Discharge base power: 80
+ Charge: 80 x2
= 160
+ STAB: 160 x1.5
= 240
+ Gem: 240 x1.5
= 360
Without Charge:
Discharge base power: 80
+ STAB: 80 x1.5
= 120
+ Gem: 120 x1.5
= 180
I added working out in case I got it wrong. This feels like a maths class. x_o
Note: this is just theoretical. I hope you're not planning to use Charge in a battle, since it makes it obvious you will be using an Electric move next, thereby alerting your opponent to switch Pokemon.