Generator Sizing with Motor Starting
Loads
ft
°F
Rating Type
Recommended Generator Size500 kW
Running Load98.0 kW
Running Load (kVA)115.3 kVA
Largest Motor50.0 kW
Starting Load409.4 kVA
Altitude Derating100.0%
Temperature Derating100.0%
Derated Required Size409.4 kW
Fuel Consumption6.86 gal/hr
Sizing a standby or prime generator is rarely just adding up the running kilowatts. The largest motor starting can demand several times its running power for a few seconds, and altitude and heat reduce a genset's usable output. This calculator aggregates your loads, models the worst-case starting surge by starting method, applies altitude and temperature derating, and recommends the next standard generator size.
Formula
kVAstart = kVArunning − kVAlargest_motor + (kVAlargest_motor × Mstart)
- kVArunning
- Sum of running apparent power across all loads (kW ÷ power factor each)
- kVAlargest_motor
- Running apparent power of the single largest motor/HVAC load
- Mstart
- Starting multiplier: 6.0 DOL, 3.0 soft start, 2.0 star-delta, 1.0 VFD
- kVAstart
- Worst-case transient apparent power demand during motor start
How it works
- Add each load with its type, rated kW, power factor, and (for motors) starting method. Demand factors are applied per load type, and running kW and kVA are summed across the list.
- The engine finds the largest motor or HVAC load and replaces its running kVA with its starting kVA using a multiplier for the starting method (6× direct-on-line, 3× soft start, 2× star-delta, 1× VFD), giving the peak transient kVA the generator must hold.
- It takes the larger of the running-kW and starting-kVA requirements, divides by the prime/standby rating factor, then divides again by the combined altitude and temperature derating to pick a standard size and estimate fuel use.
Worked example
A 75 kW direct-on-line motor (PF 0.85) plus 30 kW of lighting (PF 0.9) at 1000 ft elevation and 90 °F, rated as standby.
- Running kW: 75 + (30 × 0.9 demand) = 75 + 27 = 102 kW. Running kVA: 75/0.85 + 27/0.9 ≈ 88.24 + 30 = 118.24 kVA.
- Largest motor running kVA = 75/0.85 ≈ 88.24; starting kVA = 88.24 × 6.0 ≈ 529.4.
- Starting load: 118.24 − 88.24 + 529.4 ≈ 559.41 kVA, which exceeds the 102 kW running need.
- No derating at 1000 ft / 90 °F (factors 1.0), so required size ≈ 559.41 kW.
Required size ≈ 559.41 kW, recommended standard generator 600 kW; estimated fuel ≈ 7.14 gal/hr at the running load.
Frequently asked questions
- Why is starting kVA usually the deciding factor?
- A direct-on-line motor draws roughly six times its running current at startup, which causes a large momentary kVA demand and voltage dip. A generator must hold that transient without stalling or dropping voltage too far, so the starting kVA often dwarfs the steady running kW.
- What is the difference between standby and prime ratings?
- A standby rating is for emergency use during a utility outage and can use close to 100% of nameplate. A prime rating is for continuous operation as the main power source and is derated (here to 90%) for sustained duty, so a prime application needs a larger machine for the same load.
- How do altitude and temperature affect generator output?
- Thinner air at altitude and hotter ambient air both reduce the engine's ability to make and cool power. This tool derates roughly 3.5% per 1000 ft above 3300 ft and 2% per 10 °F above 104 °F, so high or hot sites need a larger generator.
- Does using a VFD or soft starter let me use a smaller generator?
- Yes. A VFD ramps a motor up with essentially no inrush (1× multiplier) and a soft starter cuts it to about 3×, versus 6× for direct-on-line. Reducing the starting surge can substantially shrink the required generator size for motor-dominated loads.