Generator Sizing Calculator (Standby kW / kVA)

What the calculator computes

Enter the load a standby generator must carry and the largest motor it must start, and the tool returns the size the alternator needs. It reads five inputs: the connected load in kW, the power factor, the demand factor, the largest single motor and how it starts, and a safety margin. An optional three-phase voltage lets it also report the running current. From these it works out two demands and reports the larger one as the recommended generator, in both apparent power (kVA) and real power (kW).

The first demand is the steady running load with a margin, the size the generator needs to serve everything that runs at once. The second is the motor-start peak, the brief but large draw when the biggest motor starts against the rest of the load already running. On most sites with a sizable across-the-line motor the second number is much larger than the first, and it sets the size. The result is a first-pass rating you confirm with a genset supplier, not a final specification, because a real quote also weighs transient voltage dip, harmonics, and site derating that this method does not model.

kW versus kVA and the power factor

A generator is rated in two ways, and the difference matters. Real power, in kW, is the work delivered: the heat, the light, the shaft power at the motor. Apparent power, in kVA, is the product of voltage and current the alternator and its windings must actually supply. The two are linked by the power factor, the fraction of the current that does real work. The relation is kVA = kW / PF, so a lower power factor means more kVA for the same kW.

Most mixed industrial and commercial loads run at a power factor of about 0.8, which is the value the calculator starts with. At 0.8, one kW of real load asks the alternator for 1.25 kVA of apparent power, so 120.0 kW of running load is 150.0 kVA. The engine is sized by the kW it must produce, and the alternator by the kVA it must carry, which is why a genset is quoted with both figures. Size the kVA on the apparent power the windings see, and the kW on the real power the engine must turn, and never confuse the two, because a load quoted in kW at a poor power factor can overload an alternator picked on kW alone.

The running requirement

The running requirement is the steady load the generator carries once everything is up and running. It starts from the connected load, the sum of every device’s rating, but not everything runs at full output at the same moment. The demand factor scales the connected load down to what is realistically on at once. A plant with a connected load of 150.0 kW and a demand factor of 0.8 has a running load of 150 times 0.8, or 120.0 kW.

That running load is real power, so convert it to apparent power the alternator must supply by dividing by the power factor: 120.0 divided by 0.8 is 150.0 kVA. Then add a safety margin for load growth, measurement error, and headroom, so the generator is not run at its limit. With a 25% margin the running requirement becomes 150.0 times 1.25, or 187.5 kVA. This is the size the generator would need if there were no large motors to start. On many sites a bigger number comes from the starting surge, which is why the running requirement alone rarely settles the choice.

Why motor starting is the real constraint

An electric motor draws far more current at the instant it starts than when it runs. A motor connected straight across the line pulls a locked-rotor current of about five to seven times its full-load current until it reaches speed, and it does so at a low starting power factor, often around 0.3, so the surge in kVA is large. This inrush lasts only a few seconds, but the alternator has to supply it without its voltage collapsing, and that is what usually decides the generator size.

The running load might call for a 187.5 kVA machine, yet a single big motor starting across the line can push the instantaneous demand well past 300 kVA. If the generator cannot supply that surge, its voltage sags, the starting motor may stall, and other equipment on the bus can drop out or reset. So the generator is not sized on the average load at all. It is sized on the worst instant, the moment the largest motor starts while everything else is already running, which is the real constraint on most standby installations.

The starting method and its factor

How a motor is started decides how big its surge is, and that choice can change the whole generator size. Starting a motor direct on line, switching it straight onto full voltage, gives the largest surge, about five times its running kVA in this method. A star-delta starter connects the windings in star to start and switches to delta to run, which cuts the starting surge to roughly three times the running kVA, at the cost of a torque dip at the changeover.

A soft starter ramps the voltage up over a few seconds and holds the starting draw to about twice the running kVA, with a smooth rise and no changeover step. A variable frequency drive, or VFD, ramps both frequency and voltage and can start a motor at about 1.5 times its running kVA, the gentlest option, while also giving speed control in normal operation. The calculator applies a factor of 5, 3, 2, or 1.5 to match the method you pick. Because the surge scales directly with that factor, softening the start of the biggest motor is often the cheapest way to shrink the generator.

The peak during a motor start

The sizing peak is the demand at the single worst instant: the largest motor starting while the rest of the load is already running. It has two parts added together. The first is the other loads, everything except the starting motor, drawing their normal running kVA. The second is the starting motor’s surge, its running kVA times the start-method factor. The sum is the apparent power the alternator must deliver at that moment without its voltage dropping too far.

Take the default case. The largest motor is 30 kW, so its running rating is 30 divided by 0.8, or 37.5 kVA, and started direct on line it pulls about five times that, 187.5 kVA. The other loads are the running load minus that motor: (150 minus 30) times 0.8, divided by 0.8, which is 120.0 kVA. The peak is 120.0 plus 187.5, or 307.5 kVA. That single figure, not the steady 187.5 kVA running requirement, is what the alternator has to be built for.

Choosing the genset as the greater of the two

With both demands in hand, the generator is sized as the larger of them. The running requirement with margin sets a floor: the machine must carry the steady load plus headroom indefinitely. The motor-start peak sets a second floor: the machine must survive the worst starting instant without its voltage collapsing. Whichever number is larger governs, because a generator that meets only the smaller one fails at the other.

In the default case the running requirement is 187.5 kVA and the motor-start peak is 307.5 kVA, so the peak governs and the recommended generator is at least 307.5 kVA. Converted to real power at a power factor of 0.8, that is 307.5 times 0.8, or about 246.0 kW. The calculator shows which demand won in the sizing-driver row, so you can see at a glance whether the machine is being sized by its steady load or by a motor start. When the start governs, the fastest way to a smaller genset is a softer starting method, not a smaller load.

The safety margin by application

The safety margin is the headroom added on top of the calculated load so the generator is not run at its ceiling. It covers load growth, metering error, aging, and the reserve that keeps voltage and frequency steady when a load steps on. The right margin depends on how critical the site is. A light residential or small commercial standby set can use about 15 to 20%, since a brief dip is tolerable and the loads are modest.

A general commercial or light industrial site usually takes about 25 to 30%, the range the calculator starts in at 25%. A mission-critical site, a data center, a hospital, or a process that cannot tolerate a voltage dip, takes about 30 to 40% or more, so the machine has ample reserve for step loads and future expansion. A larger margin buys stability and room to grow, but it also raises the purchase price and can push the engine to run lightly loaded, which brings its own problems, so match the margin to the application rather than reaching for the biggest number.

Running current from a three-phase voltage

When you enter a three-phase system voltage the tool also reports the running current, the steady amps the generator delivers at the running load. For a three-phase supply the current is the apparent power in volt-amperes divided by the square root of three times the line voltage: running current = running kVA times 1,000, divided by (1.732 times voltage). This is the figure that sizes the cables, the breakers, and the transfer switch on the load side.

For the default running load of 150.0 kVA on a 400 V three-phase system, the current is 150,000 divided by 1.732 times 400, which is 150,000 divided by 692.8, or about 216.5 A. Note this is the steady running current, not the starting current, which is much higher for the few seconds a motor is coming up to speed. Size the conductors and protection for the running current, but check that the breaker and transfer switch can also ride through the starting inrush without nuisance tripping.

Reading the results panel

The headline is the recommended generator in kVA, the apparent power the alternator must supply. Directly under it, the recommended kW converts that to real power at your power factor, the figure that sizes the engine. The running-load row shows the steady demand in both kW and kVA, so you can see the base load the machine carries between motor starts.

The motor-start peak row shows the worst instantaneous demand, marked to stand out because it usually governs the size. The sizing-driver row names which of the two, the running requirement or the motor start, set the recommended figure, so you know immediately whether the machine is start-limited or load-limited. The running-current row gives the steady amps for cable and breaker sizing when you have entered a voltage. Read the note beneath for a plain summary, including a reminder that a softer motor start would cut the surge and let you use a smaller unit when the start is what governs.

Five worked examples

Example 1: the running requirement

Start with the default site: a connected load of 150.0 kW at a power factor of 0.8 and a demand factor of 0.8, with a 25% safety margin. The running load is the connected load times the demand factor, 150 times 0.8, which is 120.0 kW. Convert that real power to apparent power the alternator supplies by dividing by the power factor: 120.0 divided by 0.8 is 150.0 kVA. Add the 25% margin and the running requirement is 150.0 times 1.25, or 187.5 kVA. If the site had no large motors to start, a 187.5 kVA generator would serve it. It usually does have one, so this is only the first of the two numbers to compare.

Example 2: the motor starting surge

Now look at the largest motor on its own. It is rated 30 kW, so at a power factor of 0.8 its running apparent power is 30 divided by 0.8, or 37.5 kVA. Started direct on line, it draws about five times that running rating while it comes up to speed, which is 37.5 times 5, or 187.5 kVA of surge. That surge sits on top of whatever else is running at the time. A single 30 kW motor, a modest load in steady terms, briefly asks the alternator for 187.5 kVA all by itself, which shows why one big across-the-line motor can dominate the sizing of an otherwise ordinary installation.

Example 3: the peak during the start

Combine the two. While the largest motor starts, the other loads keep running. Those other loads are the whole load minus the starting motor: (150 minus 30) times the demand factor 0.8, divided by the power factor 0.8, which is 120 times 0.8 divided by 0.8, or 120.0 kVA. Add the motor’s starting surge of 187.5 kVA and the peak during the start is 120.0 plus 187.5, or 307.5 kVA. This is the worst instant the alternator must ride through without its voltage collapsing, and it is the number that sizes the machine on this site.

Example 4: choosing the genset

Set the two demands side by side. The running requirement with margin is 187.5 kVA. The motor-start peak is 307.5 kVA. The generator must cover the greater of the two, so 307.5 kVA governs, and the recommended machine is at least that size. Convert it to real power at the power factor of 0.8 to size the engine: 307.5 times 0.8 is about 246.0 kW. So the site needs a generator of roughly 307.5 kVA and 246.0 kW, sized by the motor start rather than the steady load. The 187.5 kVA running requirement, which many quick estimates would stop at, would leave the machine unable to start its own biggest motor.

Example 5: softening the start

Change one thing: fit the 30 kW motor with a soft starter instead of starting it direct on line. The soft starter holds the draw to about twice the running kVA, so the surge falls from 187.5 kVA to 37.5 times 2, or 75.0 kVA. The other loads still draw 120.0 kVA during the start, so the peak drops to 120.0 plus 75.0, or 195.0 kVA. Now compare against the running requirement of 187.5 kVA: the two are close, and the machine can shrink from 307.5 kVA to about 195.0 kVA. Nothing about the running load changed. The start method, not the steady demand, decided the size, and softening it cut the generator by more than a third.

Three expert tips

Size on the motor-start peak, not the running load

The most expensive sizing mistake is to add up the steady load, add a margin, and buy that machine. On any site with a large across-the-line motor the starting peak is much higher than the running load, and a generator picked from steady demand alone will sag or stall when that motor starts. A single direct-on-line motor can double or triple the size you would pick from the running load, as the default case shows: 187.5 kVA of running requirement against a 307.5 kVA start. Always work out the motor-start peak and size on the greater of the two. The steady load tells you what the machine carries; the worst start tells you what it must survive.

Soften the biggest starts before buying a bigger alternator

When the motor start governs the size, the cheapest fix is usually not a bigger generator but a gentler start. A soft starter or a VFD on the largest motor cuts its starting surge from about five times its running kVA to about two or one and a half times, which pulls the peak down sharply. In the default case a soft starter drops the required machine from 307.5 kVA to about 195.0 kVA. A starter costs a fraction of the alternator capacity it saves, and it protects voltage for the rest of the plant during the start, so lights do not dim and controls do not drop out. Look at the starting method before you look at a larger frame.

Do not oversize on a blanket rule

Reaching for a much larger generator to be safe carries its own cost. A diesel genset run below about 40% of its rating for long periods wet-stacks: unburned fuel and soot build up in the exhaust, the engine glazes its bores, and reliability falls, the opposite of what oversizing was meant to buy. Size the machine so that at the normal running point, after the safety margin, it sits around 70 to 80% loaded, which is where a diesel runs cleanest and most efficiently. Let the motor-start peak set the momentary rating and the running load set the continuous loading, and avoid a blanket safety multiplier that leaves the engine loafing and fouling between starts.

Limits of the method

This calculator gives a first-pass size, not a final specification. It models the two demands that usually govern, the running load with margin and the motor-start peak, but it does not model the transient voltage dip during the start in detail. When a motor starts, the generator voltage momentarily sags, and it must stay above about 80% of nominal so contactors hold and the motor develops enough torque to accelerate. Whether a given alternator meets that limit depends on its subtransient reactance and its excitation, which only the supplier’s data can confirm.

The method also leaves out several real effects. It does not account for harmonic loads from drives, UPS systems, and electronic equipment, which heat the alternator and can force a larger machine or a special winding. It does not sequence step loads, so a site that starts several motors in a planned order may need less than a naive sum, while one that starts them together may need more. And it does not apply the derating for altitude and ambient temperature, which cuts an engine’s output at height and in heat. Confirm the alternator momentary rating, the harmonic capability, and the site derating with the genset supplier before you commit.

Where this calculator fits

It suits anyone putting a first number on a standby or prime generator: facility and plant engineers specifying backup power, electrical contractors quoting a genset and transfer switch, consultants scoping a project before a detailed study, and students working through a power-sizing exercise. It turns a load schedule and the biggest motor into a defensible kVA and kW figure in one pass, so a supplier conversation starts from a real number rather than a guess.

Because it separates the running requirement from the motor-start peak and names which one governs, it also teaches the point that decides most jobs: the worst start, not the average load, sets the size. Use it to test a starting method, to see how much a soft starter saves, or to sanity-check a quote against the load it must carry. Take the result to a genset supplier for the transient, harmonic, and derating checks the method leaves out, and the first-pass size becomes a firm specification.

Common mistakes to avoid

The first mistake is sizing on the running load alone and ignoring the motor-start peak, which leaves the machine unable to start its own biggest motor. The second is confusing kW and kVA, sizing an alternator on kW at a poor power factor so the windings are overloaded even though the engine is not. The third is using the connected load without a demand factor, which sizes the machine for a moment when everything runs at once that may never occur, and inflates the cost.

A fourth is starting every motor direct on line by default and paying for the surge with a bigger generator, when a soft starter on the largest motor would have been cheaper. A fifth is oversizing on a blanket safety rule, which leaves a diesel loafing below 40% load where it wet-stacks and wears. A sixth is treating the first-pass size as final and skipping the supplier checks on transient voltage dip, harmonics, and altitude and temperature derating. Size on the greater of the two demands, keep kW and kVA straight, apply a sensible demand factor and margin, soften big starts, and confirm the momentary rating with the supplier, and the number will hold up.

Frequently asked questions

Sources, disclaimer, and editorial transparency

The running-load and motor-start method, the starting-method factors, the power-factor and demand-factor conventions, and the loading and derating notes described here follow recognized electrical and standby-power references, including engineering guidance on sizing generators from Consulting-Specifying Engineer and practitioner generator-sizing calculator references such as Generator Source generator-sizing guidance. Starting surges are treated as a multiple of running kVA by starting method, and a safety margin is recommended by application. This calculator and guide are built and reviewed by the OpsCalculators team; see our Editorial Policy for how each tool is researched, built, and tested.

Results are accurate estimates for planning and education, not a substitute for a full electrical study or a supplier specification. The method sizes on the running load and the motor-start peak only and does not model the detailed transient voltage dip, harmonic loading, step-load sequencing, or altitude and temperature derating, any of which can change the machine required, so confirm the alternator momentary rating and the site derating with the genset supplier before a purchase or a capital decision. See our full Disclaimer. OpsCalculators.com is operated by MAFHH INTERNATIONAL LTD. Your inputs are processed in your browser and are never stored; see our Privacy Policy.