I have purchased DFR0580 which lists in the specifications

Solar input voltage 15V ~ 25V

Maximum charge current (solar) 4A

12 V lead acid battery

Features: For 18V solar panels within 100 W;

I would like the largest solar panel available (if reasonable);

The DFR0580 is designed for 18V solar panels so I would like to buy an 18V solar panel;

The most common solar panel of that kind for sale is 100 W; using (current = wattage / voltage)

(100 W / 18 V = 5.56 A) which is larger than the 4 A maximum solar charge current;

Is it safe and possible that you can hook up a 100W 18V solar panel to the DFR0580 please?

Perhaps the DFR0580 would only draw 4A even though the solar panel is capable of delivering more.

Otherwise at 18 V and 4 A is the maximum solar panel wattage you can buy

(18 V * 4 A = 72 Watts) ???? It is hard to find a panel of those specs.

You might find one rated for 18V 50 W or 18V 60 W.