Generator and UPS Sizing Calculator
Continuous kVA, the peak demand when the largest motor starts, the candidate generator rating, and a UPS runtime that accounts for the rate effect and ageing.
Calculator
From a load list to the continuous power, the peak demand when the largest motor starts and a candidate generator rating; on the UPS side, the realistic runtime of a battery string.
Result
The calculation runs entirely in your browser; nothing you type is sent to our servers. Only if you press “Turn this into a quote request” is the result written to your browser’s session storage, so it can be carried into the quote form.
Worked example
Opened with its default case — Lighting and socket load (kW): 15 · IT and server load (kW): 8 · HVAC load (kW): 20 — the calculator returns the figures below. They are written out here so the output is readable without running JavaScript: in print, with scripts disabled, or by a search engine.
| Input | Value |
|---|---|
| Lighting and socket load (kW) | 15 |
| IT and server load (kW) | 8 |
| HVAC load (kW) | 20 |
| Other continuous load (kW) | 0 |
| Diversity factor | 0.8 |
| Power factor (cos φ) | 0.8 |
| Allowance for future load (%) | 20 |
| Shaft power of the largest motor (kW) | 11 |
| Motor locked-rotor code letter (NEMA) | H — 6.3–7.1 kVA/HP (typical on IE3 motors) |
| Motor starting method | Direct on line (DOL) — the heaviest start |
| Generator rating type (ISO 8528-1) | ESP — emergency standby: up to 200 h/year, 70% average load |
| Load to be supported by the UPS (kW) | 5 |
| Battery string voltage (V) | 240 |
| Battery capacity (Ah) | 9 |
| Depth of discharge (%) | 80 |
| UPS efficiency (%) | 90 |
| Ageing and high-rate allowance (%) | 60 |
| Result | Value |
|---|---|
| Continuous load (with diversity) | 53.1 kW (80% diversity, +20% margin, motor included) |
| Apparent power required | 66.3 kVA (cos φ 0.8) |
| Peak demand at starting (skVA) | 150.4 kVA · 11 kW motor, direct on line (DOL), code H (6.7 kVA/HP) |
| Lower size — for the continuous load | 100 kVA (ESP) (calculated: 94.8 kVA) |
| Upper size — to start without a voltage dip | Same as the lower size — starting is not the governing case |
| What governs the size | Continuous load · ESP (emergency standby) |
| Average load — at the lower / upper size | 66% (100 kVA) · 66% (100 kVA) |
| Load factor assessment | Within limits — inside the 70% average load limit for ESP |
| UPS runtime | 7.5 minutes (622 Wh usable) |
| Ah needed for 10 minutes | 12.1 Ah · 240 V string |
Change any field above and the calculator recomputes; this table is the default case only.
How do you size a standby generator?
Two limits are calculated and the larger one wins. The first is the continuous load: sum the loads, apply a diversity factor, add a future allowance, divide by the power factor to get kVA (or by 0.8 if the load’s power factor is higher, since set ratings assume 0.8 lagging), then divide again by the rating type’s permitted average load — 70% for an ESP or PRP set under ISO 8528-1. The second is the starting demand of the largest motor, which for a direct-on-line start is roughly kW ÷ 0.746 × the NEMA code kVA/HP — for an 11 kW motor with an H code that is about 99 kVA of inrush, against 11 kW of running load. Starting is very often what decides the machine, which is why a soft starter or star-delta is frequently cheaper than the next generator size up. The tool reports both limits, says which one governs, and warns when the resulting set would run below 30% average load — the wet stacking zone that damages a diesel engine over time.
The two limits
| Limit | Built from | What it protects |
|---|---|---|
| Continuous | (Σ kW × diversity × (1 + future allowance)) ÷ cos φ — or ÷ 0.8 if that is larger, since set ratings assume 0.8 lagging — plus the largest motor’s running kVA, then ÷ the permitted average load for the rating type | The engine and alternator running within their duty rating |
| Starting | Motor kW ÷ 0.746 × code kVA/HP × the starting-method factor, divided by the alternator’s accepted inrush multiple | The voltage dip when the motor starts, and everything that trips because of it |
Both are reported. When starting is the governing case the tool says so explicitly, because that is the situation where changing the starter — not the generator — is usually the cheaper fix.
The diversity factor is the input people get wrong in the safe direction, which is still wrong. Entering 1 assumes every load runs simultaneously and produces a set that will spend its life lightly loaded. A measured figure from a maximum-demand recording, or a considered 0.7–0.85 for a mixed building, gives a better machine.
Starting method changes everything
| Method | Relative starting current | Trade-off |
|---|---|---|
| Direct on line (DOL) | Full locked-rotor current | Cheapest starter, largest generator; hardest on the driven machine |
| Star-delta | About one third | Cheap, but there is a transition transient and reduced starting torque |
| Soft starter | About 45%, adjustable | Smooth ramp; still draws more than running current |
| Variable frequency drive | No inrush at all | The set is instead oversized for harmonic load — commonly by a factor of around 1.4–2 |
The last row is the one that surprises people: a VFD removes the starting problem and introduces a different one. The generator is sized to carry the harmonic distortion the drive presents, which is why the calculator treats a VFD as a continuous oversizing requirement rather than as a peak.
The NEMA code letter on the motor nameplate is the single most useful number in this calculation and the one most often not looked up. It sets the locked-rotor kVA per horsepower, and the difference between an F and a K code is 60% more inrush from the same motor rating.
ESP, PRP, COP — what the rating means
| Rating | Permitted use | Average load over 24 h | Overload |
|---|---|---|---|
| ESP — emergency standby | Mains failure only, limited hours per year | 70% | Not permitted |
| PRP — prime | Unlimited hours, variable load | 70% | 10% for 1 h in 12 |
| COP — continuous | Unlimited hours, constant load | 100% | Not permitted |
The same physical machine carries different kVA figures under each rating, which is why comparing two quotes without checking the rating type compares nothing. A set quoted at its standby rating and one quoted at its prime rating are not the same size.
Too big is a fault, not a safety margin
The instinct to round generously upward causes a specific and expensive failure. A diesel engine running below roughly 30% of its rating does not reach its designed cylinder temperature; unburnt fuel and lubricating oil accumulate in the exhaust and turbo — wet stacking. The symptoms are visible smoke, oil at the exhaust and progressive loss of output, and the remedy is either a load bank exercise programme or a correctly sized machine.
This is why the calculator reports the average load factor at both candidate sizes and warns below 30%. When starting demand forces a large machine that will then idle, changing the starting method is almost always the better answer than accepting a set that will be under-loaded for twenty years.
Sizing the UPS battery honestly
A UPS battery does not deliver its nameplate capacity in a UPS discharge, and this is the most common reason a runtime calculation is wrong by a factor of two. Three corrections belong in every estimate:
- The rate effect. A lead-acid nameplate Ah is quoted at a 20-hour discharge. Over the ten minutes a UPS actually runs, the same battery delivers roughly half that energy.
- End-of-life ageing. IEEE 1188 asks for a 1.25 factor, because a battery at the end of its service life is expected to retain 80% of its rated capacity and the system must still work then.
- Depth of discharge. Taking a VRLA battery to 100% discharge shortens its life sharply. The calculator caps the input at 90% and says so in the result rather than accepting a figure that would destroy the battery.
Together the first two are why the ageing allowance defaults to 60%. Lithium batteries are a different case: their nameplates are quoted at 1C, so the rate correction is far smaller and an allowance this large would understate the runtime.
What to send us for a quote
Send the calculator output with the motor nameplate details, the fuel tank autonomy you need in hours and whether the set will sit indoors, in a canopy or on a roof. Those three decide the exhaust, the acoustic specification and the fuel arrangement, which are often more than half the installed cost. We quote the set, the transfer arrangement, batteries and the consumables — filters, oil, coolant, fuel treatment — on one list, and the maintenance parts are the same ones you will reorder for years.
Frequently Asked Questions
What size generator do I need for a 30 kW load?
It depends on the starting demand as much as the load. For 30 kW at 0.8 power factor with 80% diversity and a 20% future allowance, the continuous requirement is about 30 kVA, which at the ESP 70% average-load limit points to a 43 kVA machine. If an 11 kW motor starts direct on line on the same set, the starting case pushes it higher — which is exactly what the calculator shows.
Why is starting current so much larger than running current?
A motor at standstill looks like a short circuit to the supply until it builds back-EMF. The locked-rotor current is typically six to eight times the running current, and the NEMA code letter on the nameplate tells you which. That inrush lasts seconds but the generator has to carry it without the voltage collapsing.
Is a bigger generator always safer?
No. Below about 30% average load a diesel engine wet stacks: unburnt fuel and oil collect in the exhaust and turbocharger, producing smoke and progressive loss of output. An oversized set is a maintenance liability, not a margin.
What is the difference between ESP, PRP and COP ratings?
They are duty ratings under ISO 8528-1. ESP is emergency standby with limited annual hours; PRP is prime, unlimited hours at a variable load; COP is continuous at a constant load. The same machine carries a different kVA figure under each, so two quotes are only comparable when the rating is the same.
Why does my UPS run for less time than the battery Ah suggests?
Because the nameplate Ah is measured over a 20-hour discharge. Over the ten minutes a UPS actually delivers, a lead-acid battery gives roughly half that energy, and IEEE 1188 asks for a further 1.25 end-of-life factor. Together those explain most runtime disappointments.
Can you supply the generator consumables as well?
Yes — filters, oil, coolant, fuel treatment, batteries and the spares you will reorder on a schedule, on one account. Send the set model and the service interval and we will build the parts list against it.
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A list, a photo or an Excel file — whatever is easiest. On standard items most quotes go out the same business day. Custom production, imports or 50+ line lists can take longer; when they do, we tell you the timeline in writing on the day we receive the request.