Where does excess electricty go?

Answer 1

There is no excess "electricity". Excess (or a deficit of) electrons end up as a static charge. "Extra" current in a wire occurs at a phase angle other than the phase of the applied voltage, but this energy is largely recovered (being essentially "imaginary power").

Answer 2

Excess electricity can be stored in batteries, fed back into the grid for others to use, or dissipated as heat through resistors. In a grid-tied system, electricity can also be sold back to the utility company for credit.