Deep cycle batteries typically power inverters, that switch DC power to AC for televisions, microwaves, computers and such. DC deep cycle applications include winches, motors, and other longer term applications. Deep cycle batteries can be used as starting batteries, please do ensure that you have the CCA required for your motor.
Deep cycle batteries are measured in AH or amp hours. One amp hour, with a 12 volt battery, is 12 watt hours of power (1ah x 12v = 12wh = .012kwh or kilowatt hour).
How long would a 100ah, 12v battery, run a 200 watt heater and only discharge to 50% (50% DOD)?
100ah * .5 (DOD) * 12v = 600wh available
600wh / 200w heater = 3 hours of run time