A ton of refrigeration is defined as the rate of heat transfer that results in the melting of 1 short ton (2,000 Pound or 907 kg) of pure ice at 0 C (32 F) in 24 hours. The value is defined as 12,000 BTU per hour, or 3517 watts.
What is BTU?
BTU (British thermal unit) is defined as the amount of heat required to raise the temperature of 1 pound of liquid water by 1 degree Fahrenheit at a constant pressure of 1 atmosphere).
Following table provides details of energy calculation details in case of 1.5 Ton and 2 Ton AC.
1 Ton AC = 12000 BTU/hr  1 Ton AC = 3517 Watts* (approx.) 



Here unit Watt* is not the Rated electrical power consumption in watt. But it is the rate at which work is done or energy is transmitted. Watt* = joules/second
SEERThe efficiency of air conditioners is often (but not always) rated by the seasonal energy efficiency ratio (SEER). The higher the SEER rating, the more energy efficient is the air conditioner. The SEER rating is the BTU of cooling output during its normal annual usage divided by the total electric energy input in watt hours (Wh) during the same period. In India it is consider as ISEER (Indian Seasonal Energy Efficiency Ratio).
Lets take example of following AC with various parameters associated with it for purpose of EER calculations
Model Description  Rated EER  Capacity(Ton)  Rated Cooling Capacity (W)  Rated Power Consumption (W)  Star Rating 

Voltas SAC 185 MY  3.51  1.5T  5050  1438  5 
EER Energy efficiency ratio is measured only one at peak cooling time and maximum outside temperature.
EER = Rated Cooling Capacity (W) / Rated Power Consumption (W)
Here, Voltas SAC 185 MY, Rated Cooling Capacity (W) is 5050 instead of 5275.5 watts for 1Ton AC
EER = 5050 / 1438 = 3.51 w/w
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