Earlier the energy efficiency ratings (STAR ratings) were given by Bureau of Energy Efficiency (BEE) Ministry of power Government of India based on simple energy efficiency ratio:
Energy Efficiency Ratio = Cooling capacity / Electrical consumption
This was good enough for the Fixed Speed Air conditioners but with entry of Inverter technology in the AC market the above simple calculation would not work!
Reason? The Inverter air conditioners do not operate at fixed speeds, the output capacity (cooling) and input (Electrical energy) requirement varies continuously based on the ambient temperatures and user desired (Set) temperature.
Inverter ACs operates from 100% capacity at the startup to 0% capacity when the desired temperature is reached and continues to operate with required adjusted capacity as per the heat load.
To rate these type of AC equipment the system of Seasonal Energy Efficiency Ratio was introduced by BEE
SEER is calculated on annual temperature data across a country thus it would be different for different countries.
The ISEER calculations are based on Indian temperature conditions of numerous cities, with an assumption that the AC equipment is used for 8 hours a day for 200 days in a year.
The ISEEER is calculated as a ratio
Total amount of heat removed by the AC equipment annually / Total Electrical Energy consumed by the AC equipment annually
Presently BEE continues to provide BEE star ratings for the Fixed Speed Air conditioners and ISEER star ratings for the inverter type Air conditioners
Practical experience suggests Inverter ACs even with ISEER 2 or 3 stars would be more efficient than BEE 5 Star rated fixed Air conditioners