Do UPS inverters use Modified or Full sine wave?

I've been running my laptop, Desktop computer monitor, and a whole bunch of other things off a cheap 100w modified sine wave inverter for few years now, but have just recently started wondering if I got an old UPS that ran off of 12V for the inverter if it would be cheaper than buying a real full sine wave inverter. But I'm not sure if the inverter is Full sine wave for use with computers.  I have a small off grid system I'm using.

Anyone know for sure?

UPS inverters come in many flavors. Generally cheap inverters have square or semi-square wave output. But some advanced UPS or latest UPS uses new technology & come with full sine wave output which is much better for computers.

You have to check with the manufacturer or the dealer for exact technical detail before purchasing any UPS.
would it be efficient to run a computer power supply off a modified sine wave inverter???????????
What kind of modifications are you referring to ?