How to Size a Genset for an Apartment Complex or Housing Society

Ask five different housing society committees "what size genset do we need" and you'll often get five different assumptions about what's even being backed up — which is where most of the sizing disagreements start.

The two sizing questions societies actually face

Before any load calculation, a society needs to settle which of these it's solving for:

• Common-area-only backup — lifts, water pumps, lobby and corridor lighting, security systems, intercom, and basic common utilities. No flat-level power during an outage beyond what's already common infrastructure.
• Per-flat partial backup — a fixed allocation per unit (commonly enough for lighting, fans, and a single AC or refrigerator point), on top of common-area load, usually billed back to residents in some form.

These produce very different genset sizes for the same building, and a lot of oversizing or undersizing complaints trace back to a committee never explicitly deciding which one it's building for.

Common-area-only: a worked example

For a mid-size society (say, 100 flats across a few towers), common-area connected load typically includes 2-4 lift motors, 2-3 water pump motors, and lighting/security load across common spaces — often landing somewhere in the 40-70 kW connected range depending on the number of towers and lift count. Add a 25-30% margin for simultaneous motor starting (see below), and you're commonly looking at a genset in the 75-125 kVA range for a society this size — though every building's actual numbers should be calculated from its real equipment list, not this example.

Per-flat backup: why it changes the number substantially

Adding even a modest 0.5-1 kVA allocation per flat scales fast — 100 flats at 0.75 kVA average adds roughly 75 kVA of connected load on top of common areas, which can push a mid-size society from a 75-125 kVA genset into 160-250 kVA territory or beyond. This is the single biggest reason "we want backup for residents too" quotes come back much larger than committees expect, and it's worth modelling explicitly rather than assuming a modest per-flat allocation is a modest sizing change.

Why motor starting margin matters more here than in a single office

A society genset picking up load after an outage can face several lift and pump motors attempting to start close together — elevators recalling to ground floor, pumps kicking in as water pressure drops. This simultaneous-start surge is why a 25-30% margin over steady connected load is standard practice for society sizing, similar in principle to the higher safety margin used for hospital critical loads, though the underlying reason (motor starting current, not brownout sensitivity) is different.

Billing and metering — a brief note

Once backup extends to individual flats, someone has to decide how it's paid for — a flat monthly charge, sub-metered consumption billing, or some hybrid. This is a society governance and metering question, not a sizing one, but it's worth deciding early: the billing model residents expect often shapes how much backup capacity they'll actually try to draw, which feeds back into whether your sizing assumptions hold up in practice.

When one genset stops being the right answer

For larger developments, splitting capacity across two or more mid-sized sets configured to run in parallel — rather than one large set carrying everything — gives N+1 redundancy: if one set is down for scheduled maintenance or fails to start on demand, the remaining capacity still covers critical common-area load. This is a capital-cost and space trade-off best discussed with an electrical consultant for bigger societies, not something smaller developments typically need to default into.

Don't skip the committee decision

The technical sizing work is the easy part once the scope question is settled. Get the committee to explicitly agree on common-area-only vs. per-flat backup, run the connected load numbers for whichever applies through our load calculator, and get a dealer's sign-off on the final number before finalizing a purchase — find one through our dealer directory.