2.5 vs 4 barrel system

Excuse me for asking a very dumb question but what exactly does a 2.5 or 4 bbl system mean ? I'm sure it is to do with  volume.

Thanks in advance,

Danny

Yes it is volume.  1 barrel (bbl.) equals 31 gallons, or two of the standard 1/2 barrel kegs of 15.5 gallons each.  Obviously most home brewers get nowhere near that large a system - these usually indicate a commercial 'nano' brewery.

So by stating that a system is 2.5 barrel it means it can produce 77.5 gallons each fermentation?

Normally, a "system" is the brewhouse equipment. The size *usually* means how much wort will make it to the fermenter. Occasionally, the size refers to kettle capacity.

Fermentation will have loss to yeast, dry hop and some other factors. It depends on the fermenter geometry and features.