Two constraints, not one
A generator set is two machines bolted together, and each one limits the size independently. The engine converts fuel into shaft power and is rated in kilowatts. The alternator turns shaft power into current and is rated in kilovolt-amperes. A load that is heavy in kilowatts stresses the engine; a load that is heavy in current at low power factor stresses the alternator windings. Size for whichever runs out first.
For steady loads the two track each other and the distinction rarely bites. The moment it matters is motor starting. A squirrel-cage induction motor started at full voltage behaves for a second or two like a short circuit with a small impedance: it draws five to eight times its full-load current at a power factor near 0.3. That inrush is almost entirely reactive, so it barely touches the engine — the crankshaft hardly notices — while it collapses the alternator's terminal voltage. Undersize the alternator and lights dim, contactors drop out and the motor may never come up to speed.
So this calculator computes both requirements. The running requirement is your load in kVA plus a reserve margin. The starting requirement is the worst instant of the day — everything else already running, plus the largest motor at locked rotor — divided by the surge capability the set can deliver without an unacceptable voltage dip. The answer is the larger of the two, and the calculator tells you which one is binding.
The starting figure comes from the motor nameplate rather than a rule of thumb. NEMA assigns every motor a code letter defining its locked-rotor kilovolt-amperes per horsepower, and NEC Table 430.7(B) lists the bands.
Working through the terms
Running kVA. Divide the running kilowatts by the power factor. Ten kilowatts at 0.9 power factor is 11.1 kVA, so the alternator must handle 11% more apparent power than the kilowatt figure implies. Resistive loads — heaters, incandescent lamps, ranges — sit at unity power factor, so for them kW and kVA are the same number.
The motor's running contribution. Your running load already contains the largest motor, so you must remove it before adding the inrush, or you will count it twice. Horsepower converts to output kilowatts at 0.746 kW/hp, and the motor's electrical input is that divided by its efficiency: a 5 hp motor at 87% efficiency draws 5 × 0.746 ÷ 0.87 = 4.29 kW of input power.
Locked-rotor demand. Multiply the horsepower by the code-letter kVA per horsepower. A 5 hp motor with code letter H draws between 6.3 and 7.09 kVA/hp — 31.5 to 35.5 kVA at the instant of start, against 4.76 kVA while running, a ratio of 31.5 ÷ 4.76 = 6.6 at the bottom of the band. A soft starter typically halves that; a variable frequency drive ramps the motor up from zero frequency and the inrush essentially disappears, which is why the multiplier for a drive is so small.
Peak demand. Add the inrush to whatever else is already connected. This is a deliberately pessimistic assumption: it says the compressor kicks in while the well pump, the fridge and the furnace are all running. That is the condition a standby set has to survive.
Converting kVA to a kW rating. Three-phase generator sets are conventionally rated at 0.8 power factor, so a 100 kVA alternator carries an 80 kW nameplate. Single-phase sets are rated at unity. The calculator applies that convention and then takes the larger of the resulting kW figure and the engine's own requirement, because a high-power-factor load can exhaust the engine while leaving alternator capacity unused.
Worked example: a house with a 5 hp compressor
A 240 V single-phase standby set has to carry 8 kW of running load at 0.9 power factor. The largest motor is a 5 hp air-conditioning compressor with code letter H at 87% efficiency, started across the line. Code H spans 6.3 to 7.09 kVA/hp; this example takes the bottom of the band, 6.3, and the note under the table below shows what the top of the band costs. The owner wants 25% reserve, and the set's data sheet allows a momentary demand of twice its rated kVA. Step by step:
- Running kVA. 8 ÷ 0.90 = 8.89 kVA.
- Motor input while running. 5 × 0.746 ÷ 0.87 = 4.29 kW, which at 0.90 power factor is 4.76 kVA.
- Everything except that motor. 8.89 − 4.76 = 4.13 kVA.
- Locked-rotor demand. 5 hp × 6.3 kVA/hp × 1.0 = 31.5 kVA.
- Peak demand. 4.13 + 31.5 = 35.63 kVA.
- Alternator for the running load. 8.89 × 1.25 = 11.11 kVA.
- Alternator for starting. 35.63 ÷ 2.0 = 17.81 kVA.
- Take the larger. 17.81 kVA. On a single-phase set rated at unity power factor that is a 17.81 kW minimum, so you buy the next standard size above it from the manufacturer's range.
- Output current. 17,810 ÷ 240 = 74.2 A.
The instructive part is step 6 against step 7. The running load alone would be satisfied by an 11 kVA machine; the compressor start forces 17.81 kVA. Fitting a soft starter to that compressor halves the inrush to 15.75 kVA, dropping the peak to 19.88 kVA and the starting requirement to 9.94 kVA — below the 11.11 kVA the running load needs. At that point the running load becomes the binding constraint and an 11.11 kVA set would serve — the soft starter has taken 6.7 kVA off the requirement, which is very often cheaper than the generator size it saves.
How to read the result
Buy the next standard size up, never down. Generator ratings come in discrete steps a few kilowatts apart, and the calculated figure is a minimum, not a target. Rounding down means running at or above 100% of rating, where the set has no margin for a hot day or a future load.
Check which constraint is binding. If motor starting sets the size, the cheapest fix is usually not a bigger generator but a soft starter on the offending motor, typically an air-conditioning compressor or a submersible well pump. If the running load sets the size, the fix is load management: an automatic load-shed relay that drops the water heater or the second air-conditioner while the compressor starts lets a smaller set serve the same house.
Derate for altitude and temperature. Naturally aspirated engines lose power as air thins. Manufacturers publish derating tables, commonly a few percent per 300 m of altitude above a reference elevation and a further allowance above a reference intake temperature. Use the manufacturer's table for your model rather than a generic figure, and apply the derate to the calculated rating.
Do not run a diesel set lightly loaded for long periods. Diesel engines below roughly 30% of rating suffer wet stacking — unburned fuel glazing the cylinder walls and fouling the exhaust. Oversizing a diesel standby set is a real cost, not just a wasted one. Gaseous and gasoline sets are more tolerant.
Cross-check the load figure itself. If you are sizing a whole-house set, the load should come from a proper calculation rather than a guess: run it through the dwelling load calculation calculator. For individual motors, the motor full-load amps calculator gives the running current the NEC tables assign, and the three-phase power calculator converts between amps, kW and kVA on a three-phase supply.
NEMA code letters and locked-rotor kVA per horsepower
| Code letter | kVA per hp | Starting kVA, 5 hp motor |
|---|---|---|
| A | 0 – 3.14 | 15.7 kVA |
| B | 3.15 – 3.54 | 17.7 kVA |
| C | 3.55 – 3.99 | 20.0 kVA |
| D | 4.0 – 4.49 | 22.5 kVA |
| E | 4.5 – 4.99 | 25.0 kVA |
| F | 5.0 – 5.59 | 28.0 kVA |
| G | 5.6 – 6.29 | 31.5 kVA |
| H | 6.3 – 7.09 | 35.5 kVA |
| J | 7.1 – 7.99 | 40.0 kVA |
| K | 8.0 – 8.99 | 45.0 kVA |
| L | 9.0 – 9.99 | 50.0 kVA |
| M | 10.0 – 11.19 | 56.0 kVA |
| N | 11.2 – 12.49 | 62.5 kVA |
The third column is 5 hp multiplied by the top of each band, rounded to three significant figures. Run the worked example at the top of the H band instead of the bottom and the peak becomes 4.13 + 35.45 = 39.58 kVA and the requirement 19.8 kVA rather than 17.81 kVA — a whole generator size. Hermetic refrigerant compressors are marked with a rated-load current and a locked-rotor current instead of a code letter; multiply the locked-rotor amps by the supply voltage, and by √3 as well on a three-phase supply, then divide by 1,000 to get the starting kVA directly.
Mistakes that produce the wrong size
- Adding every nameplate together. Nameplates are maximums and loads are not all on at once. Use a realistic simultaneous running load, or a demand-factored calculation for a dwelling. Summing nameplates routinely doubles the answer.
- Counting the largest motor twice. If the running load already includes the compressor, you must subtract its running draw before adding its inrush. This calculator does that for you, but hand calculations frequently do not.
- Ignoring power factor on a three-phase set. A 100 kVA three-phase alternator is stamped 80 kW. Sizing on kilowatts alone and buying a matching nameplate leaves the alternator short whenever the load power factor is below 0.8.
- Using a starting multiple instead of the code letter. "Three times running" is a folk rule that happens to fit some motors. The code letter is on the nameplate and is specific to that machine, and the spread from code A to code N is a factor of four.
- Forgetting non-linear loads. Variable frequency drives, LED drivers, UPS rectifiers and switching supplies draw harmonic current that heats the alternator without doing work. Sets serving substantial non-linear load often need a larger alternator or a permanent-magnet exciter, which the manufacturer will specify.
- Sizing the transfer switch to the load instead of the set. The automatic transfer switch and its conductors are sized on the generator's rated output current, which is the last figure this calculator reports.
Which code articles apply
NEC Article 445 covers generators themselves: nameplate requirements, conductor ampacity, overcurrent protection and disconnecting means. Article 700 governs emergency systems, Article 701 legally required standby, and Article 702 optional standby systems — which is where nearly all residential and small-commercial backup sets land. Article 702 permits a system that serves only part of the premises, provided the load is managed so the generator cannot be overloaded, which is the code basis for automatic load-shed relays.
Article 250 and the transfer-switch listing decide whether the generator neutral is bonded, and getting that wrong defeats ground-fault protection. NFPA 110 covers emergency and standby power systems as a whole, including testing intervals and fuel supply. Confirm which NEC edition your jurisdiction has adopted; three-year cycles mean the adopted edition is often two behind the published one.
Where a generator fits against batteries and solar
The size you calculate here is a power figure. It says nothing about how long the set can run, which is a fuel question — a 20 kW set at half load burns a very different amount from the same set at full load, and the generator fuel consumption calculator converts a rating and a load factor into gallons or cubic feet per hour.
That distinction is what separates generators from batteries. A generator is cheap in energy and expensive in power: adding runtime costs only fuel, while adding kilowatts means a bigger machine. A battery bank is the reverse — inverters deliver large surges readily, while extra kilowatt-hours are the expensive part. Systems that combine the two exploit this: the inverter and battery bank handle surges and short outages, and a modest generator recharges the bank at a steady, efficient load rather than following the house demand up and down.
For a hybrid design, size the generator on its charging duty plus the largest simultaneous motor start rather than on the whole house. That is frequently half the standalone figure, because the inverter absorbs the peaks. In an off-grid installation with a well-sized solar array, the generator runs a handful of hours a month in midwinter, so purchase price matters more than fuel efficiency and a smaller set wins on both.
If you are converting between nameplate figures while building the load list, the watts to amps calculator handles single- and three-phase conversions consistently, which stops mixed units creeping into the total.