Calculating Air Conditioner

Heat Load
The amount of heat generated is known as the heat gain or heat load. Heat is measured in either British Thermal Units (BTU) or Kilowatts (KW). 1KW is equivalent to 3412BTUs.
The heat load depends on a number of factors, by taking into account those that apply in your circumstances and adding them together a reasonably accurate measure of the total heat can be calculated.

 Heating, Ventilation and Air Conditioning

Factors include: 

• The floor area of the room
• The size and position of windows, and whether they have blinds or shades
• The number of room occupants (if any)
• The heat generated by equipment
• The heat generated by lighting

Floor Area of Room
The amount of cooling required depends on the area of the room. To calculate the area in square metres:

Room Area BTU = Length (m) x Width (m) x 337
Window Size and Position
If, your room has no windows, you can ignore this part of the calculation. If, however there are windows you need to take the size and orientation into account.
South Window BTU = South Facing window Length (m) x Width (m) x 870
North Window BTU = North Facing windows Length (m) x Width (m) x 165
If there are no blinds on the windows multiply the result(s) by 1.5.
Add together all the BTUs for the windows.


Windows BTU = South Window(s) BTU + North Window(s) BTU

Occupants
You will have to take that into account people who normally working in the space. The heat output is around 400 BTU per person.

Total Occupant BTU = Number of occupants x 400
Equipment
Clearly most heat in a room is generated by the equipment. This is trickier to calculate that you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is probably safer to overestimate the wattage than underestimate it.


Equipment BTU = Total wattage for all equipment x 3.5

Lighting
Take the total wattage of the lighting and multiply by 4.25.

Lighting BTU = Total wattage for all lighting x 4.25
Total Cooling Required
Add all the BTUs together.


Total Heat Load = Room Area BTU + Windows BTU + Total Occupant BTU + Equipment BTU + Lighting BTU
 
This is the amount of cooling required so you need one or more air conditioning units to handle that amount of heat.