There is one awesome combined way about recharging any energy weapons, which is universal for all type of guns (pistol-smg) and technologies (from lasers to any plasmas).
1 - installed sun battery, light but requiring open place and sun (no roof and smoke around), and highly limited recharge per turn anyway (2-3 turns for one smg burst for example, or 1 snipershot);
2 - backpack generators mk1 mk2 mk3 etc... Requiring strenght, slots, advansed technologies and strictly recommended for powerful weapon.
So you just need to define power expense for all kind of shots for all kind of weapons, and define recharge rate for advanced generators
Imo, it's a great way to give more choice to player tactics and explain why it is not a good idea to take only laser-smg to night missions.
Never forget about good old bullets!