17.2Ah SLA Battery Datasheet:

I just found a datasheet for the SLA battery used in my powerpack. It shows that it can do deep discharge cycles and handle them very well, but I assume this is provided they are charged almost instantly after discharge. This said it'll be no good for a solar system if you use it all at night and you get no charge till 9am the next day.

100% DOD - 250 cycles
50% DOD - 550 cycles
30% DOD - 1200 cycles

I always try to do no more than 50% DOD (12.2v according to the chart). Usually its no more than 25% ish DOD (12.4v).