Free sizing tool

Generator kVA Calculator

Find the right generator size for your site in under a minute — with the correct safety margin and matching ATS panel — then send the result to our engineer on WhatsApp.

Step 1 of 4 — Load

Total running load (kW)

Load power factor

Mixed sites ≈ 0.8. Offices & IT ≈ 0.9–0.95. Motors ≈ 0.85.

Largest motor in the load (kW, 0 if none)

Pump, compressor, chiller, lift — the one that draws the most when it starts.

Starting method

UPS / drives / chargers in the load (kW)

Rectifier type

How the calculator works

1
Load
Total load or a full schedule — motors and how they start, UPS and drives.
2
Site & duty
Standby or prime per ISO 8528-1, derated for site temperature and altitude.
3
Starting & quality
Maximum voltage dip when the largest motor starts, and whether loads switch on in sequence.
4
Recommendation
Minimum rating and what sets it, then every 4U model that passes — with load %, fuel use and tank autonomy.

Formulas used

kVA = √(kW² + kVAr²)

SkVA = hp × 6.29 × starter factor

Dip ≈ X·SkVA ÷ (kVA + X·SkVA)

I @ 400 V = kVA × 1.443

Method & standards

ISO 8528-1 ratings: standby (ESP) and prime (PRP), with the 24 h average load limit of each brand's datasheet (70–80 %).
Motor locked-rotor kVA from NEC Table 430.7(B), code G (up to 6.29 kVA/hp), at 0.35 power factor while starting.
Reduced-voltage starting: star-delta ≈ 33 %, autotransformer 65 % tap ≈ 42 %, soft starter at 300 % ≈ 50 %; VFDs draw no inrush.
Voltage dip = X·SkVA ÷ (kVA + X·SkVA), typical alternator reactance X ≈ 0.12. Limits: 15 % for fire pumps, 20–30 % for general motor loads.
Non-linear loads: alternator × 1.15 (12-pulse), × 1.2 (IGBT), × 2 (unfiltered 6-pulse).
Derating: full output to 40 °C and 1000 m, then ≈ 1 %/°C and ≈ 4 %/300 m (typical manufacturer datasheet values).
Minimum 30 % load to avoid wet stacking (NFPA 110). Fuel use from the 4U model datasheets.

FAQ

Sizing questions

Yes — every product we supply carries a 12-month warranty against defects in materials and workmanship, and manufacturer warranties on engines, alternators and solar modules are passed on in full. We also offer maintenance contracts to keep your equipment covered and running.

Popular ratings are often available from stock for UAE delivery within days. Built-to-order sets and large panels typically take 3–8 weeks depending on engine and switchgear lead times — we confirm dates in writing with every quote.

Standby rating is for emergency use while mains is down, with limited annual hours. Prime rating is for sites where the generator is the main source of power and runs for unlimited hours at variable load. Prime is roughly 10% lower than standby for the same set.

Match the ATS current rating to the generator's full-load current (or the incoming mains breaker, whichever is higher). At 400 V three-phase, current ≈ kVA × 1.44 — so a 250 kVA generator needs about 360 A, meaning a 400 A ATS.

Generator kVA Calculator

Sizing questions

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What is the difference between standby and prime rating?

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