Your server room got denser
One machine can undo the sizing of a whole room. A rack that drew four kilowatts for a decade is being replaced by one drawing thirty, and the UPS in the corner was specified against the old number.
This is not a hyperscale problem. It reaches a manufacturer with one server room the day somebody buys a GPU box for inspection imaging or model training, racks it beside the file server, and connects it to the UPS that has run the room since 2019.
The number that changed
Conventional racks drew three to five kilowatts. GPU-dense racks draw thirty to fifty and beyond. A single such machine can exceed the entire original design load of a small server room, and it arrives as a purchase order from a department that has no reason to consult anyone about the electrical supply.
The UPS does not fail dramatically. It runs closer to its limit, its runtime falls because the same bank is supplying a larger load, and its efficiency drops because it is now operating at a load point it was not sized for.
Runtime is the first thing to go, quietly
Backup time falls faster than the load rises, because battery capacity derates with discharge rate. A bank giving thirty minutes at 40% load will not give fifteen at 80% — it will give closer to ten, because pulling harder extracts less total energy from the same cells.
Nobody notices until an outage lasts longer than the new, shorter runtime. The UPS reports healthy throughout, because it is.
| Before | After | |
|---|---|---|
| Room load | ~8 kW across four racks | ~35 kW with one GPU rack added |
| UPS load point | 25% of a 40 kVA machine | Near capacity |
| Runtime on the same bank | 30 minutes | Under 10, from Peukert derating |
| Heat into the room | ~8 kW | ~35 kW — usually the real constraint |
Cooling fails before the UPS does
Thirty-five kilowatts of IT load is thirty-five kilowatts of heat in a room that was cooled for eight. In a Pune summer, a room designed around a two-tonne split unit will not hold temperature, and the first equipment to suffer is usually the UPS battery, which sits in the same room and loses life at roughly half per eight to ten degrees above 25 °C.
The dense rack shortens the life of the bank protecting it, through the room rather than through the load.
The check worth doing today
Read the load percentage on the UPS display and compare it against the last discharge test. If the load has risen materially since that test, the runtime figure you are relying on is fiction. A timed discharge at the present load is the only thing that replaces it.
What to do, in order of cost
- Measure the room load now, not from the original schedule
- Run a timed discharge at the present load and write the real number down
- Check the room ambient at the hottest hour, not the average
- Extend the bank if runtime is the gap and the machine has headroom
- Replace or parallel the UPS only if the load point genuinely exceeds it
Frequently the machine is fine and the bank is not. Replacing a UPS that is running at 70% because the runtime dropped is a common and expensive misdiagnosis — the runtime dropped because the bank is the same size it always was.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
How much UPS capacity does a GPU server need?
Size it on measured draw, not the nameplate — GPU loads swing widely between idle and full utilisation, and the nameplate reflects a maximum most workloads never reach. Clamp the actual draw under a representative load for a working day, then add headroom. A single dense rack can exceed the entire original design load of a small server room.
Why did my backup time drop when I added one server?
Because battery capacity derates with discharge rate. Pulling harder extracts less total energy from the same cells, so a bank giving thirty minutes at 40% load gives closer to ten at 80%, not fifteen. The UPS reports healthy throughout, because nothing is wrong with it — the bank is simply the size it always was.
Do I need a bigger UPS or a bigger battery bank?
Different problems. If the load exceeds what the machine can carry, you need a bigger UPS. If the machine copes but the runtime is too short, you need a bigger bank — and that is the far more common case. Check the load percentage on the display before assuming which one you are looking at.
Does higher rack density affect the battery room?
Yes, and it is usually overlooked. The extra load becomes heat in the same room, and VRLA life halves for roughly every eight to ten degrees above 25 °C. A dense rack can shorten the life of the bank protecting it through the room temperature alone, without ever overloading the machine.