heat pump for domestic hot water in openstudio

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Hello all I simulated a heat pump for domestic hot water (DHW) in a single-family house in Italy using OpenStudio software. After running the simulation, I noticed that the calculated size of the heat pump is significantly larger than the real-world size typically required for this application.

What could be the potential reasons for this discrepancy? Which parameters in the simulation should I pay close attention to in order to ensure the heat pump's size is accurately calculated?