Hello - I'm a retired EE who spent a career working in and around coal-fired electric generating plants. Nowadays I make my own electricity from the sun, while selling any excess back to the grid.
I came across this forum while googling for what the acceptable standards are for US household voltage. I found a couple of old threads and most seemed to indicate plus or minus 5% of 120 volts, but in my experience here in NE Ohio, my house sees over 130 volts quite often in the springtime, before my utility changes their xfmr taps for summer loads (at least, I think that's the explanation...). The reason for my search is that my home power system has grid-tied inverters that don't like it when the voltage on either 120 volt leg rises above 130.