For a long time it was not known why Superman's powers changed. And although never fully explained, it should be noted that in the special Superman Secret Files and Origins #1 (page 51) Superman explains that he became an energy being as a direct cause of his battles with Tolos (the creator of the bottled city of Kandor). Because Kandor exists on another plane, Tolos' attempts at pulling Superman into Kandor changed him, leaving him slightly out of phase - an energy being.
In his search to find the answers for himself, Superman and Professor Hamilton conducted some tests in which the Professor confirmed that Superman's "spectrometer" readings were very similar to the phase-shift required to enter the bottled city of Kandor.
Although Wizard Magazine's Superman Special edition (March 1998) states that Superman became an energy being because of an excessive amount of solar energy intake when trying to regain his powers after the Final Night storyline. And that his return to his "classic" costume and powers came about because of his excessive outpouring of energy to save the earth at the end of the Millennium Giants storlyine (Superman #135).
Wizard Magazine's explanation is further backed up by information presented in Superman Secret Files and Origins #2 (May 1999) where Professor Hamilton explains his theory that when Superman immersed himself in the reignited sun shortly after the Final Night saga that Superman may well have overloaded his body's ability to process solar radiation, and therefore his body had no choice but to store the excess energy, eventually resulting in the transformation to a being that consisted only of energy.
It should also be noted that Superman Secret Files and Origins #2 (May 1999) also reveals that while some theorised that the eventaul return to the classic Superman resulted from the fact that Superman Red and Superman Blue were prepared to sacrifice themselves during the "Millennium Giants" saga and therefore were rewarded by being returned to the one true Superman, Professor Hamilton thinks that Superman's restoration can be attributed to the total release of the excess solar energy Superman absorbed which (he thinks) caused the transformation to an energy being in the first place.
There was originally some confusion over whether or not Superman could actually physically hold, grab or touch solid objects. Bullets and rockets were seen to pass through him, his hand had been seen to pass through objects as he was 100% energy. Yet we also saw him wearing a parachute, grabbing hold of Lex Luthor by the throat and other examples where he appeared to be solid.
To add to the confusion, Professor Hamilton stated in his interview with Jimmy Olsen (which we heard on TV reports in MOS #70, page 8) that, and I quote: "As an energy being, Superman can't physically touch anything..." Yet, in the very same comic (page 22) we saw Superman aiding Scorn who had his arms around Superman's shoulders for support!!
It seemed that these contradictions were explained by Superman's ability to form a Solid Energy Field around himself which enabled him to hold, touch and grab solid objects (as explained in MOS Annual #6, page 30).
Did Superman [in his energy form] have some kind of internal power storage that needed recharging, like a power ring and its battery, or in his energy form was he connected to something like the Flash's "speed force", so that he had a constant supply of energy available to him?
Superman was 100% energy. He was able to absorb energy from a variety of sources and convert this into energy he could use, boosting his powers. He could store this energy thanks to the containment suit.