All power supplies, whether in desktops or servers, are on average approximately 60%-80% efficient. This implies that say a 200 watt maximum output PSU would consume 333 watts of input power to supply these 200 watts on full load.

Input power: The power in Watts that is fed from the mains into a PSU. The maximum a power supply can take is often expressed as a Voltage (Volts) at a certain Current (Amps), multiplying these gives the maximum input power in Watts. See Server PSU articles for individual range specs.

Output power: The maximum power in Watts that can be drawn from the PSU by the server mainboard, hard drives, CPU, memory, etc.

For example: The 450 watts redundant PSU in SystemBase AXIII has a maximum input power of 200V @ 3.28Amps = 656 watts consumed maximum (i.e. 69% efficient).

For the UK mains 230V AC @ 3.28A gives you 754 watts and therefore 60% efficiency. i.e. the very maximum power this server will consume under 100% load will be 754 watts. Realistically, the maximum power under normal operating conditions will be around 650 watts. 

Heat output is measured in British Thermal Units (BTUs) - 1kw/hr = 3412 BTU/h or 1000 BTU/h is around 293W.

Based on the 754 watt figure, BTUs/hr = (754 watts * 3600) / 1054 = 2577 BTUs per hour, per server.

When calculating the amount of cooling (air conditioning) required in a room, you should take into account that the room should always be maintained at an ambient temperature of much less than 35 degree Celsius and that any other equipment in the room will also generate heat.

Officially, Intel recommends the following for the one SystemBase AXIII chassis (in a rack):

Important: For ventilation, the equipment rack must provide sufficient airflow to the front of the server to maintain proper cooling. It must also include ventilation sufficient to exhaust a maximum of 4,100 BTUs per hour for the server. The rack selected and the ventilation provided must be suitable to the environment in which the server will be used.

Web-based conversion calculators can be easily found (e.g. http://www.anver.com/document/company/Reference%20Guides/power.html).

There is a linear increase in heat output for multiple servers.

In the example above, each server would need cooling for a minimum of 2600BTU and a maximum of 4100BTU.

RM servers are normally rated up to a maximum ambient temperature of 35 degree Celsius. If the room temperature rises above this level, it may lead to premature hard drive failure, CPU thermal shutdown or data loss. It will also invalidate any warranty.

If possible, an ambient room temperature of approximately 20 degree Celsius is recommended.

Note: Most modern hard drives record the hard drive temperature in an internal log which can be examined at a later date by an engineer.