if your power is under 9 cents usd you stand a chance to make a profit
if your power is under 6 cents usd you stand a better chance to make a profit
if your power is 15 cents usd or higher you will most certainly lose out.
That's assuming it works perfectly 24/7, which they don't.
Expect to struggle with getting it running.
Expect to burn up the factory psu.
Expect to restart in and unplug it, sometimes several times per day, just to keep it running.
Expect it to not work for random reasons.
Expect it to work just fine, then you go and tweak one setting or change a cable, then it won't work at all or only at 50% speed.
Save your money for anything but a U3, they are crap. If someone gives you one it's worth playing with, but don't pay money for one and expect to see that money back, let alone a profit.