.
When you step back and think about it... you're (usually) only allowed to attach one energy per turn, so you (usually) don't want to "clog up" your hand with energies... one per turn is enough!
In order to have a decent chance of having one energy in your hand... how many do you need?
* * * (Please take a step back as I use a statistical formula) * * *
If you have 9 energies in your deck, there is a 73.5% chance that you will have one in your hand at the beginning of first turn (your original 7 cards plus your draw card) ---- if you run 12 energies, you up your chances to 83.1%.
So running 9 to 12 energies gives you a great chance of having one in your starting hand... how about the rest of the game?
Well... how many hands do you expect to play? That's a pretty tough question really... next time you play, see how many turns it takes for someone to win... You'll (usually) want an energy in your hand to attach every turn...
Searching for energy could be from Roseanne's Research, Draw Cards (Oak's Visit, etc.), or "Hand Refresh" Cards (Oak's New Theory, etc.)
Then, mid-game, you may need to get some energies back from the discard pile... Night Maintenance, Fisherman, Palmer's Contribution.
I guess it comes down to playing style, but decks generally run A LOT better when you have nice Trainers and Supporters in your hand... not a lot of "dead" energy.