I just installed the Xantrex 1055 in my own
boat. Love it.
Yes, it draws
power with no load attached. Not much, but some. I haven't measured mine under no-load conditions. At the very least, you're powering up some LEDs in the display along with the logic and control circuits inside the unit. I would guess that the "snooze" mode is something less than 1 Amp and that the idle mode (inverter on and no load on the output) is somewhere around 2-4 amps. The only sure way to turn off the unit is to disconnect both the DC and AC input.
Yes, transformers with no loads on the output also draw
power. Again, not much, but enough to measure.
According to the manual, the Xantrex 1055 has a maximum efficiency of 87%. Maximum efficiency is usually around 90% of full load. Below 50% of full load,
inverter efficiency drops off sharply, sometimes getting as low as 30-40%. The good news is that the output is a pure sine wave, making it much kinder to the AC loads you
plug in downstream.
One unfortunate feature of the Xantrex 1055 is that if AC power is suddenly available, the unit will turn on and try to charge the
batteries (if connected), and then if the AC power turns off, the unit will remain on until the
battery voltage drops below the limit that you can program in. This could be an issue if you leave the
boat unattended for a long period (days or weeks), as this mode of operation virtually guarantees that you will have dead or deeply discharged batteries when you get back. For this reason, I would strongly suggest that you have an on/off switch for the AC supply to the unit, along with a disconnect in the positive line going to your
battery bank.