Use Outputs->To Be Saved->Select on Schematic and select the terminal of the capacitor connected to the output net. This will then save the current through the terminal of the capacitor - I'm assuming that the voltage on the net will be saved anyway (either by saving all voltages, or by you explicitly saving it).

After running the simulation, on the direct plot form, select power. Then choose "Select: Terminal" and select the capacitor terminal as prompted. You could also choose "Select: Instance with 2 terminals" and then select the capacitor instance as prompted.

In essence you need to save both the current and the voltage, and then the power can be computed. With a port or a resistive load, you can get away with only saving voltage, as the power can be computed from the voltage and the resistance of the load; with more complex loads, you need to measure both the voltage and the current in order to compute the power.

Regards,

Andrew.