What size UPS you need: VA, watts and battery runtime

How the UPS size is calculated

The right UPS has a watt rating equal to the watts of your devices divided by the recommended maximum load, and a VA rating that follows from its power factor. The steps:

1. Power of the loads: add up the watts of everything you plug into the battery outlets. The watts are on the rating label or the power adapter; a computer's power supply rating is its maximum, so it errs on the safe side. A plug-in watt meter gives you the real figure.
2. Headroom: Schneider Electric, which makes APC units, recommends keeping the load around 80% of the UPS maximum capacity. The calculator divides the watts by 0.8, and you can change the percentage in the advanced options.
3. Minimum VA: a UPS has two ratings, VA and watts. The ratio of watts to VA is its power factor: on the CyberPower Value models at 600, 800, 1500 and 2200 VA it is 0.6. The calculator starts from 0.6 and works out the minimum VA as minimum watts ÷ 0.6.
4. Size: the smallest of 600, 800, 1000, 1500, 2200 and 3000 VA that is enough. Above 3000 VA the calculator splits the loads across several identical units.
5. Runtime: only if you enter the battery Wh from the model's spec sheet. The math is Wh × inverter efficiency ÷ watts of the loads, with the 80% EnerSys suggests when the efficiency isn't known.

Without the battery Wh the calculator gives no runtime: it depends on the model, and a made-up figure would do more harm than good. The numbers are the same in the US, where UPS units plug into 120 V outlets, and in the UK at 230 V: watts and VA are rated the same way.

Worked example: computer, two monitors and a router

These are the values the calculator opens with: a 250 W computer, two 30 W monitors and a 15 W router.

• Loads: 250 + 2 × 30 + 15 = 325 W
• Minimum power with an 80% maximum load: 325 ÷ 0.8 = 406.25 W, so 407 W
• Minimum VA at power factor 0.6: 406.25 ÷ 0.6 = 678 VA
• Size to buy: 800 VA, which at 0.6 is 480 W; the loads use 68% of it

A 600 VA unit tops out at 360 W and isn't enough. If the spec sheet lists a 12 V, 7 Ah battery, that is 84 Wh, and the estimated runtime is 84 × 0.8 ÷ 325 = 0.21 hours, about 12 minutes: enough to save your work and shut the computer down.

Which UPS for how many watts

With an 80% maximum load and a 0.6 power factor:

Loads	Minimum watts	Minimum VA	Buy
150 W	188 W	313 VA	600 VA
325 W	407 W	678 VA	800 VA
500 W	625 W	1,042 VA	1,500 VA
800 W	1,000 W	1,667 VA	2,200 VA
1,200 W	1,500 W	2,500 VA	3,000 VA
1,800 W	2,250 W	3,750 VA	2 × 2,200 VA

With 500 W of loads a 1000 VA unit looks big enough, but at 0.6 it delivers 600 W, below the 625 W you need with headroom.

Sizes on sale and the watts on the spec sheet

At the same VA, the watts change from model to model. In the CyberPower Value range:

• 600 VA: 360 W
• 800 VA: 480 W
• 1000 VA: 550 W, less than the 600 W a 0.6 power factor would give
• 1500 VA: 900 W
• 2200 VA: 1320 W

That's why the receipt also gives you the minimum watts to look for on the spec sheet. If you're looking at a specific model, enter its watts in the advanced options: the calculator tells you how hard it works and whether it keeps the headroom. For the battery Wh, multiply the voltage and amp-hours on the spec sheet (for example 12 V × 9 Ah = 108 Wh) and add the Wh up if there are two batteries.

Common mistakes

• Looking only at VA. A 1000 VA unit may handle 550 W, 600 W or more: watts are the real limit.
• Loading the UPS to 100%. Schneider Electric recommends staying around 80%: it leaves room for peaks and one more device.
• Leaving a device out. Modem, switch, NAS and computer speakers all add up: if they're missing from the count, the UPS can end up over its limit during an outage.
• Trusting a proportional runtime. In the Schneider Electric example the same UPS runs 6 min 30 s at 1000 W and 9 min 34 s at 800 W, not 8 minutes as a simple proportion would say. The maker's runtime chart beats any formula.
• Forgetting the battery. UPS batteries age and runtime drops over the years: check when the maker recommends replacing them.