Generator (DG) Size Calculator
Generator (DG) Size Calculator
Add up to six loads with their power factors and starting multiples and get the running kW and kVA, the worst-case starting demand, the set size to buy with your own margin, the running and starting currents, and the fuel it will burn.
DG set size from a load list
Ten 60 W lights and fans, a 200 W fridge (PF 0.8, 3× start), a 1 hp pump (750 W, PF 0.8, 6×), a 1.5 ton AC (1,500 W, PF 0.9, 4×), 20% margin, 230 V single-phase
Running case and starting case
Srequired = max( Srun × (1 + margin), Sstart ÷ ktransient )
I = S ÷ V (single-phase) or S ÷ (√3 × VLL) (three-phase) fuel = kW × L/kWh
- Pi, ni
- running watts of one unit of load i, and how many of them
- PFi
- power factor of that load. A generator is rated in kVA because its alternator is limited by current, not by the real power it delivers
- mi
- starting kVA of one unit of load i as a multiple of its running kVA. 1 for anything without a motor; 5–7 direct-on-line
- ktransient
- how much starting kVA the set will take briefly, as a multiple of its rating, within an acceptable voltage dip
- only one m
- only the LARGEST single starting excess is added, because only one motor is starting at that instant
Worked example
Ten 60 W lights and fans, a 200 W fridge (PF 0.8, 3× start), a 1 hp pump (750 W, PF 0.8, 6×), a 1.5 ton AC (1,500 W, PF 0.9, 4×), 20% margin, 230 V single-phase
Running: 3,050 W of real power; in kVA, 600 + 250 + 937.5 + 1,666.7 = 3,454.2 VA = 3.45 kVA
Starting excesses, one unit at a time: fridge 250 × 2 = 500 VA; pump 937.5 × 5 = 4,688 VA; AC 1,666.7 × 3 = 5,000 VA
The AC wins, not the pump — a smaller multiple on a bigger load. Worst case = 3,454.2 + 5,000 = 8,454.2 VA = 8.45 kVA
Running case needs 3.45 × 1.2 = 4.14 kVA; starting case needs 8.45 ÷ 2 = 4.23 kVA
The starting case is larger, so take 4.23 kVA and buy the next standard set: 5.0 kVA, which the running load then loads to 69%
Currents at 230 V: 15 A running, 36.8 A for the second the AC starts. Fuel at 3.05 kW and 0.30 L/kWh: 0.915 L/h
Starting multiples: where the numbers come from
| How the motor is started | Starting current, × full load | Starting kVA, × running kVA | Note |
|---|---|---|---|
| Direct on line (DOL) | 6× typical | 5–7 | The default for small motors. Kohler: “starting current is typically six times a motor’s rated full-load current” |
| Star-delta | about 2× | 2–3 | Roughly a third of DOL, at a third of the starting torque |
| Soft starter | 2–4× | 2–4 | Set by the ramp; the figure is whatever you configure |
| Inverter (VFD) | about 1× | 1–1.5 | The drive starts the motor from near zero frequency, so there is barely a starting transient at all |
| Anything without a motor | 1× | 1 | Lamps, heaters, electronics. A transformer or a big LED driver has a brief inrush that this method does not model |
Standard DG set sizes
| kVA | kW at PF 0.8 | Typical use |
|---|---|---|
| 5 | 4.0 | One home: lights, fans, fridge, one AC |
| 7.5 | 6.0 | A larger home or a small shop |
| 10 | 8.0 | Small commercial, two or three ACs |
| 15 | 12.0 | Small office floor |
| 25 | 20.0 | Apartment block essentials |
| 62.5 | 50.0 | Commercial building, lift and pumps |
| 125 | 100.0 | Larger building with full backup |
Sizing a diesel generating set
Two numbers size a generating set, and they are not the same number. The first is the running load: every appliance’s apparent power added up, in kVA rather than kW, because an alternator is limited by the current in its windings and not by the real power that current happens to deliver. The second is the starting case, and it is usually the one that decides.
Why the starting case is the largest motor, not all of them. An induction motor started direct on line draws several times its running current for a second or so while it comes up to speed, at a very poor power factor. The realistic worst moment is one motor starting while everything else in the building is already running — the pump cutting in on a summer evening with the air conditioner already on. Assuming every motor starts at the same instant is not conservative, it is wrong: it never happens, and designing for it buys a set that then spends its life lightly loaded. So this page adds only the single largest starting excess to the running total.
The largest motor is not always the one with the biggest multiple. In the default list the 1 hp pump starts at 6× and the air conditioner at 4×, but the pump’s excess is 4,688 VA against the air conditioner’s 5,000 VA, so the air conditioner sets the size. The chart sweeps that multiple to show how much of the answer hangs on it: fitting a soft starter to the worst offender is often cheaper than the next set size up.
What the set can actually take. A set will supply far more than its rated kVA for a moment, at the cost of a voltage dip. How much depends on the alternator, not the engine, and the manufacturer publishes a motor-starting table for it; the transient-capability box on this page is a stand-in for that table. Most contactors and control relays tolerate a 35% dip, and many specifying engineers hold sets to 20%, which is why an alternator is often oversized relative to the engine on a motor-heavy site.
Loading, fuel and derating. A diesel engine wants to be worked. Below about 30% of rating it runs cool and wet-stacks — unburnt fuel and soot glaze the bores and the exhaust weeps oil. Above 80% there is nothing left for the load you add next year. The default list loads a 5.0 kVA set to 69%, which is a comfortable place to be. Fuel is roughly proportional to the kWh generated: 0.30 L/kWh gives 0.915 L/h at this load and 1.200 L/h at the set’s own rated output. Finally, published ratings are at ISO reference conditions; output falls with altitude and with the temperature of the air the engine breathes, roughly 1% per 100 m above 1,000 m and a few per cent per 10 °C of cooling air above 40 °C — but the only derating worth designing to is the one on your own set’s data sheet.
For a battery inverter or UPS rather than an engine, use the inverter / UPS size calculator, which applies the same starting logic to a different machine; for the kVA and kW arithmetic on its own, the kVA to kW calculator; for a single motor’s full-load current, the motor full load current calculator.
Frequently asked questions
What size generator do I need for a house?
Add every load’s kVA, add the largest motor’s starting excess on top, and take the larger of that and the running load plus your margin. The default list here — lights, fans, a fridge, a 1 hp pump and a 1.5 ton AC — comes to 3.45 kVA running and 8.45 kVA at the worst start, so a 5.0 kVA set.
Why is a generator rated in kVA and not kW?
Because the alternator is limited by the current its windings carry, which is apparent power. The kW figure on the nameplate is the kVA at a power factor of 0.8, the value sets are conventionally rated at. A load at a worse power factor draws more current for the same watts, and the set feels it.
Why does the largest motor starting set the size?
Because it is the moment of greatest demand that actually occurs. A motor started direct on line draws several times its running current at a poor power factor; if the set cannot supply it, the voltage collapses, the motor stalls and every contactor on the board drops out. Assuming all motors start together is not conservative, it is unrealistic.
How much diesel does a generator use per hour?
Roughly the kWh it generates times its specific consumption, usually 0.28–0.33 L/kWh near three-quarters load. 3.05 kW at 0.30 L/kWh is 0.915 L/h. Light loading makes it noticeably worse, which is another reason not to oversize.
Can I run a generator at 20% load?
Not for long. A diesel engine at light load runs below its design temperature, burns fuel incompletely and wet-stacks, glazing the bores with soot and unburnt fuel. Keep it above about 30%, and exercise it under real load if it normally sits idle.
Related calculators
References
- Kohler Power Systems, Sizing generators for motor starting — a practical guide to understanding how motor starting loads affect generator performance: “starting current is typically six times a motor’s rated full-load current”; motor-starting kVA from the NEMA code letter on the nameplate (Code A, 0–3.14 kVA/hp, through Code V, 22.4 and up); and the voltage-dip limits a set is judged against (35% tolerated by most contactors, 20% preferred by many specifiers).
- Jubaili Bros, How do ambient temperature and altitude affect generator performance: engine output reduced about 1% per 100 m above 1,000 m, and alternator output (Leroy-Somer guidance) by 3% per 10 °C of cooling air above 40 °C and 0.4% per 100 m above 1,000 m. Treat these as indicative; the derating that counts is the one on your own set’s data sheet.
- ISO 8528-1, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance: the COP/PRP/LTP/ESP rating definitions and the reference conditions ratings are quoted at. Cited for the rating vocabulary; the standard itself is paywalled and not quoted here.
- Hughes E, revised by Hiley J, Brown K, McKenzie Smith I. Hughes Electrical and Electronic Technology, 12th ed. Pearson, 2016 — three-phase power, apparent vs real power, and induction motor starting current.