The backpack pumps that John Hewlett supply are small low current draw units which only need a small battery.
Once you get to the larger pumps, they draw a lot more power. Our Shurflo pumps draw around 4 - 5 amps. So if you have a battery that is a 20 amp then theoretically it will last 4 to 5 hours of continuous running.
But the less time we spend on the glass the longer the same battery should last (if that makes sense.)
We use Varistream controllers which allow you to lower the cut-out pressure of the pump to just above what you are need. The last time I checked this, our controllers were set at around 60psi. This reduces power consumption as the pump doesn’t have to use extra current pumping up the 100 psi to cut the pump off via the pressure switch.
A lot of cheap controllers on ebay don’t have this second function as they just reduce the speed the motor turns at and hence reduces the flow. If you reduce the flow too much then the motor will want to stall at higher pressures and it acts like a short which draws more current.
We have found that the best battery is an 85 amp leisure battery for a single operator.