Undersize a generator and it trips on the first motor start — or worse, limps along overloaded until the windings cook. Oversize it and you pay more upfront, then keep paying through wet stacking and terrible fuel economy at 20% load. Correct sizing is a five-step calculation, and this guide walks through each step with a real 500 kW facility as the worked example.
Step 1: Build the load schedule
List every load the genset must carry, with its running kW and — critically — its starting behaviour. Separate three categories:
- Resistive loads (lighting, heaters, IT): starting ≈ running demand
- Motor loads (pumps, compressors, HVAC, lifts): direct-on-line starting draws 5–7× running current for a few seconds
- Non-linear loads (VFDs, UPS rectifiers): modest starting demand but harmonic content that may require alternator oversizing
Step 2: Apply the diversity factor
Not everything runs simultaneously. A realistic diversity (demand) factor for mixed commercial/industrial facilities is 0.7–0.9 of connected load. Use measured maximum demand from utility bills where available — it beats any assumption.
Step 3: Check the largest motor start
The genset must hold voltage while its largest motor starts with the rest of the essential load already online. Rules of thumb:
- DOL start: motor kW × 6 = starting kVA surge
- Star-delta: × 2.5–3
- Soft starter: × 2–2.5
- VFD: × 1.2–1.5
Generators typically tolerate a brief 30% voltage-dip surge of ~2.5× rating. If a big DOL motor dominates the load, either the genset grows or the motor gets a soft starter — the soft starter is almost always cheaper.
Step 4: Derate for site conditions
| Condition | Derating |
|---|---|
| Altitude above 1,000 m | −3–4% per additional 300 m |
| Ambient above 40°C | −2–3% per additional 5°C |
| High humidity (tropical coastal) | −1–2% |
| Restricted container ventilation | per enclosure test data |
Step 5: Set the operating band and add margin
Target the genset to run at 50–80% of prime rating under normal duty. Below 30% invites wet stacking; above 90% leaves no headroom. Add 10–20% for load growth, and decide the redundancy model — for critical sites, N+1 with paralleled units beats one oversized machine.
Worked example: 500 kW facility
- Connected essential load: 500 kW → diversity 0.8 → 400 kW running demand
- Largest motor: 75 kW chiller compressor, soft-started (×2.2 ≈ 165 kVA surge)
- Site: 800 m altitude, 42°C design ambient → combined derating ~5%
- Calculation: 400 kW ÷ 0.8 PF = 500 kVA → with surge check and 5% derating → 625 kVA required prime capacity
- Selection: 1 × 650 kVA prime unit, or 2 × 350 kVA paralleled for N+1 — running load lands at 62% of rating either way, squarely in the efficiency sweet spot
Check the fuel implications of your sizing in our fuel consumption chart, and make sure the rating class matches the duty — see prime vs standby ratings.
Econo Solar supplies gensets from 5.5 to 4,000 kVA across eight engine platforms, and our engineers run this sizing calculation free against your load schedule. Send it over for a same-day recommendation.