It all depends on the transformer. On the label of the transformer you will see specifications and it will say Primary / Secondary or Input / Output. Primary should always be 120VAC 60 hz. The Secondary will be the voltage that you should be seeing (within a few volts, it is not uncommon for an 18v transformer to read 20v) on your multimeter. Pay close attention to AC or DC because most multi-meters need to be set to AC or DC and if you check on the wrong setting you will read 0 volts. Even if you have a DC gate opener many times they use an AC output transformer. The board rectifies it to DC, they do this because AC travels a longer distance.