I made a similar inquiry 3 or 4 years ago and was presented with. 2,500 word response from one player, we'll call him playerX. I'll try to spare you.
As pointed out both your current progression and your style of play will effect energy usage, and time is/will be the limiting factor.
I always try to keep my pool empty, or at least below cap.
Any energy that goes directly to the pool, I dont collect unless I'm almost out and intend to use it immediately or it's about to expire. I.e. Daily, advanced, playtime. Any energy that goes to inbox, leave until I need it. I seldom use it outside fusion/frag unless about to expire.
This also means, usually, letting energy rewards from Tourny/event/cvc going to inbox.
Youll get 3 refills from logging in each week, Tuesday, Thursday, and Sunday. They go to inbox and are huge resources for fusions if you let them sit till needed.
Things that don't require energy I dont do if my pool is full or close to full. Arena, DT, FW, upgrading/selling equipment, etc.
This all seems like basic stuff. I guess it revolves around keeping your pool below cap, and saving anything you can for a rainy day.