Emergency Generator Calculator
Size emergency generators with altitude and temperature derating, fuel consumption, and runtime estimates.
About this calculator
Sizing an emergency or standby generator means starting from the load it must carry and then correcting for site conditions that reduce a genset's real output. This calculator sums your Total Critical Load with the largest Motor Starting Load (motor inrush typically runs about 3x running kW, and a generator must survive that transient without collapsing voltage) to get a base kW requirement, then adds a flat 20% sizing margin. That margin is then divided by two multiplicative derating factors that model how a diesel or gas engine loses capacity in the real world, per ISO 8528-1: an altitude derating of roughly 3.5% per 300m above 1,000m elevation (floored at 60% capacity), and a temperature derating of about 2% per 5°C above a 40°C ambient (floored at 70%).
The result is the Required Generator Size in kW; dividing by the standard 0.8 power factor NFPA 110 assigns to standby generators gives the Required KVA nameplate rating you'll actually shop for. Fuel consumption is estimated at a rule-of-thumb 0.27 liters per kWh for a diesel engine running at 75% load — real consumption curves are non-linear and manufacturer data sheets should be consulted for a final fuel plan — and Runtime is simply how long a 500-liter day tank lasts at that burn rate. Because the floors on both derating factors cap worst-case capacity loss, this calculator will understate required size for truly extreme combinations of altitude and heat; always cross-check the result against the manufacturer's derating tables for the specific model being purchased.
Inputs
NFPA 110 / IEEE Std 446: standard generator 0.8 PF rating; kW = kVA × 0.8; size in kW not kVA
ISO 8528-1: no derating ≤1,000 m; ~3–4% per 300 m above 1,000 m; at 2,000 m ~10–14% derating
Results
Required Generator Size
300 kW
≈ 20 homes' peak draw
Required KVA
375 kVA
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 |
|---|---|---|
| 100 | 180 kW | 225 kVA |
| 150 | 240 kW | 300 kVA |
| 300 | 420 kW | 525 kVA |
| 500 | 660 kW | 825 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 Parameters4 parametersTotal Critical Load = 200, Largest Motor Starting Load = 50, Power Factor = 0.8, Site Altitude = 200 = 5 input(s) provided
- Calculate Required Generator SizeRequired Generator Size300 = 300
- Calculate Required KVARequired KVA375 = 375
- Calculate Derated CapacityDerated Capacity300 = 300
- Calculate Fuel ConsumptionFuel Consumption60.75 = 60.75
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is the Required Generator Size larger than my Total Critical Load plus Motor Starting Load?
The calculator adds a flat 20% margin on top of your base kW (critical load plus motor starting load), then divides that by the altitude and temperature derating factors combined. Since derating only ever reduces effective capacity (the factors are 1.0 or less), dividing by them always inflates the required nameplate size above the raw margin-adjusted load — the generator has to be oversized to still deliver the full load after real-world derating is applied.
Why does raising the Site Altitude input change my Required Generator Size but not until above 1,000 m?
The altitude derating formula only activates when altitude exceeds 1,000 m, applying roughly 3.5% capacity loss per 300 m above that threshold, floored at 60% of nameplate capacity no matter how high the site is. Below 1,000 m the altitude derating factor stays at exactly 1.0, so the input has zero effect on the result until you cross that line.
How is the Runtime figure calculated, and what does it assume about my actual fuel tank?
Runtime divides a fixed 500-liter tank size by the estimated Fuel Consumption, which itself assumes a diesel engine running at 75% load and burning about 0.27 liters per kWh of the Required Generator Size. If your actual tank isn't 500 liters, or your generator runs at a different load fraction than 75%, scale the reported Runtime proportionally rather than trusting it as-is.
Why does the Power Factor input change my Required KVA but not my Required KW?
Required KW is derived purely from your critical load, motor starting load, margin, and derating factors — power factor never enters that calculation. Required KVA is simply Required KW divided by the Power Factor, so a lower power factor (further from 1.0) produces a larger KVA nameplate rating for the same kW requirement, which is why generators are typically shopped for by KVA rather than KW.
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