What is the energy decay speed for different resonator levels?
All portals decay 15% per day.
Source: Official Niantic documentation.
Portal Decay
Resonators are powered by XM. Resonators start with a full charge, but the XM store is consumed at a rate of approximately 15% per day.
(Emphasis mine)
Portals used to drain at 10% per day, but it was increased in an update early 2013 to be 15%