Power is one of the primary budgets for your satellite, and is a hard limit that must be obeyed. If you exceed your bandwidth budget, you lose some (but not all data), but if you exceed your power budget, your entire satellite cannot function.

Fortunately, the calculations for your power budget are relatively straightforward. Solar cells will charge the batteries so they are X full on average, with a minimum power of Y (during dark periods, at maximum predicted discharge). Your satellite will use Z power.