My EV mode is always "unavailable"

I thought so at first too, but now I'm learning how great the Prius is at recharging. Two-three good stop lights (where I can cruise in with braking at 60-80% recharge) and the battery is charged back up to 80-90%! Since I had to use the gas engine anyways to get up to speed (or guys behind me get annoyed), the energy was wasted anyways. But the fact that I can recover 80% of an entire battery in two-three stop lights is amazing.

 

If you overly-deplete your battery running on EV more than needed, the engine will run harder (burn more gas) when it does run. Regenerative braking only recovers a modest proportion of the kinetic energy of the car, but if the computer sees the battery too low, it will work the engine to charge it back up.

So, no, you are not using only wasted energy to bring the battery back up. You are recovering some wasted energy, but you are also burning extra gas.

However, if you use EV mode judiciously when the battery is high, then you are indeed using energy that would otherwise be wasted.