UPS sizing: VA versus W, power factor and how to calculate runtime

When choosing a UPS there are two separate questions: can it carry the load (power), and for how long (energy)? They are calculated separately, and most wrong sizing comes from mixing the two up.

1. VA and W are not the same

UPS units are quoted in two values, VA (apparent power) and W (real power). The power factor lies between them: W = VA × power factor. For example, a UPS rated 1000 VA with a power factor of 0.9 carries at most 900 W. The power factor varies from model to model; look at the real power rating in the catalogue. On the load side it is the same: if a server's power is stated in W you need not convert it to VA, but the UPS's W limit must be larger than the load total.

2. Add up the load properly

Adding the nameplate (maximum) power of the devices gives an exaggerated result; measuring the real consumption or reading it with a power meter is more accurate. Still, account for start-up loads (disks spinning up, cooling fans) and devices to be added later. Leaving a margin instead of running the UPS at its full rated power is a common internal yardstick; set the margin according to your own policy.

3. Runtime: energy divided by power

The simple approach goes through the steps below. Example: load 300 W; two 12 V, 9 Ah batteries in series (24 V, 9 Ah).

• Nominal energy: 24 V × 9 Ah = 216 Wh
• With 90% inverter efficiency, power drawn from the battery: 300 / 0.9 = 333.3 W
• Battery current: 333.3 / 24 = 13.9 A
• The whole battery cannot be used: with 80% usable depth and an 80% ageing or temperature correction, usable capacity is 9 × 0.8 × 0.8 = 5.76 Ah
• Time: 5.76 / 13.9 = 0.415 hours, about 24.9 minutes

Shorter than you expected? That is normal: only about half of the nominal 216 Wh is actually usable.

4. Working backwards from a target time

You can turn this around. If 30 minutes is wanted for the same load, the usable capacity needed is 13.9 A × 0.5 h = 6.94 Ah, and in nominal terms 6.94 / 0.64 = 10.85 Ah; so a battery string of about 11 Ah or more is needed. The next standard size up must be chosen.

5. Limits of the calculation

• **The load is not constant.** Runtime lengthens non-linearly as the load falls; at high current the battery delivers less capacity (the Peukert effect). The constant-power assumption is approximate.
• **Batteries age.** A battery a few years old has less capacity than at the start; a hot environment shortens its life too.
• **The manufacturer's curve is what counts.** The calculation is for a preliminary decision; for the exact time use the manufacturer's load-runtime table.
• **End voltage.** The UPS shuts down when it drops below a certain voltage; this "depth" must be consistent with the DoD you entered in the calculation.

Use the tool

Enter your own load, battery and target time into the UPS backup runtime calculator: it calculates runtime from VA or W, power factor, inverter efficiency, an ageing margin and an optional Peukert exponent, shows sensitivity when the load changes, and in reverse mode gives the capacity needed. For the cooling load of the whole server room there is the server room power and cooling calculation. To review your backup power design, get in touch.