Press 4 to access your right panel -> Choose the modules panel -> make sure that your power usage is not exceeding your power output( the percentage bar at the bottom with percentage and numbers 1 2 ).
So if you figure out that you dont have enough power, try to reduce it with several ways
- better power plant
- power management. If you notice it in a modules tab, there is a priority number. this number represents the order of shutting down of modules if your power plant is damaged or just not have enough megawatzz. Perhaps you should turn off your cargohatch, since you are not using it while the weapons are deployed. also you can lower the priority of FSD and FSD interdictor, so they will turn off automaticly when you deploy you weapins and the power usage jumps up.
- try another equipment. for example the high grade warrant scanners take quite a lot of power. there is not much need for giving a lot of power to this, so use the cheap ones. same with other equipment like shields and such.
In this game you cant really put anything you want on a ship, so you need to choose... or engeneer your A powerplant to crazy limits)