Can I work 30 of these Solenoid Valves from a battery?
Assumiung the average (mean)  number of solenoids on over time is 15, the average current draw will be 7.6A

so for 1 hour between charges you need at-least a 7.8Ah or larger battery.
for 2 hours get at-least twice that etc...

But get a much larger battery like 12Ah or more for 1 hour, the extra headroom results in a much longer battery life (longer time before you need to replace the battery because it's worn out and can't hold enough chgarge)

the calculation is you take the current needed by the solenoid 6.2W at 12V is 6.2/12 = 
0.52A,  multipy that bt 15 for average 15 solenoids 7.8A

then you multiply that by the number of hours between recharges to get the battery capacity needed 7.8A  times 1 hour is 7.8Ah 

then you increase that a bit for reliability and choose a battery size that's suitable.