Air Conditioner Cooling Load and BTU Calculator (Rough Sizing)

How is air conditioner capacity estimated?

The cooling load of a room is the sum of the heat that enters and the heat produced inside. A real calculation finds walls, roof, glazing, ventilation and internal gains separately. For rough sizing the room area is multiplied by a unit load and the obvious internal gains are added: Q = (A × q × h / 2.7 + N × q_k + P_c + A_c × q_c) × (1 + p).

Here A is room area, q the load per unit area, h the ceiling height, N the number of occupants, q_k the heat per person, P_c the equipment power, A_c and q_c the glazing area and its solar gain, and p the safety margin.

How BTU relates to kW

Air conditioner capacity is often quoted in BTU/h. 1 kW = 3412 BTU/h, so 12 000 BTU/h is about 3.5 kW. This is cooling capacity, not the electrical power the unit draws.

Example

A 20 m² room with a 2.7 m ceiling at 120 W/m² has a room share of 2400 W. Two occupants add 240 W and equipment adds 300 W. With a 10 percent margin the total is 3234 W, about 11 000 BTU/h. The next common size up is a 12 000 BTU/h unit.

Limits of this method

The result gives an idea, not a design value. Load per unit area is a rule of thumb and varies widely with climate, facade and insulation. For top floors, living rooms with large windows, kitchens, shops and offices have a heat gain calculation done. An oversized unit also cycles on and off, removes moisture poorly and reduces comfort.