Generator Sizing Calculator

A generator has to survive two moments: the steady hum of all loads running together, and that instant when the biggest motor slams in and demands many times its running current. The machine that fits one may fail the other.

Generator Sizing Calculator builds a running-kVA base from your loads, adds the largest motor start as a momentary demand, applies a starting sequence and reserve, and returns the kVA rating to specify at standby and prime duty.

It is a preliminary estimate. Final selection follows the manufacturer rating tables and site conditions.

• Running loads: Enter total kW and kVA with power factor.
• Largest motor start: Its starting kVA as the surge test.
• Starting sequence: Adds starts one at a time or stacked.
• Reserve capacity: 10-25% headroom applied to the size.
• Standby and prime: Both duty ratings shown.
• Current at voltage: The feeder current for the chosen set.
• Export: Copy or download the sizing as text, CSV or JSON.
• Learning guide: Why the start wins the sizing argument.

1. Enter the running load in kW and its power factor.
2. Enter the largest motor starting kVA (or its kW and multiplier).
3. Choose the starting sequence — single or stacked starts.
4. Set the reserve percentage for headroom.
5. Read the generator kVA at standby and prime duty.
6. Export the sizing sheet if needed.

Example 1 — Fire plant. 60 kW base at 0.85 pf, 50 kVA largest start, single sequence, 10% reserve: about 110 kVA standby.

Example 2 — Pump station. 100 kW of pumps, largest start 90 kVA, 20% reserve: roughly 230 kVA if starts stack.

Example 3 — Small standby. 25 kW essential load, 15 kVA motor start, 15% reserve: about 48 kVA, size 50 kVA.

Example 4 — Prime. A 150 kVA standby set at 80% derating renders about 120 kVA continuous — confirm the duty curve.

• Both moments covered: Running load and the start surge both checked.
• Sequence-aware: See the difference between staggering and stacking starts.
• Duty correct: Standby vs prime ratings kept separate.
• Explained maths: The kVA build-up shown live.
• Free and private: No uploads, no accounts, no logging.