The simple explanation: Your electric devices have stickers on them which say how many amps they draw. Add the amps, multiply by hours you plan to use each device; total from use of all devices has to be less than about 50 percent of the amp-hours stored by your batteries. (Percentage varies with type of battery; most say don't draw down to below 50 percent.) These numbers give you a clue of what you can use and for how long. If your TV is not 12V, you will need an inverter to run it, and the inverter will waste some amp-hours.