Once the batteries charge, not a lot. It doesn't actually quote the input power.
Though for a rough estimate, the thermal output (power wasted as heat by internal electronics) is 100 BTU/hr, (equates to about 30Wh). Given we're not accounting for all the input power using the upper end of leccy prices @ 13p/kWh, about 10p per day if it's fully loaded.
You can get a better estimate if you can get hold of one and look at the input power rating and subtract 675W from it.