Which Volvo? 240, 740. 850?
I had a 240 estate when I was trad and now I've got a Honda accord which is similar but the back slopes like a Mondeo.
I carry 200 litres in 25 litre barrels. 2 in passenger footwell, 6 in rear estate space along with trolley, pump and battery. Trad gear on back seat and light ladders on roof.
So - me 80kg, water 200kg, trolley, pump, pole and battery 25kg, trad stuff 5kg, ladder 20kg
All up 330kg. I could physically squeeze another two barrels in, but car would wallow like a whale and ground tow-bar and exhaust at times. I reckon with your passenger you're pushing it a bit. Iwouldn't exceed 200 litres if its a 240, may be the full 250 if a 740?