I see the recommended AH on the Victron Multiplus 12/3000 120-50 120V is 400ah for 12v set ups.
I'd like to try 270ah to start and upgrade to 540ah down the road. What are the risks involved with only having 270ah battery bank instead of the recommended 400ah?
Lithium or lead?
A 3kVA MultiPlus can deliver 2400W continuously, at 12V that's 200A.
It also can deliver 6000W for a sort peak, that's 500A!
Your battery also should be able to deliver that current. You need to look in to the datasheets.
e.g. the 200Ah Victron Smart Lithium battery has a recommended continuous current of 200A and a max. continuous of 400A.
Assuming lead acid batteries, the rate of discharge from the batteries is higher which results in a shorter battery life, less capacity available (look up peukert effect), the voltage under load drops to a lower value so for a given ac power the DC current is higher causing more heating and lower efficiency in the inverter, also drawing the batteries down even faster. The inverter may well stop working on low DC voltage alarm prematurely. As a stop gap, if you limited your power draw to less that 2000VA (1600W) until you get your extra batteries then you would be prorating your output to your smaller battery bank.
If the batteries are lithium then some of the above may not apply, but the discharge limits of the battery cells and BMS would need to be checked.
