I'm sorry to pour cold water on this discussion but microwave emission/absorbtion is not directly due to the electrostatic potential energy of an electron in the field of the nucleus.
Most atomic spectra transitions take place at much higher energies ie visible light, uv light and x rays.
Microwave and infra red transitions lie in the province of bond activity in molecules.
That is more than one nucleus is involved.
These transitions may be due to molecular rotations, bond stretching, bond waggling and so on.
Here is a very basic chart illustrating this.
If you think about it, the energy can't come from the atoms or they would fall apart.
So we now answer the OP's rather more perceptive question.
As we know when the electrons are excited they move from higher energy state to ground state causing electromagnetic radiation. Now EM radiation in form of microwaves is Energy. Where has this energy come from. From the mass of the electron?
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The energy comes from what ever makes the molecules vibrate about their bonds.
This may be thermal, an external field as in NMR and so on.