then he needs to recharge by absorbing sunlight.

Technically speaking, Scott's true power is the ability to generate portals to the "Punch Dimension" of unlimited kinetic energy which never runs out. It's his own ability to sustain that portal which is limited however though with experience and training he has been able to improve on this. It's important to understand that the kinds of portals he opens isn't the traditional "hole in space" variety but rather a per-particle one that allows individual quanta of energy through. Part of his mutation is his own immunity to this energy, given it's kinetic nature when a quanta of the energy impacts his body the kinetic it is completely nullified via some unknown effect that effectively cancels out the entire event back to the source like it never occurred. This is why even though his powers are "always on" there isn't actually any drain on his portal generation ability until the actual kinetic effect of the punch dimension has carried through. Another way to think of it is like a phone charger that is plugged into the wall but doesn't have anything connected to it. It's always on, but it isn't until an actual charging load is connected to it that there is a current drain. Ruby Quartz just so happens to be a material that exhibits the same nullification effect as his own body so the end result is the same.