Chris Hobson

Hi Paul I run an off - grid system and there are pros and cons for both.
3 kVA
Pros
Cost of the unit (marginal R 2-3K). Can have 24V battery bank (depending on size could be a considerable saving). Low quiescent current ~ 20W as opposed to the 4kW which has a quiesecent current of 40-50W. Friend's 3kVA system ran perfectly on my Chinese no-name gennie. Cons
3kVA limited capacity. (I thought a 4kW system would be adequate and it is but on occasions 5-6 kW would be nice. - Seems the the more you have the more you want). If the battery bank is a 24V system then each individual battery has to work twice as hard to supply a set 2kW load as compared to a 48V system. Limited to a nominal 1500W solar array . You can oversize one's array but the recommendation is for this to be by only 30 - 40%. 5kVA
Pros
4kW  I can bake bread, weld and do other serious sh#t if I want to (and I do). 3000W solar array - I can run some heavy loads direct off the panels and not discharge the batteries in fact on a good day I can heat a geyser and charge batteries  at the same time. This array can also be oversized. If you think that you can get by with a 3kVA you probably will not need to upgrade a 5kVA. Cons
A more expensive 48V DC battery bank.
 
I will check tomorrow if my Chinese gennie works with my 5kVA and report back.
If the size of your battery bank is determined by required backup capacity rather than budget then the 48V battery bank beats the 24V battery bank. A 24V 200Ah battery bank is ½ the size of a 48V 200Ah battery bank.
 

Hi TTT
Have a look here
 

Charging is limited to 15A so @ 12 V maximum is 200W and @ 24V 400W. Extra batteries and you will get better production out of your panels.