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Generator Size Calculator

Generator Capacity Calculator

Calculate the required generator size based on continuous loads, non-continuous loads, and motor starting surges using standard electrical sizing practices.

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Enter Load Details

kW

Loads operating for 3+ hours (requires 125% sizing).

kW

Intermittent loads (sized at 100%).

kW

Additional surge kW required by the largest motor.

Usually 0.8 for 3-phase, 1.0 for single-phase.

%

Recommended: 20% to 25% for future expansion.

How It's Calculated

This calculator follows National Electrical Code (NEC) sizing guidelines for establishing a safe baseline for generator capacity.

1. Required Base kW

Base kW = (Continuous kW × 1.25) + Non-Continuous kW + Motor Surge kW

2. Adding Safety Margin

Total kW = Base kW × (1 + (Margin % / 100))

A 20-25% margin ensures the generator isn't running at 100% load, extending its lifespan and accommodating future additions.

3. Converting to kVA

kVA = Total kW / Power Factor

Generators are often rated in kVA. Three-phase systems typically use a 0.8 Power Factor (PF).

Common Generator Capacity Chart

A quick reference chart for common generator kVA ratings and their equivalent kW based on power factor.

Generator Rating (kVA) 1-Phase kW (PF=1.0) 3-Phase kW (PF=0.8)
10 kVA 10 kW 8 kW
25 kVA 25 kW 20 kW
50 kVA 50 kW 40 kW
100 kVA 100 kW 80 kW
250 kVA 250 kW 200 kW
500 kVA 500 kW 400 kW
1000 kVA 1000 kW 800 kW