I got more and more energy stockpiling up and no use for it.
I conquered 27 shards in 27 turns without spending energy on a single one and I doubt I will start soon...

So what are the other uses of energy?
So far I found only 2:
1. IIRC I paid zarr to reveal info about Stenriya
2. I had the option to pay beleth for info about doh-gor's amulet (but didn't need to because I got it for free from another master)

I know you can pay Stenriya to tell your fortune. There's a few other choices like that which pop up from time to time.

Also, you may want to spend some of that energy when attacking another master. They have a sizable advantage on their own shard, so starting with a library, workshop, etc. can make a big difference.

Other than shard bonuses there is no reliable energy sink. 100-300 here, there and that's it.

Keep in mind that the benefits of some of the bonuses depend on your difficulty level. On beginner you get an extra 5 gem income per turn from the start. On expert you get zero... so even +1 or 2 gem income becomes noticeable.