Two distinct constraints, one room
A UPS room has to satisfy two unrelated thermal-management constraints. Heat extraction means removing the kW of waste heat that the UPS and battery charger put into the room every second; failing means the room temperature climbs until something shuts down on over-temp. Hydrogen ventilation applies only to lead-acid (VRLA and flooded) batteries, which evolve hydrogen during charging; failing means a potentially explosive atmosphere develops in the worst case. Sealed lithium chemistries don't need the second check.
Where the heat comes from
UPS heat is its electrical loss: P_loss = P_load × (1/η − 1). A 94%-efficient UPS at 20 kW load dissipates ~1.3 kW continuously. Battery float charging adds another few hundred watts to a few kW depending on capacity and chemistry. Total heat scales with UPS loading, not nameplate — under-loaded UPS rooms often surprise dealers with how cool they stay.
IEC 62485-2 ventilation formula
For lead-acid batteries, IEC 62485-2 (and EN 50272-2) specifies the airflow needed to keep hydrogen below 4 % of the lower explosive limit: Q = v × q × s × n × I_gas × C_rt × 10⁻³, which the standard collapses to Q = 0.05 × n × I_gas × C_rt × 10⁻³ (m³/h) — n cells,I_gas in mA per Ah from the standard's table (VRLA float 1, flooded float 5, VRLA boost 8, flooded boost 20), and C_rt the C10 capacity in Ah. For natural ventilation the inlet and outlet opening area is A = 28 × Q (cm²); for forced ventilation the fan must move at least Q of fresh air to the outside. The standard does not set an “air changes per hour” criterion — an earlier version of this tool implied one, and it has been removed. The charge-mode selector matters: if the charger can ever enter boost, the ventilation must cover it.
Sizing the cooling unit
Cooling capacity is usually expressed in kW (SI), BTU/hr (US), or refrigeration tons (HVAC convention: 1 ton = 3.5168 kW). For small UPS rooms (under 5 kW heat load), a domestic split AC is usually adequate. Above that, look at dedicated computer-room AC (CRAC) units with N+1 redundancy. Always add a design margin of 15–25% for duct losses, future expansion, and worst-case ambient days.