Generator Sizing Calculator
Estimate the kW generator size needed for your load list.
| Total Running Load | 5.00 kW |
|---|---|
| Total Running kVA | 5.00 kVA |
| Required kW (with safety factor) | 6.25 kW |
| Peak Starting Surge | 5.00 kVA |
| Recommended Standard Size | 7.5 kW |
· Total running load: 5.00 kW / 5.00 kVA.
· With 1.25x safety factor: 6.25 kW.
· Peak starting surge (largest single load): 5.00 kVA.
· Generator must also handle starting surge. Consult generator manufacturer for transient performance.
· This estimate does not account for derating for altitude, ambient temperature, or fuel type.
How It Works
Generator sizing involves two checks: steady-state running load and starting surge. Running load is the sum of all connected loads, multiplied by a safety factor (commonly 1.25) to allow for growth and inrush. Each load's kVA is calculated as kW divided by its power factor. Starting surge is the short burst of current a motor or compressor draws on startup, often 3 to 6 times its running current. The generator must be able to handle the largest single starting load without the voltage collapsing. This calculator gives an estimate. Real generator sizing projects also account for altitude, ambient temperature, generator subtransient reactance, and automatic transfer switch sizing.
Example
10 kW at 0.8 PF = 12.5 kVA running. With 1.25 safety factor = 12.5 kW required. Starting surge = 12.5 kVA x 3 = 37.5 kVA peak. Recommended size: 15 kW.
Frequently Asked Questions
What is a typical starting surge for a motor?
Most induction motors draw 3 to 6 times their running current during startup. Soft starters and VFDs reduce this significantly.
Should I include the A/C in my load list?
Yes, always. Air conditioners have compressor motors with significant starting surge and are often the largest individual load in a residential or light commercial system.
Does this account for altitude?
No. Generator output decreases at higher altitudes due to thinner air. Derate approximately 3.5% per 1,000 feet above 3,300 feet elevation.