Got this from the ParkedInParadise van conversion blog:
How To Size Your Campervan Charge Controller:
[font=Roboto,][font=Roboto,][size=large]At least 7.5 Amps for every 100W of solar[/font][/font][/size]
[font=Roboto,]This is easier for campervans because we already know our batteries are at 12V. Vandwellers are not usually using above 1000W of solar, so [size=medium][font=Roboto,]use the above for a good rule of thumb[/font]. Going bigger isn’t an issue with this, other than price, and allows you to add more panels later if your setup proves to not be enough.[/font][/size]
[font=Roboto,]Example: if you’re getting a 300W panel setup, you need at least a 22.5A charge controller. No reason to cut it close so get the common 30A charge controller size. This is big enough to add another 100W if you find you’re not getting as much sun as you were expecting.[/font]