I have around 450 GJ on my capacitator. Theres no module currently on it but the second I fit in a small armor repair, which takes about 40 GJ, my capacitator goes red? I still haven’t even fit in my weapons yet. Why is that. My ship is a small frigate.
What that display is telling you is how long your capacitor can run, with everything running all the time.
As you said the small armor repair module takes 40GJ, but it cycles every 6 seconds. Your capacitor is constantly refilling, but it’s possible that modules will take capacitor faster than you can generate. This is what the system is saying here, there should be a readout saying how long the capacitor will last.
Now this is not a bad thing. It’s fairly rare at the start especially to have a ship that is cap stable as we call it, especially when using repair modules. To minimise this there are a couple of solutions. Firstly only run the modules when you need them. If your ship has full armor there is no need to run the armor repair module. You can also run it in bursts as and when you need it to save capacitor.
Secondly look how long the capacitor will last. If you make sure your fights are less than that then you can keep everything running. You can always warp out if you really get low on capacitor and you need the armor boost.
The small armor repairer uses 40GJ every time it cycles - a cycle is about 5 or six seconds (the detail is in the attributes tab of it’s info page). While the capacitor in a ship does recharge over time, it’s not quick - although there are ways of improving that recharge rate.
So the rate at which the repairer is using capacitor is greater than the capacitor re-charge rate. The fitting window shows that as red, with a length of time the ship can run all the active modules before the capacitor empties (and life becomes a lot more awkward!)
You don’t have to be “cap stable” - i.e. in the green - because you won’t need all the modules running at the same time. For example, you will use a propulsion module, such as a microwarpdrive (MWD) to close range with a target, then turn it off. Like wise, you’ll only use the repairer in bursts.
Let me know what ship you are looking at and what you are thinking of as a fitting and I’ll see if I can help “small frigate” covers a lot of ground.
Of course, if this is a combat ship for use against other players, cap stability isn’t important - the fight won’t last that long, one way or the other!
and @ISD_Tipene just beat me to it!