What size UPS do you need?

What is the difference between VA and watts on a UPS?

Watts are the real power a device uses; volt-amps (VA) are the apparent power, the volts applied multiplied by the amps drawn. For computers and UPSs the VA figure is always equal to or larger than the watt figure, and their ratio (watts ÷ VA) is the power factor. Schneider Electric (APC) says that neither the watt rating nor the VA rating of a UPS may be exceeded by the equipment connected to it. The watt rating sets the heat and the power bought from the utility; the VA rating sets the wiring and breaker sizes. A UPS labelled 1500 VA / 900 W, for example, has a power factor of 0.6, so the VA figure on the box can overstate what it will carry in watts by a lot.

How big a UPS do I need for my computer?

Add up the real draw in watts of everything that will sit on the battery outlets: the computer, the monitor, the modem and the router. A tower PC with a monitor, a modem and a router that together pull 200 W, kept under 80% of the rating, needs a UPS rated for at least 250 W. At a watts-to-VA ratio of 0.6 that is 417 VA, so the next size up. If the model's ratio is 0.9, the same 200 W needs 278 VA. Use a plug-in power meter or the power-supply label, and measure under load; the supply's rated wattage says what it could draw, not what it does.

Should I load a UPS to only 60% or 80%?

The sources disagree on the reason, so the field is yours to set. Schneider Electric's White Paper 15 gives 60% as a conservative rule: unless you are sure of the watt figures, keep the sum of the nameplate ratings of the loads under 60% of the UPS's VA rating. That rule exists because nameplates often hide a power factor of 0.6 to 0.7 in computer power supplies, and the paper itself says it tends to give an oversized UPS. If you have measured the watts, 80% of the watt rating is a common working limit, not one from a standard. Either way, a lighter load also gives a longer runtime and more room for new equipment.

Why is the real runtime shorter than the formula says?

A battery gives less energy at a high current than at a slow one. Peukert's law describes this: runtime = H × (C ÷ (I × H))^k, where C is the capacity in amp-hours at the H-hour rate (usually 20 hours), I the discharge current and k the Peukert exponent. Wikipedia lists k as 1.1 to 1.3 for lead-acid batteries, 1.05 to 1.15 for AGM and 1.2 to 1.6 for flooded types. Take a 12 V, 100 Ah battery at the 20-hour rate and a 120 W load with no losses: the current is 10 A, which is the slow 20-hour rate, so k does not matter much for the first estimate of 10 h 0 min. Draw ten times more and the answer shrinks. The calculator also takes off the inverter efficiency and the share of the battery the UPS lets you use.

Can I run a laser printer, heater or fridge on a UPS?

Usually not on the battery outlets. A space heater draws as many watts as the heating element is rated for, often more than the whole watt rating of a desktop UPS, and the watt rating cannot be exceeded. Laser printers and fridges draw far more at start-up than while running, so the figure on the label understates what the UPS must carry for a moment. Keep them off the battery side unless the manual of your model says otherwise; put the computer, monitor, network gear and storage there.

What does a UPS do for a NAS or a router?

A NAS has hard drives that can be damaged, or a file system left half written, if the power cuts out. A UPS keeps it on long enough to shut down cleanly: many NAS units and operating systems talk to the UPS over USB or the network and start a shutdown after a set time. For this job you need minutes, not hours, so the runtime can be small. A router and modem on the same UPS keep the connection up through short outages, and they draw only a few watts, so they add little to the load.

How the size is worked out

The calculator first turns your load into the watt rating you need: the load divided by the share of the rating you are willing to use. Then it turns watts into VA by dividing by the power factor of the UPS itself, the ratio of its watt rating to its VA rating, which is printed on the label or in the data sheet. For 200 W, an 80% limit and a ratio of 0.6, the watt rating is 200 ÷ 0.8 = 250 W and the VA rating is 250 ÷ 0.6 = 417 VA. Both numbers are limits, and the UPS has to meet the larger of the two when checked against your load. The second mode does the check the other way round: your load against the ratings of a UPS you own or are considering.

Worked example: checking a UPS you already have

Take a 1500 VA, 900 W UPS carrying 300 W and 450 VA. The watt load is 33.3% and the VA load 30.0%: plenty of room. A smaller model of 700 VA and 405 W carrying 450 W and 560 VA is at 111% of its watts and 80% of its VA. The watt limit is crossed first, and the UPS will trip or shut down when the power fails. This is why the page asks for both ratings: a load that fits the VA on the box can still exceed the watts.

Runtime: the part that is only an estimate

Runtime depends on the battery, its age and its temperature, and on how the UPS converts power. The page uses the simple model of battery voltage, amp-hours, inverter efficiency, the share of the battery that can be used and a Peukert exponent. For a small UPS with a 12 V, 9 Ah battery and a 100 W load at 85% efficiency, the model gives 55 min with an exponent of 1 and 35 min with 1.15. These are our own estimates; the maker's runtime chart for your model, which is measured, is the better guide. Batteries also lose capacity with age, so a real unit gives less than a new one; the maker's guidance for your model says when to replace them.

Limits of this method

First, the watts on a power-supply label are the most it can deliver. A computer with a 750 W supply may idle at 60 W, so measure the real draw. Second, the draw changes: a gaming PC under load, or a NAS that spins up all its disks at once, may use several times its idle figure, so size for the peak. Third, the power factor of a UPS depends on the model. Schneider Electric states that its on-line models from 6 kVA up have a power factor of 1.0 and its smaller newer on-line models 0.9 or higher, while White Paper 15 puts the watt rating of small UPS systems near 60% of the VA rating, so never convert watts to VA with one fixed number. Fourth, a small UPS is built to bridge minutes, not hours; for a long outage you need a larger battery system, and the battery runtime calculator sizes one.

Sources: Schneider Electric (APC), blog “VA vs watts: What’s the difference anyway?” (February 20, 2015) and White Paper 15, “Watts and Volt-Amps: Powerful Confusion” (Rasmussen; the 0.6 to 0.7 power factor of computer supplies and the 60% rule were read in copies of the paper, not the original page); Wikipedia, “Peukert’s law” (exponent ranges, a secondary source). These are vendor documents. Related: battery runtime calculator, watts to amps calculator and electricity cost calculator. How we check our calculators.