The Standard Penetration Test (SPT) is the most popular example of dynamic probing, a large category of soil testing techniques. Understanding and interpretation of these tests is hampered by the difficulties of reproducing them under controlled laboratory conditions. The virtual calibration chamber technique, based on the Discrete Element Method (DEM), may supplement or substitute this complex experimentation. In this paper SPT in sand are analyzed considering the energy transfer involved. Energy balances are written for the penetrating rod and for the material in the chamber. All the terms are computed for a number of cases in which the main variables controlling test response in the field -initial density and stress level- are systematically varied. The analysis confirms previous field observations indicating that, when an energy-based interpretation is used, SPT provides a value of equivalent penetration resistance that is the same that would be obtained with a static cone penetration test. The analyses also provide an unequivocal explanation for this observation: although the impacting rod shows complicated dynamics the response of the sand is quasi-static.