Calculate the kilowatts per BTUs of the air conditioning unit. This is the unit's EER, or energy efficiency ratio.
It is calculated by dividing the unit's BTUs per hour (cooling capacity) by the wattage used per hour (energy usage).
Measure the square footage of the area to be cooled. Often this information can be found on blueprints or lease agreements.
Otherwise, use a surveyor's wheel.
Multiply the square footage to get the base number of BTUs needed for the space. Since there is no single variable to use in the equation, this can be tricky.
In general, larger areas of 700 to 1000 square feet are multiplied by 18 to 20. Smaller areas of 100 to 150 square feet are multiplied by as much as 33.
Refer to your contractor for the correct variable for your space.
Divide the number of BTUs needed for the building by the EER of the unit. This gives you the number of kilowatt hours needed to reduce the temperature of the building one degree.
Multiply the kilowatt hours needed by the cost of kilowatt hours in your area.