– EER - Energy Efficiency Ratio
The Energy Efficiency Ratio (EER) is a key measurement of the energy performance of air conditioning equipment in cooling mode. It is the ratio of the output cooling energy produced vs. the input electrical energy used during full load operation.
The higher the EER, the more efficient the air conditioning equipment is in cooling mode, which means that it consumes less electricity to deliver the same amount of cooling to the room. This is important when choosing an air conditioner, as models with a lower EER will result in higher electricity bills.
It is calculated as the ratio of cooling energy output vs. the electrical energy input, based on 2 fixed conditions: (1) the equipment is operating at full load (which means it is operating at maximum cooling capacity), (2) it is measured under fixed ambient conditions (35 °C / 95 °F outside air temperature, 27 °C / 80 °F inside air temperature and 50% relative humidity).
In reality, air conditioning equipment rarely operates at full load, and is generally less energy efficient when running at part load - especially important in the context of larger (normally commercial) buildings. This is one reason why Hitachi have developed SmoothDrive technology to improve real-world energy efficiency under part load conditions.
An alternative EER measurement is SEER (Seasonal Energy Efficiency Ratio) which tries to address the issue about load calculation by combining different load factors for different proportions of time - somewhat replicating seasonality. The precise calculation combines data when the AC equipment is running at 100% (full) load for a certain proportion of the time, and then also lesser load factors (most commonly 75%, 50% and 25%) for different proportions of the time. The exact load factors, ambient (outdoor temperatures) and indoor (set temperatures) vary slightly by country or region. Although it is still a theoretical calculation, for most situations it probably reflects a more realistic comparison of energy efficiency between different units.
In summary, EER is not a perfect measurement of energy efficiency, but it is a good relative measure for comparing different pieces of air conditioning equipment under fixed conditions.