Guide
How to size a UPS
Sizing goes wrong in one of two directions: a unit that cannot carry the load, or a unit bought two sizes larger than it needed to be. Five checks get it right.
Start with watts, not VA.
Equipment is rated in watts; UPS units are marketed in VA. The two are only the same at unity power factor. Add the wattage of everything on the protected circuit — servers, switches, storage, monitors, the KVM, anything that must stay up — and work from that total.
Then check the power factor gap.
A UPS rated 6000VA at 0.9pf delivers 5400W. The same nameplate at 0.8pf delivers 4800W. Optima Series runs at power factor 1, so 6000VA is 6000W. When you are close to the limit, this is the number that decides whether you need one unit or two.
Add headroom.
Size for roughly 75–80% of the UPS watt rating at installation. That leaves room for equipment added later, and keeps the unit out of the overload region during inrush when several devices power up together.
Decide the runtime you actually need.
There are two honest answers: enough time to shut down cleanly, or enough time to ride through until a generator picks up. Internal-battery (S) models are built for the first. Long-run (L) models have no internal batteries and are designed around an external string sized for the second.
Do not forget the environment.
Battery life halves for roughly every 10°C above 20°C ambient. A UPS in a hot comms cupboard will need batteries sooner than the same unit in a cooled room. On the Infinity 6–10KVA range, also derate 1% per 100m of altitude above 1000m.
Putting it together
A worked example.
A comms room with two servers at 450W each, a switch stack at 300W, storage at 250W and monitoring at 100W totals 1550W. Add 25% headroom: 1940W. On a 0.9pf unit that needs a 2200VA-plus rating, so the Infinity Series T3000S (3000VA/2700W) is the choice. The 1940W design point is 72% of its watt rating, and the 1550W actually installed is 57%. The next size down, the T2000S at 1800W, cannot carry the design point at all — which is why the step up is right even though it lands under the 75–80% target.
Send us your load list and we will size it.
A written specification covering model, battery configuration, runtime and monitoring — back to you as soon as we can.