Sizing a UPS

5 min readLast reviewed: January 2025

If a supplier quotes you a UPS after one phone call, they're guessing. Here are the four numbers that need to be on the table first.

1. The actual load, measured, not estimated

Nameplate ratings on IT equipment lie by 30-50%. A rack labelled 5 kW may draw 2 kW at typical utilisation. Clamp meters on the existing circuits, or a power quality logger for a week, get you the number that matters. If the equipment isn't installed yet, size on measured similar systems, not on nameplates.

2. Power factor

UPS ratings are given in kVA. Your real load is in kW. The ratio between them is power factor. Modern IT loads run at 0.9-0.99 leading, old servers and inductive loads run lower. A 100 kVA UPS at 0.8 PF gives you 80 kW; at 1.0 PF it gives you 100 kW. Assume the wrong PF and you've sized 20% wrong.

3. Autonomy, how long, at what load

'15 minutes at full load' is not the same as '15 minutes at half load', battery discharge curves are non-linear. Decide the autonomy at the load you'll actually run at, at the temperature the battery room will actually reach in August. Then check the runtime table, not the marketing sheet.

4. Growth, the load in year five

A UPS is a 10-15 year asset. What does the load look like in five years? Modular UPS lets you grow inside a single frame. Static UPS gets replaced. Decide up front which philosophy fits your growth path, retrofitting the wrong choice is expensive.

A worked example

A small data room: two racks, measured at 11.4 kW total with a clamp meter over a working week, peaking at 12.6 kW. Power factor measured at 0.95. Step one, add headroom for the peak and never plan to run a UPS above 80 percent: 12.6 divided by 0.8 gives 15.8 kW required. Step two, convert to kVA: 15.8 divided by 0.95 gives 16.6 kVA, so a 20 kVA unit. Step three, autonomy: a generator is on site, so 10 minutes at 15.8 kW is the target, which is 2.6 kWh delivered. Lead acid is limited to 50 percent depth, and high rate discharge reduces available capacity by a further 20 percent or so, so the string is sized around 6.5 kWh nominal. At 480 V DC that is roughly 40 blocks of 12 V 12 Ah, or a smaller lithium string with the same delivered energy. Step four, growth: a third rack is planned within three years, so a 20 kVA modular frame that accepts a second power module beats a fixed 20 kVA unit that would have to be replaced.