How the calculation works
Every source is converted to BTU per hour (1 W = 3.41214 BTU/hr):
- IT equipment = IT kW × 1,000 × 3.41214. The power IT equipment draws becomes heat in the room.
- UPS losses = IT watts × (100 ÷ UPS efficiency % − 1) × 3.41214, only when the UPS is in the room. A UPS delivering 8 kW at 94% efficiency draws 8.51 kW; the 0.51 kW difference is heat.
- Lighting = lighting watts × 3.41214.
- People = people × BTU/hr per person. The default of 450 BTU/hr is a planning assumption for seated or light work, not a measured figure; the ASHRAE Handbook—Fundamentals gives figures by activity.
- Other heat = an allowance you enter for heat through walls, windows and the roof, sun, and outside air. The calculator does not model the building.
- Cooling to plan = total × (1 + margin %). Tons = BTU/hr ÷ 12,000.
AC units. Units for the load (N) = cooling to plan ÷ one unit’s capacity, rounded up; N+1 adds one. IT load one unit supports = (unit capacity ÷ (1 + margin) − lighting, people and other heat) ÷ (3,412.14 × (1 + UPS losses as a share of the IT load)).
Limits. This sizes the cooling for the heat load. It doesn’t check airflow, where the cold air goes or how hot air returns, which decide whether racks stay cool in practice. Compare the result with the unit’s sensible capacity at your room temperature and humidity, and have an HVAC contractor confirm anything with outside walls, windows or a roof above. To convert one figure without the room, use watts to BTU.
Worked example
A server room with 8 kW of IT, the UPS in the room at 94% efficiency, 300 W of lighting and two people working there most of the day, with a 20% margin and 2-ton units in N+1:
| Source | BTU/hr |
|---|---|
| IT: 8,000 W × 3.41214 | 27,297 |
| UPS losses: 8,000 × (100 ÷ 94 − 1) = 510.6 W × 3.41214 | 1,742 |
| Lighting: 300 W × 3.41214 | 1,024 |
| People: 2 × 450 | 900 |
| Total | 30,963 (2.58 tons) |
| With 20% margin | 37,156 (3.1 tons) |
- Units: 37,156 ÷ 24,000 = 1.55, so 2 units carry the load and 3 are installed for N+1.
- One 2-ton unit supports (24,000 ÷ 1.2 − 1,924) ÷ (3,412.14 × 1.0638) = 4.98 kW of IT; the two working units support 10.49 kW.
Why server closets overheat
Small rooms have little air to absorb heat, so the temperature climbs within minutes of the cooling stopping. Closets often have no return path for hot air, racks recirculate their own exhaust, and office systems cycle off after hours. A room that has grown one server at a time may be well past what its cooling was sized for. If the heat load keeps growing, compare the cost of more cooling, power and UPS capacity with moving the equipment to colocation.
