Any time you change the equipment onboard your ship, go to your systems panel tab along to modules and check power group 1 is below 100%, set weapons to group two, then put things like FSD interdictor, fuel scoop cargo hatch etc to priority 3. Then test this in action before going out on your travels. Best way to test power management after a change of ships configuration is to launch from the station where you did the outfitting and fly 10km out from the station to leave the no fire zone, then deploy hardpoints and fire weapons for a bit to make sure your ship doesnt black out...
Its all too easy to get a rake of credits and upgrade a few modules and discover you are now beyond your powerplants capability either in terms of MW of power or thermal capacity. The result of going over on power is disastrous, thrusters and shields go offline, life support goes to back up oxygen, fsd offline, in short you are a sitting duck. Obviously its better to discover this in a controlled environment a few km out from the station with no hostile ships present than it would be to discover this when you deploy hardpoints after getting interdicted for the first time with the new load out.
The result of going over on thermal load is bad but not disastrous, you'll find after a few seconds firing and manouvering that your temperature is critical, so you need to stop firing and let the ship cool down, and resume fire and stop to let it cool again and again... Or you can launch heatsinks, up to the amount you are carrying. I often find a ship that is running too hot to sustain fire is less effective in combat than the same ship with a cooler running load out, changes such as substituting beam lasers with pulse lasers for reduces thermal and electrical load.....