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.