If you need to count on your backup power supply, you better be testing your fuel and polishing it on  a regular basis. Never assume the fuel is good to go and then find out in an emergency that the fuel has a problem.

For testing use  http://dieselcraft.com/fuel-test-kits/

Once you have established a testing procedure you need to call a service to fix the problem or buy the equipment.

Portable Fuel Polishing

Fuel Maintenance Services