Hi A.G. yes the "squarewave" inverters are almost always a "modified squarewave" that has a pause between power pulses. Making it hard to calculate, because you don't know how many degrees the pause is and it can vary from product to product.
To MrAl; I would just try it and see. The makers of the inverter have surely considered that a lot of users will run smallish mains transformers from their inverter, so it will probably be close enough to spec to work fine without harm.
And you could always do a heat test comparison, ie testing for X minutes/hours with each power source and checking how hot the scope gets.