We played through that last night. We didn't have to count shots, but I basically said that the militia got close to running out of "ammo" for the lasers after a 2-day trek by the players (to rescue the general).
So, it was thoroughly abstracted. But, it gave them a reason to go secure the power plant, which is what that section of the adventure was supposed to do.
So, it all worked out.
By the way, my players have a working ship, so they're not stuck like the players in the adventure. I'm appealing to their basically good nature to keep them there. I've convinced them that if they leave, the whole planet will be wiped out. And, they're rising to the occasion.
But, with a working ship, they can charge the lasers as often as needed. So, it's only for when the players take their ship on an excursion that the power plant is needed. But, they're being far more clever than the adventure seems to have expected. So, it's working out.