Why are electrons in solids always considered to be in energy eigenstates? When studying the properties of solids we always say that electrons are in (stationary) energy eigenstates. The theory of conduction for example (with conduction bands and stuff) follows from the assumption that electrons are in energy eigenstates but why are they in such states in the first place? What prepared the electrons in energy eigenstates? And if they are not in energy eigenstates how is it possible that the assumption of them being in energy eigenstates works so well?
Example:
I have a block of aluminum. It has never had the energy of its electrons measured by me nor anybody else: it just came out of production. If I use it as a conductor it has a resistance. Assuming its electrons are in energy eigenstates I can calculate its resistance. Why am I allowed to assume that its electrons are in energy eigenstates? If nothing ever measured the energy of the electrons of the aluminum they might be in a much different state. 
 A: I think you may be mistaken that the electrons are in energy eigenstates when looking at valence/conduction bands. The energy eigenstates are a complete basis, so we use them to express the state of the electrons, but that doesn't mean they're in an eigenstate.
However, when talking about the valence and conduction bands, it's just easier to talk about the eigenstates rather than explicitly talk about the wavefunction of a single electron that is a superposition of different eigenstates. After all, we're interested in the energy needed for electron to jump bands (at which point it will have collapsed to a single energy band), and also the properties of the bulk material, not a single electron.
Also, in the comments you ask why not momentum eigenstates, but energy eigenstates. Here, these are the same thing. In fact quite often in QM they're the same since the Hamiltonian and momentum are simultaneously diagonalisable in many systems. 
A: I'm not sure if this explains it, but the electrons only fall into a particular eigenstate upon observation; before that, electrons exist in a state that may be expressed as a probabilistic superposition of all available eigenstates.  Eigenstates exist due to the quantisation of angular momentum.  (edit)
Also, possible duplicate: Why do electrons tend to be in energy eigenstates?
A: I think it is not usually assumed that an electron (or system of electrons) is an energy eigenstate, but that at low temperatures the electron (or system of electrons) is in its ground state, which happens to be an energy eigenstate. In fact, for general temperatures a system at thermal equilibrium is generally assumed to be in thermal state, see e.g. http://militzer.berkeley.edu/diss/node13.html.
The point is that systems try to be in the state of lowest energy for low temperatures and most of the time low temperature is a good assumption.
This also why the Bohr model for the atom works so well. Here one essentially assumes that the atom is in its ground state. Most low energy phenomena can be described by valence electrons being excited etc and this is clear since energy eigenstates where the valence electrons are excited are the next to lowest energy eigenstates. Of course the atom, at moderate temperatures, can be considered to be in a superposition of the ground state and maybe some of the lowest excited states. So the dynamics usually happens "close to the ground state", unless of course we prepare the system in some way, by shooting a laser at it or something (I am very ignorant about applied physics). Of course this discussion applies equally well to solids or Fermi gas etc.
I want to also point out a confusion that I had with regards to this. The description of ground state, i.e. the state of lowest energy, applies to the lowest energy eigenstate of the single-particle system (e.g. lowest energy shell of atom) as well as to the lowest energy eigenstate of the $N$-particle system, i.e. the state where all fermions are "inside" the Fermi sphere. This should not be mixed up.
(Actually wanted to post this as a question, I am looking for agreement or disagreement for this take)
