at which pressure ACH is reported in EP?

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(It sounds like you are using the ZoneInfiltration:EffectiveLeakageArea object, as opposed toAirflowNetwork. In that case, EnergyPlus does not explicitly calculate a pressure for each timestep. Rather, it directly calculates infiltration volumetric flow rates (m3/s) from your user inputs, wind speeds, and outdoor temperatures. The ACH output that it also provides is simply converted from the volumetric flow rate and the zone volume.)

So to your question, the calculated flow rates would correspond to a pressure closer to 4 Pa (often used as an assumed annual-average natural pressure) as opposed to 50 Pa (the pressure during a blower door test). I think you are correctly interpreting the output. Hard to say why your results are off by a factor of ~5, you should double check the zone volume, height, terrain, stack/wind coefficients, etc. You should probably also output the infiltration as volumetric flow rate directly. If you still can't figure it out, you might need to provide your IDF file for someone to take a look.