Emergency Generator Calculator

Size emergency generators with altitude and temperature derating, fuel consumption, and runtime estimates.

Inputs

Total Critical Load

Largest Motor Starting Load

Power Factor

NFPA 110 / IEEE Std 446: standard generator 0.8 PF rating; kW = kVA × 0.8; size in kW not kVA

Site Altitude

ISO 8528-1: no derating ≤1,000 m; ~3–4% per 300 m above 1,000 m; at 2,000 m ~10–14% derating

Maximum Ambient Temperature

°F

Tab to move between fields

Results

Required Generator Size

300 kW

≈ 20 homes' peak draw

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.

How the result changes with Total Critical Load

Total Critical Load	Required Generator Size	Required KVA
1,001	1,261.2 kW	1,576.5 kVA
3,501	4,261.2 kW	5,326.5 kVA
6,500	7,860 kW	9,825 kVA
9,000	10,860 kW	13,575 kVA

What each input means

Total Critical Load: Sum of all critical loads that must be powered during an outage.
Largest Motor Starting Load: Additional KW demand from the largest motor starting (typically 3× running KW).
Power Factor: Generator rated power factor per NFPA 110 and IEEE Std 446 (Orange Book). Standard emergency generators are rated at 0.8 PF per NFPA 110. kW = kVA × PF; a 0.8 PF generator produces more kVA than kW nameplate.
Site Altitude: Elevation above sea level. Generator capacity decreases with altitude due to reduced air density per ISO 8528-1. Derating: ~3–4% per 300 m (1,000 ft) above 1,000 m (3,281 ft); consult manufacturer for exact correction factors.
Maximum Ambient Temperature: Highest expected ambient temperature. Derating starts above 40°C.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Total Critical Load = 200, Largest Motor Starting Load = 50, Power Factor = 0.8, Site Altitude = 200 = 5 input(s) provided
Calculate Required Generator Size
Required Generator Size
300 = 300
Calculate Required KVA
Required KVA
375 = 375
Calculate Derated Capacity
Derated Capacity
300 = 300
Calculate Fuel Consumption
Fuel Consumption
60.75 = 60.75

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