Generator Sizing for a Data Center: A Worked Example
A data center generator has to carry the whole site, not just the IT load. Here is the arithmetic from UPS input to generator kVA, with every assumption written down and the number of sets checked for a failed set.
The example and its assumptions
This is an illustrative example, not a project. It continues the UPS sizing example (1,200 kW critical load, 96% UPS efficiency, 60 kW battery charging). Replace every figure with project data and manufacturer datasheets.
| Input | Assumed value |
|---|---|
| UPS input power | 1,200 ÷ 0.96 + 60 = 1,310 kW |
| Cooling plant electrical load | 400 kW |
| Lighting, small power, BMS, fire and security | 60 kW |
| Design margin | 10% |
| Generator rated power factor | 0.8 |
Step 1: Total running load
Running load = 1,310 + 400 + 60 = 1,770 kW. With the 10% margin: 1,770 × 1.10 = 1,947 kW.
Step 2: Convert to kVA
Required generator capacity = 1,947 ÷ 0.8 = 2,434 kVA.
Step 3: Number and size of sets
One 2,500 kVA set would cover 2,434 kVA, but it would be a single point of failure. For N+1 with two working sets, each must be at least 2,434 ÷ 2 = 1,217 kVA, so choose 1,250 kVA sets and install three.
| Check | Result |
|---|---|
| Installed capacity | 3 × 1,250 = 3,750 kVA |
| Load per set, all three running | 2,434 ÷ 3,750 = 65% |
| Load per set with one set failed | 2,434 ÷ 2,500 = 97% |
The post-failure case passes, but with very little headroom. That is a design finding, not a pass: it is a reason to revisit the margin, the load estimate, or the set size. A project with strict step-load or ambient derating requirements may need larger sets or a fourth set.
Check the starting and step loads
Running load is not enough. The generator must also accept the largest motor start (typically a chiller or pump, often reduced with a soft starter or drive) and load steps as the transfer sequence picks up UPS and mechanical loads, within the voltage and frequency dip limits set by the specification. This usually decides the size for sites with large motors. See the generator sizing calculator, which sizes on the larger of running load and the starting requirement per ISO 8528.
Derating and rating basis
Output falls with high ambient temperature and altitude, and the same engine has different standby, prime and continuous ratings. Size from the manufacturer's rating at the site's conditions, not from the nameplate alone.
Common mistakes
- Sizing from the IT load instead of the whole site load.
- Mixing kW and kVA, or standby and prime ratings.
- Checking only running load and ignoring motor starts and UPS behaviour on generator supply.
- Not checking loading with one set out of service.
- Ignoring ambient and altitude derating.
Related
How the generator relates to batteries and runtime: UPS vs diesel generator. How the system fits a redundancy target: redundancy explained.
Written by the Vision Matrix Institute editorial team. Worked examples use stated, illustrative assumptions; check them against your project data, the applicable standards and manufacturer datasheets before use.
Frequently Asked Questions
How do you size a generator for a data center?
Add the electrical input of every load the generator must carry: the UPS input (including battery charging and UPS losses), cooling plant, lighting and small power, and other essential services. Add a margin, convert kW to kVA using the generator's rated power factor, check the largest motor starting step, then choose the standard rating and the number of sets for the redundancy target.
Why is generator capacity larger than the IT load?
Because the generator also feeds the UPS losses, battery recharge, cooling and building services that keep the IT load running. The IT load is only part of the site demand.
Is generator size in kW or kVA?
Both are quoted. A set is commonly rated in kVA at 0.8 power factor, so kW is kVA times 0.8. Always check which rating basis (standby, prime or continuous, per ISO 8528) the quoted figure uses.
Do UPS loads need a bigger generator?
Often yes. A UPS rectifier is a non-linear load that can cause harmonic voltage distortion and has a ramp-up behaviour on generator supply, so generators serving UPS systems are frequently oversized or paired with UPS walk-in and input filter features. Confirm with the generator and UPS manufacturers.
Want to learn this properly?
This topic is covered in depth in our Electrical Design – Data Center Specialist program.
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