Generator Sizing Calculator
Plan your electrical engineering project with our free sizing calculator. Get precise measurements, material lists, and budgets.
Reviewed by Abdullah, Technical Content Specialist
Formula
Generator kVA = (Total Running Watts + Largest Starting Surge) x (1 + Safety Margin%) / (Power Factor x 1000)
Generator size is determined by adding the total continuous running load to the largest single starting surge, then applying a safety margin and dividing by the power factor to convert from kW to kVA. The starting surge is the difference between a motor starting wattage and its running wattage.
Worked Examples
Example 1: Residential Backup Generator
Problem:Size a generator for a home with 5 kW HVAC (3x start), 2 kW water heater, 1.5 kW kitchen appliances, and 1 kW lighting.
Solution:Running total: 5 + 2 + 1.5 + 1 = 9.5 kW\nHVAC starting surge: 5 x 3 = 15 kW (surge = 10 kW)\nPeak demand: 9.5 + 10 = 19.5 kW\nWith 25% margin: 19.5 x 1.25 = 24.38 kW\nAt PF 0.8: 24.38 / 0.8 = 30.5 kVA
Result:Recommended: 25 kW / 31.25 kVA generator (select 30 kW standard size)
Example 2: Small Commercial Office
Problem:Size a generator for 15 kW HVAC (3x start), 5 kW lighting, 8 kW computers/servers, 3 kW elevator (4x start).
Solution:Running total: 15 + 5 + 8 + 3 = 31 kW\nHVAC surge: 15 x 3 - 15 = 30 kW, Elevator surge: 3 x 4 - 3 = 9 kW\nLargest surge: 30 kW\nPeak: 31 + 30 = 61 kW\nWith 25% margin: 61 x 1.25 = 76.25 kW
Result:Recommended: 80 kW / 95.3 kVA generator
Frequently Asked Questions
How do I determine what size generator I need?
To determine generator size, you need to add up the running wattage of all equipment that will operate simultaneously, then account for the highest starting surge among motor loads. Motors typically require 2 to 6 times their running wattage to start. Add a safety margin of 20-25% to the peak demand to determine the minimum generator size. This ensures the generator can handle unexpected load additions and operates efficiently without being overloaded. Always round up to the next standard generator size available from manufacturers.
What safety margin should I add when sizing a generator?
A safety margin of 20-25% above the calculated peak demand is recommended for most applications. This margin accounts for future load growth, load calculation uncertainties, and degradation of generator output over time due to altitude, temperature, and aging. Operating a generator at full rated capacity continuously reduces its lifespan and leaves no room for transient overloads. For critical applications like hospitals or data centers, a higher margin of 30-35% may be appropriate to ensure reliable power under all conditions.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy