You'd be surprised.  

Toward the end of this corn harvest I stopped the combine one night and as I was about to exit the cab  I hear over the radio "Whoevers in the combine that just stopped east of me, there's a fire chasing you".....talk about getting one's attention...I looked in the mirror and, sure enough, there's flames 4' high underneath the back of my machine.....I dive back in the cab and shove the hydro balls to the walls for a second, then stop and bail, grabbing the 2 gallon water extinguisher as I go down the steps.  By the time I got to the fire and got the safety pin out I was looking at an area 15' in diameter with flames 5' high.   I had it knocked down in seconds and completely extinguished shortly later with better then half my water left.  Emptied most of that on the machine near the bearing which caused the incident.    Both the other guys arrived on the scene as it was over carrying similar extinguishers from their machines but they were not used.  Until you start a fire and try it, it's hard to believe how much fire 2 gallons of water out of one of those extinguishers at 100 psi will knock down.