If you remove all of the equipment plugged into the UPS, is it still lit?
Are you sure this is an overload light, and not a replace battery light? If this is a smart-ups unit, the overload light is next to a picture that looks like an unbalanced scale. The replace battery light is a square with an X through it.
If it is, in fact an overload light that is lit, and it stays lit with less than the maximum load on it, or without any load on it, then you have a critical internal failure and the unit needs to be replaced or repaired.
You can also find your user manual here for more information:
http://www.refurbups.com/Knowledge-Base