In the early previews, people seem confused about why the crown/energy exchange rate varies.
The Massively guy didn't seem to know about it at all, and even the guy explaining it in the comments was confused.
"And also interestingly, excess energy can be swapped back for Crowns. This system is dynamic. You might get over 3000 Crowns for 100 Energy one minute and only get 1900 or so the next. I'm not really sure how the system determines the conversions."
Shameless Wiki Plug:
If anyone wants they could co edit the Energy Depot page to make it sound more clear! There's currently only a little blurb on how the offers and trades work.