In 6 years detailing, never have I come across a situation where power was not available. I carry 200 feet total of extension cords ( 100 feet of 12 gauge and 50 feet of 14 gauge and 50 feet of 16 gauge). As for water, it has happened that it was not avaialbe and I just do a rinseless wash. Anyway this year I have switched to rinseless on most vehicles. I still use the pressure washer for wheels and to do a pre-wash on the bottom half of the vehicle as well as rinse off the chemicals for all the decontamination steps if water is avaiable.
It's good to be prepared for anything, but buying and carrying around a generator when it is never needed is not something I feel I need to do. If someone tells me they have no power where they want me to do the work, I will simply ask them if it can be done somewhere else. Never been a problem. If eventually I do loose a job because of it, it will no even put a dent in the savings I have made not having to buy the generator and the extra gas needed to carry around a heavy piece of equipment.
Same goes for a water tank. I carry around a 5 gallon bucket with a gamma lid full of ONR. I also have about 5 more gallons of distilled water in the truck in case I need to mix chemicals or use pure water for something. Never needed a heavy 100 or 150 gallon tank.
I only do 1 client per day, I don't take wash jobs so in my reality, my equipment is more than adequate. So to come back to your original statement, I would say people who are car washers, and not detailers need water tanks and generators.