Shop for batteries. The higher the amp-hours, the longer they'll last. Weigh the batteries, if that's a factor that'll influence your purchase choice. You'll also need a power inverter to change the battery's DC voltage to the 220 volt AC that's required for your projector, amplifier, and dvd player.

See how long you can run the equipment before the battery dies! If the run time is too short, buy another battery to double the time, and so on. Connect the batteries in parallel.

Buy solar panels and use them to charge the batteries, or to keep the batteries running longer between charges.