Measuring performance (EER or COP) for a modulated system is a very challenging task.  For a fixed speed a/c system, performance is measured as EER or COP.  To calculate EER or COP, the a/c equipment has to be tested in a psychrometric room as per the ARI standard.  The standard cooling test for ARI specifies that the outdoor should be maintained at 35°C and indoor should be maintained at 27°C dry bulb and 19°C wet bulb.  Under such operating conditions, the cooling capacity and input power is measured.  Dividing the cooling capacity by input power gives the EER of the machine.  As can be seen, this is a fairly straight forward measurement as only one operating point is involved. 

A capacity modulated system can operate at various levels of cooling capacity.  Depending on the compressor technology used, the cooling capacity can range from 10% of full load capacity to 100% capacity.  Given the wide operating range, the questions would be:  At which point do you measure the cooling capacity?  Should we select the 100% capacity point, the 50% capacity point or any other point?  Similarly, at what ambient should the machine be tested?  Since capacity modulated systems are sold on the value of seasonal efficiency, most of the times they operate at ambient that are much cooler than the 35°C (as used in the single point EER measurement).

Integrated Part Load Value (IPLV)

To ensure consistency of measurement, the ARI 210/240 method of calculating IPLV is most appropriate.  As per this standard, EER at 4 capacity points are measured – 25% of full load, 50% of full load, 75% of full load and the full load point.  The ambient is kept at 27°C and the indoor is also kept at 27°C.  Once the EER at the 4 points have been measured, a weighted average formula is applied to calculate and get the IPLV.  The chart below shows the weights for each point.
(Click image to enlarge)

The Digital Scroll technology has a very high IPLV number (3.56) and it has been tested in an approved government laboratory in China.  The attached chart shows the capacity versus EER curve and the calculation of the IPLV (Cooling) and IPLV (Heating).

Other Rating Systems

European Seasonal Energy Efficiency Ratio - Europe

Similar to IPLV, ESEER is a weighted formula taking into account the variation of EER with the load requirement and change in ambient temperature.

The ESEER calculates a unit's yearly energy efficiency based, in the case of an air cooled packaged chiller at nominal conditions of 7 / 12°C water, 35°C ambient, on a small range of ambient temperatures. The higher the ESEER, the more efficient is the cooling system.

The calculation equation is:
ESEER = A*EER100% + B*EER75% +C*EER50%+D*EER25%

Integrated Energy Efficiency Ration  - US

By January 1 2010, IEER supersedes IPLV metrics. This new standard based on AHRI 340 / 360 is a significant improvement over IPLV as it follows for uniform rating of all product including single and multi-stage units.