I too have 750 VA Ferrups in my basement. Even went to the trouble of
bringing the battery connections out of the box using those nice Anderson
connectors for a BIG external battery. But as others have stated, it is
inefficient because it is a FERRO-RESONANT transformer. Notice that there
is a cooling fan inside just to remove heat. I hooked it up to the AEMC
Power Logger and found it eats 100W doing nothing. With no AC input, and no
load at all on the output, about 8A flows from the 12V battery. Yes, it is
virutally bullet-proof. But for my hobby application, I didn't want to pay
100W of energy 7 x 24 so it sits in the corner disconnected while I work on
a switch mode inverter system. YMMV.