Stored wattage can be estimated using the amp-hour rating.

I decided that knowing 12 volts X 1 amp for 180 minutes is a rating pertinent to calculating stored energy in watts.

Assume a .1 volt drop resulting from testing the battery charged at 12 volts delivering current through 12 ohms for a duration of 180 minutes. Power lost from the battery is 12 x 180/60 = 36 watts.

It is then assumable the battery will perform in like manner through a linear range of the discharge period when in use. Thus for a deep cycle battery charged to 13.5 volts the available power in watts is estimated as:

10 x (13.5 - 11.5) x 36 = 720 watts