Got a  Screen : 7 inch. resolution: 1280*800, brightness rating max 1200 nit (cd/m2) . very bright.        enough for ourdoor.
 power consumption. 
       the Screen tested result:   
              brightness 100% 1.26A @  11.25V  14.2W ;
              brightness  50% 0.87A @  11.25V  9.7W ;

              brightness  30% 0.72A @  11.25V  8.1W ;    
     Power Supply I will need :
           Lipo Batteries s 18650 9pc,  3s3p to make 11.1V
           A voltage regulator to 5V output for the Raspberry Pi

           3S Lipo Charge/Protection Board

others: wireless KeyBoard/Mouse       

I think this setup can hold up to 6 hours.