for motor inrush current (typically 1.25–2.0; use 1.0 if no motors).
Typical value: 0.8 for mixed loads; 1.0 for purely resistive loads.
Recommended 20–25% headroom above calculated load.
Single-phase: 120 or 240 V; Three-phase: 208, 480 V, etc.
Formulas Used
1. Adjusted Load (W)
Adjusted Load = Connected Load (W) × Motor Starting Factor
2. Base Apparent Power (kVA)
kVAbase = Adjusted Load (W) ÷ (Power Factor × 1000)
3. Required kVA with Safety Margin
kVArequired = kVAbase × (1 + Safety Margin ÷ 100)
4. Required Real Power (kW)
kW = kVArequired × Power Factor
5. Full-Load Current
Single-phase: I = (kVA × 1000) ÷ V
Three-phase: I = (kVA × 1000) ÷ (√3 × V)
Assumptions & References
- A power factor of 0.8 is the industry standard for mixed resistive/inductive loads (IEEE Std 446).
- A 20–25% safety margin is recommended by NFPA 110 and most generator manufacturers to prevent overloading.
- The motor starting factor accounts for inrush current, which can be 1.5–3× running current for motors (NEC Article 430).
- Standard generator sizes follow common commercial/industrial ratings; actual availability varies by manufacturer.
- This calculator provides an estimate only. Actual sizing should account for load growth, altitude derating, temperature derating, and harmonic loads.
- References: NFPA 110 (Emergency Power Systems), IEEE Std 446 (Recommended Practice for Emergency Power), NEC Article 445 (Generators).