What size UPS do I need?

Sizing a UPS starts with two numbers: how much power your equipment draws, measured in watts, and how long you need it to keep running during an outage, the runtime. Once you have those, three more factors decide the final specification: whether the supply is single-phase or three-phase, the environment the UPS will live in, and how much you expect the load to grow. Get all of this right and the UPS is invisible, which is the point. Get it wrong and you have either overpaid for capacity you never use, or fitted a unit that gives out at the worst moment. This guide works through each part, with worked examples, so you can brief a supplier properly or sense-check a quote. You can also look at this guide for Total cost of ownership

How do you work out the load a UPS has to carry?

The load is the total power your connected equipment draws, and the figure that matters is watts (W), the real power. List everything you want to protect and add up the draw of each item: each server, the storage, the network switch, the firewall, the comms gear. You will usually find the figure on the device label, in its spec sheet, or from the power-supply rating. The nameplate rating is the maximum the device could draw, which is often well above what it actually pulls in normal use, so where you can measure the real draw with a meter or a PDU, use that instead, because sizing to nameplate alone tends to oversize the UPS.

There is one trap worth knowing. UPS units are rated in both VA (volt-amps, the apparent power) and watts (the real power), and the watt rating is the one that limits you. A UPS sold as “3000VA / 2700W” can carry 2700W of real load, not 3000W. Older equipment had a power factor around 0.8, so VA and watts differed a lot; modern IT runs close to a power factor of 1.0, which is why current Eaton units quote watts and VA almost equal. Always size against the watt rating, and against your real measured load where you have it.

How long does the runtime need to be?

Runtime is how long the UPS holds the load on battery, and the right target depends on what you need to happen during an outage. There are three common goals. The first is a clean shutdown: just long enough for servers to close down gracefully or staff to save and stop, often five to ten minutes. The second is ride-through: long enough to carry the load across the short outages that make up most real-world events, so the equipment never goes down at all, perhaps fifteen to thirty minutes. The third is bridge-to-generator: enough runtime for a standby generator to detect the outage, start and take over, usually only a minute or two, after which the generator carries the load.

Runtime is not free. More runtime means more battery, which means more cost and more space, so be honest about what you actually need. Paying for an hour of battery when your real requirement is a clean shutdown is waste; beyond a point, a generator is the better and cheaper answer than ever-larger battery banks. Decide the goal first, then size the battery to it.

How do you extend runtime with battery modules?

When you need more runtime than a UPS gives on its internal batteries, you add extended battery modules. An extended battery module (EBM), sometimes called a battery cabinet or pack, is an external set of batteries that plugs into the UPS to increase how long it can hold the load. This is the usual way to reach fifteen, thirty or sixty minutes of runtime without moving to a much larger UPS. Eaton units are built for it: the 9SX Gen2, 9PX and 5PX Gen2 all accept one or more add-on EBMs, and larger systems like the 93PS use external battery cabinets sized to the runtime you need.

Two practical points come with EBMs. First, the UPS charger has to be rated to recharge the extra batteries in a sensible time; add several modules and the standard charger may take a long while to bring them back to full after an outage, so a larger charger or a unit designed for the battery count is sometimes needed. Second, more battery means more weight and floor space, and the runtime you gain is not perfectly linear, so the right number of modules is a calculation, not a guess. We use the manufacturer runtime tables to match the EBM count to your target rather than rounding up.

Single-phase or three-phase: which supply do you have?

Most racks, offices and small server rooms run on a single-phase supply, and a single-phase UPS like the Eaton 5P, 5PX, 9SX or 9PX is the right fit. Larger sites, bigger loads and many purpose-built server rooms run on a three-phase supply, where a three-phase UPS such as the 93PS is built to match. There is also a middle option: some installations take a three-phase input and deliver a single-phase output (a 3:1 configuration), which can balance the draw better across the building supply and suits sites that have three-phase available but mostly single-phase equipment.

Phase matters for more than just the connector. Three-phase systems generally suit larger loads, scale further, and pair more cleanly with standby generators, which are usually three-phase themselves. If you are not sure what you have, the incoming distribution board will tell you, and it is worth confirming before sizing, because it changes which model range you are choosing from. We check the supply, the load profile and any generator on site, and specify the phase configuration around them.