Field guide · Racks & rooms
UPS Sizing: VA, Watts and Runtime
Why a 1500 VA unit does not deliver 1500 watts, how runtime actually behaves as load rises, and what belongs on battery in a comms room. Covers the power factor everyone trips over, sizing for the whole path rather than one device, why runtime curves are not linear, and the maintenance item that makes half the installed base useless.
Diagram
The number on the box is not the number you need.
A 1500 VA rating delivers about 900 W.
Measured at the plug, not read off a nameplate. A switch rated 500 W with no PoE load might sit at sixty.
400
watts, real power0.6
power factor667
VA, before headroomBuy on the watts figure, not the VA on the front of the box. Two units with the same VA rating can differ by a third in the real power they will carry, and the one that fails is the one sized on the bigger number.
Procedure
Doing it
From measured load to a unit that survives the cut, with the two numbers people confuse handled first.
Add up watts, not the ratings on the labels
Nameplate ratings are the ceiling a device may draw at maximum. Actual draw is usually far below it. A switch rated 500 W with no PoE load might sit at 60. Measure or use vendor figures for the actual configuration, and treat the nameplate as a ceiling rather than an estimate — sizing to nameplates is how a room ends up with three times the UPS it needs.
Convert to VA using the real power factor
Watts divided by power factor gives VA, and that is the number the UPS is rated in. Older units assume around 0.6, modern ones are nearer 0.9 to 1.0, and the difference is large: a 1500 VA unit at 0.6 delivers only 900 W. Check the unit's own watt rating, because both numbers are printed and only one is the limit you will hit.
CautionSizing on VA alone is the classic error. A load can be within the VA rating and over the watt rating, and the UPS will shut down under it.
Size for the whole path, not the rack
IP phones need the switch, the router, and the internet circuit all on battery to place a call. Protecting the switch and leaving the modem on an unprotected outlet is a common and entirely self-defeating arrangement. Decide what has to keep working and follow it end to end.
Read runtime as a curve, not a number
Runtime falls faster than load rises, because battery capacity is not linear with discharge rate. A unit that claims thirty minutes at half load may give well under ten at full. Use the manufacturer's runtime chart at your actual load, and if the answer is thin, add capacity rather than hoping.
Decide what runtime is actually for
Riding out a two-minute cut and surviving a two-hour outage are different products. If the goal is a graceful shutdown, five to ten minutes plus working shutdown signalling is enough. If it is staying up, size for the real outage length or accept that this is a generator problem rather than a UPS one.
Leave headroom, and put the batteries in the calendar
Run at around seventy to eighty percent of capacity so the unit is not permanently at its limit and there is room for one more device. Then diarise the batteries: sealed lead-acid is typically three to five years, less in a warm room, and an untested UPS with tired batteries provides no protection while appearing to. Test under load rather than trusting the panel light.
CautionA UPS nobody has load-tested is an assumption. The failure mode is silent — it looks healthy right up to the cut it does not survive.
Questions
Questions people actually ask
- What is the difference between VA and watts?
- Watts is real power consumed; VA is apparent power, which is what the UPS's electronics have to handle. They differ by power factor. A 1500 VA unit with a 0.6 factor delivers 900 W, and one at 0.9 delivers 1350 W from the same VA rating. Both numbers are on the unit, and the watt figure is the one your load will actually hit first.
- How long should a UPS run for?
- It depends entirely on what you want it to achieve. For a clean shutdown, five to ten minutes plus working shutdown signalling is plenty. For riding out short cuts, fifteen to thirty is reasonable. For staying up through a real outage, you are into generator territory and a UPS is the bridge to it rather than the answer.
- Why does my UPS last far less than the rated runtime?
- Two usual reasons. The rating is quoted at a fraction of full load and you are running nearer capacity, and runtime falls faster than load rises. Or the batteries are simply old — capacity drops steadily from about the third year, and a unit with tired batteries still shows a healthy panel until it is asked to do the job.
- What should be on the UPS in a comms room?
- Whatever has to keep working, followed all the way through. For phones to keep working that means the switch feeding them, the router, and the internet circuit's own equipment. Protecting some of the chain and not the rest gives you a rack that is powered and a service that is down.
- How often do UPS batteries need replacing?
- Sealed lead-acid typically three to five years, and towards the shorter end in a warm room — heat is the main thing that ages them. Lithium units run considerably longer. Either way, put it in a calendar and load-test rather than relying on a self-test light, because the failure mode is silent.
Other guides
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