Cap Boosters

With a med cap booster witch is better 400 or 800 ?

It depends.

Short answer:

Bigger (800) allows you to inject capacitor a bit faster, while smaller (400) gives you more control over when you want to have capacitor, since you can now choose to inject 400 now and another 400 much later.

Long answer:

Cap boosters inject as much GJ into your ship as it says in the name, with a Cap Booster 400 injecting 400 GJ and a Cap Booster 800 injecting 800 GJ.

The other important aspect of cap boosters is their size, which is 4m3 for every 100 GJ (or 3m3 for the more compact Navy cap boosters).

Capacitor booster modules have a limited capacity, so it can hold and use half as many cap booster 800s as it can hold cap booster 400s, before it needs to reload.

Your cargo hold also has limited capacity and only can hold a limited amount of extra capacitor in any booster form.

With bigger cap boosters you can inject it faster, but have fewer cap injections in your cargohold, so if you’re under neut pressure and need to inject a cap booster every few seconds to keep a few modules running, you may do better with smaller cap charges as you then can spread your cap injections more.

Usually it’s better to pick the biggest size cap charge though. And if you think the added cost is worth it, consider the more compact Navy cap boosters.

Alternative situation answer:

If you happen to use cap boosters not for a capacitor booster module but to fill your Anciliary (Remote) Shield Repair module, always pick the smallest cap booster you can use. For the anciliary shield repair module the amount of cap doesn’t matter, only that it has any cap charge (if it runs dry it sucks a huge amount of capacitor from your ship), so then you want to fill it with the smallest possible charges to fit more of them.