ModulesOutfittingUser InterfaceWeapons
At right user interface you can set any priority from 1 to 5 to any module - 1 being the most important and 5 being the least important. That means if there is not enough power then first modules with priority 5 will be shut down, then 4, 3, 2 and modules with priority 1 will be shut down last.
By not setting any priorities even if you exceed your total power output by 0,1% ALL modules will go offline. So having all modules at same priority doesn't make any sense.
Most important modules are ones that will make your ship inoperable or limit your options - in regular flight those would be thrusters, life support, sensors, shields, power distributor etc., while in super-cruise frame shift drive, fuel scoop, sensors, discovery scanners etc.
You can't use frame shift drive in regular flight while your hardpoints are deployed and you can have extra shield booster or better weapon instead, same goes for fuel scoop - you can't scoop in regular flight, so always use very low (4-5) priority for Frame shift Drive, Fuel Scoop, Cargo Hatch and other modules that are meant to be used only in super-cruise.
You can set priorities that way that when your hardpoints are retracted shield cell banks come online, but more practical way is to set higher priority to shield cell bank than to weapons and disable it, when you want to charge shields just enter right user interface and enable shield cell bank. By doing this some of your weapons automatically will go offline but you can use shield cell bank. When shield charge is finished you open right user interface again and disable shield cell bank module and by doing that previously disabled weapons automatically will come back online.
When you start the fight you care more about your and enemy shields, so you can disable weapons that you plan on using against enemy hull only and thus have Kill warrant Scanner, Shield Cell Bank or even extra Shield booster enabled for time being and when enemy loses shields then you can switch on that weapons and Kill warrant Scanner, Shield Cell Bank or Shield booster will go offline but you do not need them that much at this point.
This is especially important on ships with limited power plant output which are most fighting ships like Viper, Vulture, Fer-de-Lance. Multi purpose ships usually have big enough power plant for most stuff, but even from Python or Anaconda you can squeeze out extra power by setting priorities, just think which modules you do not need when you are fighting and set them to low priority or even disable (like cargo hatch).