With what we know we can agree that the battery bank is to small.
Here's a suggestion seeing were you work you have a wealth car batteries at hand .
Car batteries are not the thing to use over the long term but can be used as a learning tool
Find out what your employer gets for a battery it won't be to much and offer him the same.
For testing you will need a volt meter a cheap one from HF will do an battery charger .
The rest you will be able to get at work. A way to discharge the battery some head lights and a amp meter
And cables to wire them together .
A likely candidate will be from a car or truck with a V8 engine .
Check the voltage if its below 11 volts move on find four as a start that read 11 to 12 volts and have good water level . When you get them home see if they will take a charge should read around 13 volts fully charged if not take that one back and trade it for one that will.
The next test is to discharge them using the car head lights as a load .
Using the amp meter you can find the wattage of the load by using Ohms' law amps X volts = watts
For a quick and dirty battery amp hour test with a known amp draw from a load time how long it takes for the battery to go from full charge volts to full discharge volts about 10.5
This is what's called battery capacity The time it takes a known amp draw over time can be in amp hours or amp minutes this is a very quick over view.
There is a lot more to it an I will let others help fill in the blanks.
When you have four batteries that have been charged an discharged at lest two times to be called good
You wire them up an trade them with the batteries your using now if your still getting only six hours run time it means that you have maxed out you charging source solar panel and need to add more charging capacity from wind or solar
I know the above seem like a lot of work but a long the way you will gain some first hand knowledge
On battery banks an there care an feeding on the cheap.
There are a lot of good folks here who will help fill in the blanks
And if nothing else your hubby will be in tinkering hog heaven an not under foot for a wile
There could be a lot said for that alone : )
Old F