Do APC units run small?

APC UPS units are not made smaller than their labels suggest, but many buyers feel they run small because they read the VA number as watts. A UPS labeled 600VA cannot power 600 watts. The usable figure is the watt rating, which is lower, and runtime drops quickly as the load rises. The same is true of other UPS brands. This guide covers APC by Schneider Electric uninterruptible power supplies, not the A.P.C. clothing brand.

Why APC ratings look small: VA vs watts

A UPS shows two capacity numbers. VA (volt-amperes) is apparent power. Watts is real power, and it is the number that matters. The two are linked by the power factor, so watts equal VA multiplied by that factor. Many APC models have a power factor well below 1, which is why the watt rating is lower than the VA rating.

Your devices list their power use in watts, so compare your load to the UPS watt rating. Never compare it to the VA number.

Real VA to watt ratios for APC models

These figures come from retailer listings of current APC models. Always confirm the exact rating on the label or datasheet for your model:

As a rule of thumb for APC:

• Back-UPS (consumer standby or line-interactive models): about 50 to 65 percent of the VA rating. A 600VA model supplies roughly 330 to 360 W.
• Smart-UPS line-interactive models: often 60 to 90 percent of VA, depending on the model.
• Smart-UPS Online and many rack models: often about 90 percent or more of the VA rating.

Runtime also looks small

Runtime does not fall in a straight line as the load grows. It collapses. APC publishes runtime charts, and the chart for the Back-UPS Pro BX1500M shows the pattern:

The load rises 16 times from 50 W to 800 W, but runtime falls by about 35 times. A UPS that runs a router for hours may run a gaming PC for only a few minutes. A UPS is designed to give you time to save work and shut down safely, not to run a full desk for an hour.

Battery age also cuts runtime. Batteries wear out with heat and use, and most need replacement every few years. If a unit seems to run small after several years, a battery replacement may restore its runtime.

Do APC units run small in physical size?

Consumer Back-UPS units are compact enough for a desk or shelf. Higher-capacity models are larger and heavier because of the batteries inside. Rack models are measured in rack units, such as 2U. Measure your space before you buy, including ventilation clearance and cable access.

How to size an APC UPS so it does not run small

1. Add up the watts of every device you want on battery. Check labels or use a plug-in power meter.
2. Add 25 percent headroom for growth and for battery aging.
3. Choose a UPS with a watt rating above that number, not a VA rating.
4. Check the runtime chart for the minutes you need at your actual load.
5. Plan for battery replacement, since runtime shrinks as the battery ages.

Here is an example. A router (15 W), a switch (30 W), a NAS (40 W) and a desktop PC (200 W) add up to 285 W. With 25 percent headroom the target is about 360 W. A 600VA Back-UPS at roughly 330 W is too small. A 1000VA unit rated at 600 W fits, and it also gives longer runtime at that load.

If you are between sizes, buy the larger one.

Do other UPS brands run small too?

Yes. Every UPS brand lists VA and watts, and the gap between them is similar for comparable product types. Comparing watt ratings and runtime charts is the fair way to judge any model.

Frequently asked questions

Is a 1500VA APC UPS really 1500 watts?

No. The Back-UPS Pro BX1500M, for example, is rated at 900 W. Always check the watt rating.

Why is my APC UPS runtime shorter than expected?

Most often the load is higher than you think, the battery is aging or the runtime chart was read at a lower load. Measure the real watts and check the chart for that number.

What size APC UPS do I need for a router and a network switch?

A router and a small switch often draw under 60 W together. Even a small Back-UPS can run them, and the runtime can be long at that load. Add up your own devices to be sure.

What is the difference between Back-UPS and Smart-UPS?

Back-UPS models target homes and small offices. Smart-UPS models target servers and network equipment, with features such as sine wave output, management cards and, on some lines, higher watt-to-VA ratios.

Does an APC UPS lose capacity over time?

The battery does. Runtime shrinks as the battery ages, and replacement restores it. The watt rating of the UPS itself does not change.

Should I load a UPS to its full watt rating?

It is safer not to. Keep the load below about 80 percent of the watt rating, which matches the 25 percent headroom advice.