said by lutful:You will need to purchase a separate 28V/175VA rated AC transformer for the 24V/4A DC output. That is also quite bulky and heavy.
Only if the OP needs 4 amps, which at 30 watts from 24 vdc is considerably lower than 4 amps, therefore that oversized transformer is not needed in this application, nor applicable...
You need to remember when sizing up power supplies to match the input to the load and as such in this case, a much smaller plug-in will do the job just fine.
Finally, it only trickle charges at 300mA which means it will take more than 60 hours to recharge your 18000mAh battery bank after each 6 hour outage.
Not sure how you came up with 18000mAh, the OP mentioned using two 12 volt 9 amp hour batteries in
series which is still 9 amp hours, not 18 amp hours, in order to achieve 18 amp hours the batteries would need to be wired in
parallel.
Wiring in series increases voltage while wiring in parallel increases capacity.