400 Battery Capacity and Inverter
The TAB 400 comes with a 1200 watt AIMS Power pure sine wave inverter with a surge capacity of 2400 watts.
The battery capacity in the TAB 400 is 220 amp-hours which is roughly 2640 watt-hours. (amp-hours x voltage)
These are AGM batteries and are rated for a maximum depth-of-discharge of 80%, but to get the most life out of a battery it's usually best not to discharge below 50% capacity.
A fully charged, healthy battery should give you 1320 watt-hours to work with on paper. (Keep in mind that battery capacity will gradually decrease naturally with usage & time as the internal chemistry degrades.) Using the inverter incurs a 10% efficiency loss, so multiply this number by .9 for a total of 1188 usable watt-hours.
You can take the wattage listed on the appliance's AC Adapter and divide that number into 1188 to get a rough idea of how many hours your appliance will run (if it's the the only thing drawing power from the battery.)