Achievement unlocks when the generator is started.
The solution is to take the hose hanging right next to the generator, as well as the empty water bottle from inside the building, then going back to the car and using the hose and then the empty water bottle on the car's fuel lid. Then use the bottle filled with gasoline on the generator back, and then click on the generator to try and start it. It doesn't work on the first try, so try clicking again and it should start.