Depends how intensively you want to run the tool. I've only got a 50 litre air tank, but it's not a problem because there's only me, and whatever I'm doing I need to stop and change position, get the next item lined up, change tools and so on, so the motor can usually keep up with me. Plus I can lift the 3HP 50 litre compressor single handed and sling it in the back of the Land Rover for work away from home. If you're doing repeated assembly work in a commercial workshop you'd probably spend a lot of time waiting for the motor to get you back up to pressure.