Generator Size Calculator

How to Choose the Right Generator Size

Selecting an undersized generator causes voltage sags, tripped breakers, and potential damage to motors and electronics. An oversized generator wastes fuel and capital. This calculator finds the minimum kVA/kW rating for your load.

Key Concepts

• Running (continuous) wattage: Power a load consumes during normal operation.
• Starting (surge) wattage: Motors and compressors draw 2–6× their running power when starting. This peak lasts milliseconds but the generator must supply it.
• Power factor: Generators are rated in kVA. Actual usable power = kVA × power factor (typically 0.8 for general use).

The Formula

Required kVA = Total Running Watts + Largest Motor Starting Watts / 1000 / Power Factor

How to Use This Calculator

List all loads, enter running watts and any motor starting surge requirements. The calculator totals running and starting demands and recommends a generator rating with a 20–25% safety margin.

Practical Examples

Example: Home with air conditioner (3,500W running / 7,000W start), refrigerator (150W running / 900W start), and lights (500W). Peak demand = 3,500 + 500 + 150 + 7,000 (start surge) = 11,150W → 15 kVA generator recommended.

Generator Selection Tips

• Add 20–25% to the calculated requirement for future loads and efficiency degradation.
• Inverter generators provide cleaner power (lower THD) for sensitive electronics.
• Diesel generators are more fuel-efficient for loads above 5kW.