Emergency Generator Calculator
Size emergency generators with altitude and temperature derating, fuel consumption, and runtime estimates.
Inputs
kW
kW
NFPA 110 / IEEE Std 446: standard generator 0.8 PF rating; kW = kVA × 0.8; size in kW not kVA
ft
ISO 8528-1: no derating ≤1,000 m; ~3–4% per 300 m above 1,000 m; at 2,000 m ~10–14% derating
°F
Results
Required Generator Size
300 kW
Required KVA
375 kVA
Derated Capacity300 kW
Fuel Consumption (75% load)60.75 L/hr
Runtime (500L tank)8.2 hours
How to Use This Calculator
- Enter the Total Critical Load in kW — sum all loads that must remain on during a power outage.
- Enter the Largest Motor Starting Load in kW — motor inrush is typically 3× running kW and determines the generator's transient kVA requirement.
- Set the Power Factor — standard standby generators are rated at 0.8 PF.
- Enter the Site Altitude in meters above sea level — derating begins above 1,000 m at approximately 3.5% per 300 m.
- Enter the Maximum Ambient Temperature in °C — derating begins above 40°C.
- Read the Required Generator Size in kW and kVA, then check Fuel Consumption (L/hr) and estimated Runtime from a 500L tank to plan fuel logistics.
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