The Master MFG Spot Sprayer 15 Gal 1.8GPM (SSC-01-015A-MM) comes with red (positive) and black (negative) wires to connect to an exterior 12 VDC power source. Since 18 feet would be about the maximum verticle spray height with the standard hose, you likely would not be able to reach this high if you added more length to the hose frome the same starting point, since you would lose pressure in the hose.
We would not recommend using the Master MFG Spot Sprayer 15 Gal. 1.8GPM (SSC-01-015A-MM) to pump diesel fuel or any other combustible materials. You could possibly use the tank for transport, but we would recommend using a pump system that is especially designed for the use you are needing.
The Master MFG Spot Sprayer 15. Gal 1.8GPM (SSC-01-015A-MM) does not have its own battery. It comes with a wire harness that would need to be connected to an exterior 12 volt battery.
If you are asking if you can only fill 1/4 of the tank of the Master MFG Spot Sprayer 15 Gal. 1.8GPM (SSC-01-015A-MM) because that is all you need to treat the desired area then yes, the tank should still perform as it should.
If you are asking if you can leave and store 1/4 of the solution left in the tank then no, we would recommend to only mix what is needed and discard what you did not use. Most pesticides recommend using the mixed solution within 24 hours.