Anton, now that I'm thinking about it I think the fatigue change with transferEnergy() may have unintended side effects. Imagine I have a link and storage next to each other, with a creep sitting there whose role is to just take energy out of the link and put it into storage. Right now I can just have a creep with 1 MOVE and several CARRY parts to transfer the energy. If I want to transfer energy from link -> creep -> storage I would need to have equal MOVE and CARRY parts.
I know other users do something similar, check AzuraStar in W5N7. He has a chain of CCCCCM creeps from his miner + link to storage. I remember in the EP there was a player (I think maybe dt7?) who would build a maze of extensions and fill the maze with CCCCM creeps and they would transfer the energy deep into the extension maze to fill it up. In those cases we'd have to add MOVE parts even though the creep never actually moves, which seems a little counterintuitive. I guess the creep is MOVEing energy so it kind of make sense. Anyway, just food for thought.
The cost of the parts isn't too bad, but right now I'm noticing that my biggest bottleneck isn't actually getting the energy, it's building the creeps. My spawns are running non-stop to create enough courier creeps, so any time I need to spend more time spawning I get grumpy. That's actually why I turned off my Source Keeper code, it was just too many parts to make sense without several RCL8 rooms.