How to Size a Generator for a Building (kW Calculation)

An undersized generator trips or stalls the first time the largest chiller tries to start. An oversized one wastes capital, burns fuel inefficiently, and — run too lightly loaded for long — wet-stacks and shortens its own life. Getting the size right is a calculation, not a guess, and this guide walks through it step by step.

We will cover what the generator actually needs to carry, the difference between running and starting loads, why kW and kVA are not the same number, and the margins that separate a dependable standby plant from a nameplate that only works on paper.

Step 1 — Decide What the Generator Must Carry

Generator sizing starts with a scoping decision, not a formula. Broadly, backup schemes fall into three tiers:

• Life-safety / essential only: egress and emergency lighting, fire alarm, smoke control fans, at least one lift, security and communications. This is the regulatory minimum in most codes.
• Business-critical: the above plus server rooms, refrigeration, critical process equipment, fuel and water pumps — whatever an outage would turn into direct losses.
• Full building backup: everything, including comfort HVAC. Common for hospitals, data centres and premium residential, and dramatically more expensive than the other two.

Every load that makes the list goes into a schedule with its running power and, for motors, its starting demand. Loads that stay off the generator should be physically prevented from running — via the transfer switch arrangement and load-shedding controls — or they will find their way onto it during the first real outage.

Step 2 — Running Watts vs Starting Watts

Resistive loads — lighting, heaters, IT equipment — draw roughly their nameplate power from the moment they switch on. Motors do not. A direct-on-line motor draws an inrush current of roughly 3–10 times its full-load current for the second or two it takes to reach speed, and the generator has to supply that surge without its voltage or frequency collapsing.

Three practical consequences follow:

1. Identify the largest motor on the standby system — usually a chiller, fire pump or lift — because its starting kVA often sets the generator size, not the total running load.
2. Stagger the starting sequence. Automatic transfer and building controls should bring loads on in steps, largest motor first while the set is least loaded, so surges never stack on top of one another.
3. Soft starters and VFDs change the answer. Reduced-voltage starting can cut starting kVA by half or more, and is frequently cheaper than the next generator frame size.

Step 3 — kW, kVA and Power Factor

Generator sets carry two ratings: real power in kW (what the engine can do) and apparent power in kVA (what the alternator can do). They are linked by power factor:

kW = kVA × power factor — standby sets are conventionally rated at 0.8 power factor, so a 500 kVA set delivers 400 kW

Your load schedule must be summed in both terms. A building full of motors and transformers at poor power factor can run out of alternator (kVA) before it runs out of engine (kW). Voltage-sensitive electronics also mean the alternator's transient performance — how far voltage dips during motor starting — needs checking against manufacturer data, not just the steady-state numbers.

Step 4 — The Sizing Calculation

1. List every load the generator will carry, with running kW (and kVA where power factor differs).
2. Sum the running load, applying realistic diversity — not everything peaks at once.
3. Establish the starting kVA of the largest motor and where it sits in the start sequence.
4. Check each step of the sequence: load already connected + the next starting surge must stay inside the set's transient capability.
5. Add a margin of 20–30% for load growth, measurement uncertainty and alternator health.
6. Select the next standard rating up, in the correct class — standby, prime or continuous — and derate for site altitude and ambient temperature where the installation demands it.

Worked example: a small commercial building with 160 kW of essential running load, whose largest motor is a 30 kW pump starting direct-on-line. Sequenced sensibly, the controlling condition is roughly 130 kW already connected plus a ~180 kVA starting surge. With a 25% margin on the running load, a 250 kVA / 200 kW standby set with strong transient response is the right neighbourhood — but the final selection must be verified against the manufacturer's starting-capability curves.

Standby, Prime and Continuous Ratings

The same engine-alternator package carries different ratings depending on how it will be used, and mixing them up is a classic sizing error:

• Standby (ESP): the highest rating — for emergency backup running limited hours per year during utility outages, with variable load. This is the correct basis for most building backup sets.
• Prime (PRP): a lower rating for sets that run as the main source of power for long periods with variable load — remote sites, unreliable grids, extended load-shedding regimes.
• Continuous (COP): the lowest rating, for constant load around the clock — baseload and process applications.

This matters enormously in markets with routine load-shedding: a set bought on its standby rating but run four hours every day is operating outside its rating class, and its service life and warranty will both reflect that. If the generator will be a working power source rather than an insurance policy, size it on the prime rating — effectively one step larger.

Fuel, Runtime and the Space Around the Set

The kW number is not the end of the engineering. Three companion decisions ride along with it:

• Runtime and tank size — a standby set is sized in hours of autonomy as well as kilowatts. Day tanks plus bulk storage are calculated from fuel burn at expected load; critical facilities commonly hold 24–72 hours.
• Ventilation and cooling air — a generator room needs large volumes of combustion and cooling air; undersized louvres de-rate the set as surely as altitude does.
• Exhaust, acoustics and vibration — exhaust routing, attenuators and isolation are part of the same design package, and they consume real space that must be coordinated with the architecture early.

Rules of Thumb — and Their Limits

Quick estimating figures exist — watts-per-square-foot allowances by building type, or "add 50 kW and 5–10 W/ft²" formulas for early budgeting. They are useful for exactly one thing: a feasibility-stage cost placeholder. They know nothing about your motor sizes, starting sequence, power factor or growth plans, which is why two buildings of identical area can need generators a full frame size apart.

The load schedule that drives all of this is the same one that sizes your transformer, cables and switchgear — it is a core deliverable of any electrical design package, and the first thing we build when a client sends us drawings. If you are new to how the disciplines fit together, our guide to MEP engineering covers the bigger picture.

Conclusion

Generator sizing comes down to four numbers: the running load you must carry, the starting surge you must survive, the power factor that links kW to kVA, and the margin that keeps the plant dependable for the next decade. Establish them from a real load schedule and the selection is almost automatic; skip them and the generator becomes the most expensive guess on the project.