Power = (energy) / (time)
0.5 W = (energy) / (0.25 sec)
Multiply each side by (0.25 sec):
Energy = (0.5 W) x (0.25 sec)
Energy = 0.125 W-sec
Energy = 0.125 Joule
We can ignore the Christmas trinket, the RC discharge, the capacitor, the resistor, and the voltage. We don't need any of them in order to calculate the energy of each flash. They're only there to confuse us, distract us, and see whether we're paying attention.