How to Size a Generator: A Step-by-Step Guide

Undersize a generator and it will stall, overheat, or fail to start your motors; oversize it and you waste money and run it inefficiently (which causes "wet stacking" in diesels). The right size covers your running load plus the surge when your biggest motor starts — with a sensible margin on top.

The short version: add up the running watts of everything that runs at once, add the extra surge of the single largest motor at startup, divide by the power factor to get kVA, then add ~25% spare capacity. The interactive calculator below does this for you.

Step 1 — Total your running load

List every appliance, motor, light and device that will run at the same time and add up their running (rated) watts. Nameplate ratings are on the equipment; for a whole building, a rough estimate is about 5 watts per square foot plus 50 kW for general commercial use.

This running total is the power the generator must supply continuously.

Step 2 — Add the largest motor's starting surge

Motors are the reason sizing isn't just simple addition. When an electric motor starts, it briefly draws far more than its running power — the inrush current. You don't add the surge of every motor (they rarely start at the same instant), but you must cover your single largest motor starting while everything else is already running.

The surge depends on how the motor is started:

Start method	Startup draw	Typical use
Direct-on-line (DOL)	~3× running	Most small/medium motors
Soft starter	~2× running	Pumps, conveyors
Variable frequency drive (VFD)	~1.5× running	Modern HVAC, controlled motors

So a 45 kW DOL motor adds about 45 × (3 − 1) = 90 kW of extra startup demand on top of its running power.

Peak demand = total running load + (largest motor running × (start factor − 1))

Step 3 — Convert kW to kVA with power factor

Generators are rated in both kW (real power) and kVA (apparent power). Convert with the standard 0.8 power factor:

kVA = kW ÷ 0.8

(For a deeper explanation, see kVA vs kW vs kWe explained.)

Step 4 — Add a spare-capacity margin

Never size a generator to run flat-out. Industry practice is to leave headroom — typically 25% — so the set normally runs at 70–80% load. This protects against voltage dips (keep them under 15%), absorbs small future additions, and keeps the engine in its efficient, healthy operating band.

Recommended rating = the larger of: peak startup demand, or running load × 1.25

Worked example

A small factory:

• Running load (lights, controls, small machines): 200 kW
• Largest motor: a 45 kW compressor, started direct-on-line (×3)

1. Starting surge of the compressor: 45 × (3 − 1) = 90 kW
2. Peak demand at startup: 200 + 90 = 290 kW
3. Continuous with 25% margin: 200 × 1.25 = 250 kW
4. Required rating = max(290, 250) = 290 kW → ÷ 0.8 = ≈ 363 kVA

You'd then look for a genset with a prime rating of at least 290 kWe / 363 kVA — browse the 300–600 kWe range to find matching engines.

Common mistakes to avoid

• Forgetting motor surge — the most frequent cause of undersizing. A generator that's fine on running watts can still fail to start a single large motor.
• Adding every motor's surge — too conservative; only the largest matters.
• Sizing to 100% load — leaves no headroom and shortens engine life.
• Confusing kW and kVA — always check which unit the spec sheet uses.

Next steps

Once you know your target rating, the rest is matching it to real hardware. Browse the catalog by power range, compare prime vs standby ratings, or filter diesel and gas engines by brand. When in doubt on a critical installation, have a qualified engineer confirm the load study.