Techniques are described that allow an implantable device to sense activity data from a patient and estimate the patient's amount of energy expended based on the sensed data. The implantable device may alternatively determine an amount of time the patient has been active based on the activity data. For example, the implantable device may be an implantable gastric stimulator. A system may provide feedback to the patient, a family member, or a doctor about the patient's activity or energy expenditure. The data may be provided in table or graphical format, and may show daily or weekly energy data or may show a trend of the daily or weekly energy data.