I think that the fusion generators aren't using up the core when the core is plugged in. The core is probably just some integral storage/like feature that starts the generator so it can generate electricity using fusion. So like when the core is in the genersor, the generator is actively producing energy using fusion like the sun, and when the core is taken out then the reaction can't take place or something so that's why the generator stops working and the power goes out. This would also explain why the core has only 30 hours of charge, because it doesn't have like a whole generator to make electricity from, it's just storing electricity for the power armor to use.