What is considered to be maximum deviation plus and minus direction from nominal voltage in an ac generator?

Hi,
I think it is 10%. It is not truly that for a power plant, but I think it is for a portable generator. Most things plugged in depend on power (watts) which is the product of voltage times amperage. The less volage, the more amps it will want (need) to work. And, of course, vice versa.
Peace,
Clarke