The battery indicator on my Axpert 5kva Inverter (48v - 4x100ah new Batteries) goes down to about 25% immediately after utility input fails, regardless if the load is at 280watt or 1000watt at the point of input failure. My inverter is not connected to any PV panels and the DC low cutoff is set to 48.4v. 

Anyone know why it would do this? I would expect the inverter to start at 100% Battery reading and then gradually to go down to 25%. 

On the last test I did, the inverter read a current load of about 3amp to 4amps at the time of AC input failure, which should last a long time with the 400ah battery bank, but it didn’t seem like it would even make 3 hours. I didn’t have a chance to run it for 3 hours to confirm. 

Do You think I need more batteries? If I do, could I ad another bank of 4 batteries a month after the first bank was installed? 

17 hours ago, Wayno said:

The battery indicator on my Axpert 5kva Inverter (48v - 4x100ah new Batteries) goes down to about 25% immediately after utility input fails, regardless if the load is at 280watt or 1000watt at the point of input failure. 

The SOC on the inverter is rather inaccurate; it makes a guess based on instantaneous battery voltage, ignoring load and charge currents.

17 hours ago, Wayno said:

the DC low cutoff is set to 48.4v. 

This value actually affects the SOC reading. You can set it lower to make the SOC reading more realistic, but then you're going to get deep discharges, which is not good for the battery. So leave it there, and get a Victron BMV or other way of estimating the true SOC of your battery.

17 hours ago, Wayno said:

I would expect the inverter to start at 100% Battery reading and then gradually to go down to 25%. 

That would be great, wouldn't it? But the Axperts don't have a current sensor on the battery current, so they can't estimate SOC properly.

17 hours ago, Wayno said:

On the last test I did, the inverter read a current load of about 3amp to 4amps at the time of AC input failure, which should last a long time with the 400ah battery bank,

Bzzt. That's a common beginners's error 😮. Four 12 V 100 Ah modules in series gives a 48 V 100 Ah battery. It's still four times the capacity (in watt·hours) of a single module, but it's not 400 Ah.

17 hours ago, Wayno said:

Do You think I need more batteries? If I do, could I ad another bank of 4 batteries a month after the first bank was installed? 

100 Ah is rather light for a 5 kW inverter. Of course, if you keep your loads low most of the time, that's not too bad. Because of a thing called the Peukert effect, large loads on a lead acid battery cause the available energy to be reduced. For example, if you run the battery at 50 A (C/2), then your nominally 100 Ah battery (measured at C/20) might only behave like a 70 Ah battery. But less than 5 A drawn from the battery is less than C/20. Note that with lead acid, for long life, you really don't want to discharge more than 50%, preferably no more than 20%, so you really only have 50 Ah or even 20 Ah available, and that's when brand new.