Uninterruptible power supplies convert some input power into heat through electrical losses, so UPS plant is part of the server room cooling load even when IT racks are the visible heat source. Designers should include UPS heat at realistic load factors, consider where that heat is released, and check airflow so UPS intakes are not starved. Cooling capacity based only on IT nameplates without UPS and distribution losses understates the continuous process load.
Count UPS losses as continuous heat
A UPS delivers protected power to IT equipment but is not perfectly efficient. The difference between input and output power appears mainly as heat in the UPS modules and associated transformers or distribution gear. That heat is present whenever the UPS is energised and loaded.
Efficiency varies with load fraction and topology. A UPS running lightly loaded can be less efficient than the same unit near its preferred operating band. Use manufacturer efficiency data at the expected operating point, supplemented by measured electrical values where available.
- Include UPS rooms or in-row UPS cabinets in the heat inventory.
- Add power distribution unit and transformer losses where relevant.
- Design for normal operating load, not only empty-room conditions.
- Revisit figures after UPS replacement or IT growth.
Locate heat rejection and airflow paths
Some UPS systems reject heat into the server room; others sit in a dedicated plant space. If both spaces share cooling plant, confirm the combined load. If the UPS room relies on separate comfort cooling, check that this plant runs whenever the UPS is loaded, including nights and weekends.
UPS equipment needs clear intake and exhaust paths. Storing boxes around UPS cabinets or placing them in dead air pockets creates local overheating that room averages will miss.
Coordinate cooling with power resilience
Cooling that depends entirely on unprotected power can fail precisely when the UPS is supporting IT through a mains outage, unless generators or another arrangement restart cooling quickly enough. Uptime Institute guidance on higher-resilience facilities emphasises stable thermal conditions through infrastructure events, not only redundant IT power.
For smaller UK rooms, at least estimate temperature rise during a cooling restart delay and decide whether shedable IT load, larger thermal mass, or backup power for cooling fans and pumps is required. Document the accepted risk if cooling is not on backed-up supply.
Feed accurate loads into the cooling calculation
A robust design basis lists IT load, UPS losses, lighting, people and any other continuous gains, then applies diversity and growth with stated assumptions. Oversizing from inflated nameplates can cause poor part-load behaviour, while ignoring UPS heat causes chronic high return temperatures.
After installation, validate with measured power and inlet temperatures. Trend UPS load against room heat removal during commissioning so operators know what normal looks like before an alarm arrives.
Frequently asked questions
Does a more efficient UPS reduce cooling need?
Yes, lower losses mean less heat to remove at the same IT output, but the UPS heat term should still be calculated and not assumed to be zero.
Can UPS heat be ignored if the UPS is outside the server room?
It can be omitted from the server room total only if that heat is truly rejected elsewhere and the UPS space has its own adequate continuous cooling strategy.
Should cooling units be on UPS power?
It depends on autonomy, UPS capacity and business risk. Cooling on UPS can exhaust battery stores quickly. Many designs prefer generator-backed cooling with a clear ride-through plan instead. Assess the specific site.
What data should I give a cooling designer?
Measured or expected IT load, UPS type and efficiency at that load, equipment locations, growth plans and the required response to mains failure.
Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.