EnergyPlus model vs raw calculation for AHU cooling

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I'm trying to compare the energy outputs of cooling air in a 100% Outdoor AHU connected to an air cooled chiller without reheat. I'm trying to compare a simplistic enthlpy calculation where i use the dry bulb RH and set-point to get the enthalpy difference and than divide it by the COP which is assumed to be at constant value of 3.1.

I found a 20% difference from the EnergyPlus output when loking at the yearly consumption in kwh, where my calculation is lower. In both models i assumed a set-point of 18C, constant CFM and use the same weather file (mainly dry ball temp and RH values)

I can also tell that the enthalpy calculation is more sensitive to change at high temperatures and is less accurate there. and that in low temperatures i get a result I'm trying to think where is this change in results is coming from, maybe from using a constant cop? but 20% is a lot i would expect less for a yearly consumption

. anyway any insights will be welcomed

I'm adding some pictures compering the models, this is a general yearly view.

this one shows behavior in high temperatures:

and this one is for low temperatures