A single 500 kW motor starting can require a generator nearly 2.5 times its running load, yet many preliminary sizing models ignore this fact. The Generator Sizing Calculator uses a fixed screening model that multiplies connected load by a sizing factor to estimate capacity. While simple, understanding the math behind it is essential for early-stage planning.
The Formula
The screening model is straightforward:
Generator Size (kW) = Connected Load (kW) × Sizing Factor
If the connected load is given in MW, first convert:
Connected Load (kW) = Connected Load (MW) × 1000
Generator Size (kW) = Connected Load (kW) × Sizing Factor
Connected Load is the total running electrical load in kW. It represents the steady-state demand that the generator must supply continuously. This is not the same as the nameplate rating of all equipment; it's the load actually running at a given time. The Sizing Factor is an allowance multiplier (dimensionless, typically > 1) that accounts for future growth, voltage drop, and non-linear loads. It is not a safety factor for starting transients—that's a common misconception. The generator size output is in kW, which for preliminary screening is assumed to be the prime power rating. The formula is intentionally linear: doubling the load doubles the generator size, making it easy to scale but also easy to misapply.
Worked Example 1: Industrial Facility
A small industrial facility has a connected running load of 250 kW. The sizing factor chosen is 1.25 to allow for a future expansion.
Generator Size = 250 kW × 1.25 = 312.5 kW
The calculated generator size is 312.5 kW. According to the Result Intelligence System, this falls into the STANDARD category (typically 100–500 kW). The engineer would then check motor starting requirements, which might push the selection to a larger unit.
Worked Example 2: Large Commercial Building
A commercial building has a connected load of 1.2 MW. Convert to kW: 1.2 MW × 1000 = 1200 kW. Using a sizing factor of 1.15 (conservative for commercial):
Generator Size = 1200 kW × 1.15 = 1380 kW
This 1380 kW generator is classified as LARGE (500–2000 kW). The engineer must verify block-load acceptance, as the building may have large HVAC motors starting simultaneously.
What Engineers Often Miss
First, motor starting current is not captured by the sizing factor. A 100 kW motor can require 600 kVA for starting, which may dictate generator size even if running load is low. Second, the sizing factor should vary by application: standby generators can use a lower factor than prime power units because overload capability differs. Third, load sequencing—starting loads in a specific order—can reduce the required generator size, but the screening model assumes all loads run simultaneously. Experienced engineers use this calculator as a quick check, not a final design tool.
Try the Calculator
Estimate your own generator size quickly with the Generator Sizing Calculator to get a preliminary capacity and category classification.
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