How the calculation works
Generator ratings. Generator sets are commonly rated at a power factor of 0.8 (the ISO 8528-1 convention; see the kVA to kW calculator), so kVA = kW ÷ 0.8. Current = kVA × 1,000 ÷ voltage (÷ √3 more for three-phase, line-to-line voltage). 1 hp = 0.7457 kW; that is a unit conversion, and the engine driving a generator needs more horsepower than kW ÷ 0.7457 because of alternator and cooling-fan losses, so take the engine rating from the data sheet.
Loads. Each load row gives running watts and, for a load with an inrush, starting watts. Its kVA = watts ÷ the load power factor.
Motors. For each motor:
- Running kW = hp × 0.7457 ÷ 0.9 and running kVA = running kW ÷ 0.85, assuming a full-load efficiency of 90% and a power factor of 0.85; your motor’s nameplate may differ.
- Across-the-line: starting kVA = hp × the top of the code letter’s range in NEC Table 430.7(B) (code G: 6.29 kVA/hp). For sizing, the motor counts at 1 kW per hp, the lower end of the rule of thumb of 1 kW of generator per 3/4 to 1 hp given by L. Bey (Cummins Onan) in Sizing Gen-Sets for Large Motor Starting, EC&M.
- Soft starter: starting kVA = running kVA × the current-limit setting, capped at the across-the-line figure. The same article gives settings of 150% to 600% of full-load current and says a 300% setting cuts starting kVA by 50%.
- VFD: no starting step; the drive counts at 1.4 × its running kW (PWM or filtered) or 2 × (six-pulse), the generator sizing factors in the same article.
Size.
- Running kW and kVA = the sum of every load and motor as counted above
- Generator kW = the larger of running kW × (1 + headroom) and running kVA × (1 + headroom) × 0.8; generator kVA = kW ÷ 0.8
- Motor-starting kVA = running kVA + the largest starting step: for each row, quantity × (starting kVA − running kVA). The units in one row are assumed to start together, as they do when they all restart after an outage, and different rows one at a time. If a transfer switch with load sequencing or staggered starts brings units on separately, put them in separate rows
The motor-starting kVA is compared with the generator set’s data-sheet figure, not added to its rating: a set can usually start more kVA than its continuous rating for a few seconds, and how much depends on its alternator and the voltage dip allowed. Bey’s article targets a starting dip of roughly 20% to 40%.
Fuel. Runtime = usable fuel ÷ consumption per hour, in the same unit, with the consumption from the data sheet at your expected load. No fuel curves are built in.
Limits. This is a planning estimate. Final sizing belongs with the generator maker’s sizing software and your electrical engineer, who will account for altitude and temperature derating, nonlinear loads, step loading through the transfer switch and local code.
Worked example
A small data room on a 480 V three-phase standby generator: a UPS drawing 40,000 W from its input, 6,000 W of lighting and controls, two 15 hp code G cooling compressors started across the line, and a 10 hp pump on a PWM drive. Load power factor 0.8, 20% headroom:
| Load | Counted for sizing | Running kVA |
|---|---|---|
| UPS and lighting: 46,000 W | 46 kW | 57.5 kVA |
| 2 × 15 hp across-the-line (12.43 kW running each) | 2 × 15 kW = 30 kW | 2 × 14.62 = 29.24 kVA |
| 10 hp on a VFD (8.29 kW running) | × 1.4 = 11.6 kW | 9.75 × 1.4 = 13.65 kVA |
| Total | 87.6 kW | 100.39 kVA |
- By kW: 87.6 × 1.2 = 105.12 kW. By kVA: 100.39 × 1.2 × 0.8 = 96.37 kW. The kW figure sets the size.
- Generator: 105.12 kW / 131.4 kVA, drawing 158.05 A per phase at 480 V at full rating.
- Largest start: the two compressors restarting together, each 15 × 6.29 = 94.35 kVA, a step of 2 × (94.35 − 14.62) = 159.46 kVA. Motor-starting kVA needed: 100.39 + 159.46 = 259.85 kVA. A candidate set listing 300 kVA of motor starting at the allowed dip covers it. With the compressors on staggered starts (two rows), the step halves to 79.73 kVA and 180.12 kVA is needed.
- With 500 gallons usable and 7.5 gallons an hour at the expected load (example figures from a data sheet), the tank lasts about 66.7 hours.
Choosing the set
The next standard size up from the result is the usual pick. Match the rating class to how the set will run: standby ratings apply to emergency use for the length of an outage, and a set that will run often or for long periods needs a prime rating. The UPS sizing calculator covers the ride-through until the generator takes the load, and the server room cooling calculator sizes the cooling the generator has to keep running.
