Wiktionary lists isoenergetic as "Having the same, or constant, energy". You might peripherally introduce a definition, e.g. "... the diagram illustrates quantum-mechanical energy isosurfaces, or isoenergy surfaces ...".
The terms isoline and isosurface are used generically for 2D and 3D equal-quantity cases, respectively. After a phrase like "... the diagram illustrates quantum-mechanical energy isosurfaces ..." has introduced your terminology, references thereafter to isosurfaces would be understood as isoenergetic surfaces.
If only certain units of energy are used in the particular problem, tack iso on the front and see if it works. That seems to be what was done for isobel.