How to Size a Standby Generator

A standby generator is sized against four questions, not one. Can it carry the running load? Can it pick that load up in the blocks it will actually arrive in? Can it start the largest motor without an unacceptable voltage dip? And is its rating valid for how long, and how often, it will run? The running load is the easy part. The other three are where transfers fail.

Step 1: Pick the right rating

The same engine and alternator carry different published ratings depending on the duty. ISO 8528-1 defines them:

A standby rating comes with conditions, typically a limit on average load over the run and on annual operating hours. The exact limits are set by the manufacturer, so read the rating definitions on the datasheet, not just the kW figure. For data centers this matters twice. First, a facility that treats the generators as its long-duration source is running them outside what "standby" assumes. Second, data center tier certification schemes generally expect generators rated to run without a runtime limit at the design load.

Step 2: Build the running load

List what the generators must carry after transfer, in kW:

• UPS input, not UPS output. The UPS draws its own losses plus battery recharge current after an outage, at exactly the moment the generators take over. Use the rated input from the UPS datasheet. The UPS sizing guide covers why.
• Mechanical load: chillers, CRAHs or CRACs, pumps and cooling towers. In a data center, this is usually the second-largest block, and the one with the hardest restart.
• Building and life-safety load: lighting, controls, fire pumps and anything else on the emergency or standby systems.

Generators are rated in kW at 0.8 power factor, which fixes their kVA. A 2,000 kW set is a 2,500 kVA set. Check the load in kVA too, and watch for a leading power factor at light load, which generators tolerate poorly. The kW vs. kVA guide covers both.

Step 3: Check block loading

After an outage, load arrives in blocks as transfer switches close and equipment restarts. Each block causes a transient voltage and frequency dip while the engine governor and alternator regulator respond. Step-load acceptance, the largest block a set can take within its transient limits, varies widely with engine design. It's often what decides the generator size in a data center, rather than kW. ISO 8528-5 defines performance classes (G1 to G4) with progressively tighter transient limits. Specify the class, then check the manufacturer's step-load data against your actual blocks.

Timing matters too. NFPA 110 Type 10 systems must pick up emergency load within 10 seconds of an outage. Loads that aren't on the emergency system are usually sequenced in afterwards, with transfer-switch time delays, so no single step exceeds what the sets can accept. UPS rectifiers typically ramp their input up over several seconds (walk-in) rather than stepping it on, which softens the largest electrical block.

Step 4: Check the largest motor start

A generator is a much softer source than a utility transformer. Its short-circuit capacity is set by its subtransient reactance (X″d), typically a few times higher than a transformer's %Z. The same screening formula used in the Motor Starting Voltage Dip Calculator applies:

Motors on drives or soft starters draw far less starting current than across-the-line starts. That's one reason modern chillers are rarely the problem they used to be. Harmonic distortion is also worse on a generator bus for the same reason, covered in the harmonics guide.

Worked example

A data hall with the following running load after transfer: 1,850 kW of UPS input (including recharge, from the UPS datasheet), 700 kW of mechanical load and 50 kW of building load, for 2,600 kW in total. Sets are 2,000 kW / 2,500 kVA with an assumed X″d of 0.15, in N+1.

• Sets for N: 2,600 ÷ 2,000 = 1.3, so 2 sets. N+1 is 3 sets, 6,000 kW installed.
• Running loading: 2,600 ÷ 6,000 = 43% with all three sets. With one set out, 2,600 ÷ 4,000 = 65%.
• kVA check: at 0.9 load power factor, 2,600 kW is 2,889 kVA, well inside the 5,000 kVA of two sets.
• Largest electrical block: the UPS input is 1,850 ÷ 6,000 = 31% of three sets, or 46% of two. Both are to be checked against the chosen sets' step-load data, and softened by the UPS walk-in.
• Largest motor: a chiller started across the line at 3,000 kVA locked-rotor. On three paralleled sets, short-circuit kVA ≈ 3 × 2,500 ÷ 0.15 = 50,000 kVA, so the dip is about 3,000 ÷ 53,000 = 5.7%. With one set out, 33,333 kVA and about 8.3%.

For comparison, the same motor on a 2,500 kVA, 5.75% utility transformer (43,478 kVA short-circuit) dips the bus about 6.5%. Losing one generator makes the motor start worse than on utility power. That's the case to design for, not the all-sets-running case.

Step 5: Derate for site conditions

Published ratings apply at reference conditions. At high altitude and high ambient temperature, engines lose output, and the manufacturer's derating curves say by how much. A generator room or enclosure also runs hotter than outdoors, and radiator airflow has to be checked at the site's design temperature, not the rating's.

Step 6: Fuel and runtime

NFPA 110 classifies emergency power systems by how long they must run at rated load without refueling: Class 2 is two hours, Class 48 is 48 hours, and so on. The class is set by the application and the authority having jurisdiction. Data centers commonly store a day or more on site. Fuel burn depends on actual load, so a set running at 43% burns far less per hour than at full load. The Generator Runtime Calculator works this out from the set's own consumption data.

Common mistakes

• Sizing on UPS output. The generator feeds the UPS input, including losses and recharge.
• Sizing on running kW alone. Step-load acceptance and motor starting often need a bigger set, or more sets.
• Checking motor starting with all sets running. Design for the N+1 case with one set out.
• Using a standby rating for long-duration duty. Read the rating's hour and average-load limits.
• Forgetting derating. Altitude, ambient temperature and enclosure airflow all reduce available kW.
• Ignoring leading power factor. Lightly loaded UPS input filters can push the generator leading.

Frequently asked questions