Backup Power (Generator / UPS)

Quick pick (TL;DR)

Classify the load: critical/no-break (control, IT) → online UPS + battery autonomy; emergency/life-safety (lighting, F&G) → generator on ATS within 10 s, often UPS-backed; essential (utilities, key pumps) → diesel generator.
Size the generator at load ÷ 0.8 then add ~25% margin, and verify the largest motor can start on it.
Pick autonomy (UPS minutes, fuel hours) and redundancy (N+1).

Step 1 — Classify the load

Class	Allowable break	Solution
Critical	0	Online UPS
Emergency	≤ 10 s	Gen on ATS
Essential	Seconds–min	Diesel gen
Non-essential	Until restore	None

Step 2 — Generator sizing

800 kW essential + motor start

Given running load = 800 kW , PF = 0.8 Step 1 — Apparent power kVA = 800 / 0.8 = 1000 kVA Step 2 — Margin ×1.25 (spare + step load) = 1000 × 1.25 = 1250 kVA Step 3 — Motor-start check largest motor 90 kW DOL inrush gen must hold volt-dip ≤ 15% → OK at 1250 kVA Answer → 1250 kVA diesel, ATS, N+1

Generators are limited by transient dip on motor start — often the governing case, not steady load.

Step 3 — Autonomy & transfer

UPS battery for 15–30 min ride-through.
Generator fuel 8–72 h; ATS ≤ 10 s.
Online UPS for no-break; N+1 for critical.

Generator sizing calculator

Standby genset rating

kVA = load/PF × (1+margin) → standard set; then a motor-start check.

Running load (kW)

Power factor

Margin (%)

Largest motor (kW)

Running-load + rule-of-thumb start check. A transient study confirms the dip.

Common mistakes

Sizing on running kW only.
Generator alone for no-break loads.
Under-sized UPS battery.

Requisition & datasheet

Type, load class, load, autonomy, transfer.
Rating, motor-start, scheme, redundancy.

Frequently asked questions

What should I know about Backup Power (Generator / UPS)?

Classify the load: critical/no-break (control, IT) → online UPS + battery autonomy; emergency/life-safety (lighting, F&G) → generator on ATS within 10 s, often UPS-backed; essential (utilities, key pumps) → diesel generator. Size the generator at load ÷ 0.8 then add ~25% margin, and verify the largest motor can start on it. Pick autonomy (UPS minutes, fuel hours) and redundancy (N+1).

Which standards or codes apply?

ISO 8528 (generating sets), IEC 62040 (UPS), NFPA 110 (emergency power), IEEE 446 (emergency/standby).

Standards — ISO 8528; IEC 62040; NFPA 110; IEEE 446.