Question-and-Answer Resource for the Building Energy Modeling Community
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This answer is based on the v8.2.0 example file RefBldgFullServiceRestaurantNew2004_Chicago. EnergyPlus sizes supply airflow rates to meet the peak zone sensible load (for this model cooling will set the airflow) and the sizing factor is applied to this flow rate. Then the coil is sized to meet the specified cooling supply air temperature at that flow rate at the design mixed air conditions. All of this sizing is based on the SizingPeriod:DesignDay conditions which in this case is 31.1C (88.2F) outdoor dry-bulb.
The reported rated (nominal) total cooling capacities for the two DX coils are 68.82kW and 18.19kW. With COPs of 3.23 and 3.67, this results in a rated total power input of 26.26kW. The power input for a DX coil at any point during the simulation is (RatedCapacity/RatedCOP)CapfTCapfFlowFracEIRfTEIRfFlowFrac*(PartLoadRatio/PLFCurve). All of the curve values can be reported as output:variable "Performance Curve Output Value". For an annual simulation with Chicago O'Hare TMY3, the peak hour cooling electricity is 26.65kW which is about 1.4% higher than rated.
The peak cooling electriicity occurs on July 19 16:00, with an outdoor dry-bulb temp of 34.3C (93.7F) which is well above the design day condition, but still below the rating conditions of 35C (95F). So, if you look at the curves, they are showing CapfT >1.0, EIRfT <1.0, which multiplied together give a total power draw for coil1 of 0.99, and coil2 1.03. Because this day in the TMY3 file is hotter than the cooling design day, the extra capacity is being put to use, so there is no discount for the 1.2 oversizing.
