The 30 day window seems reasonable and should be enough time to get past a blizzard, ice storm, hurricane, forest fire or an earthquake. During this 30 day window you can assess your situation and make a decision to continue to stay or leave your home for a safe area and wait out the rebuilding and restoration of utilities.
Most people buy the common and relatively inexpensive 5,000 watt generators with a 220 volt and 110 volt outlet. If you have your own fresh well water to pump then the 220 volt outlet may be necessary for water. These generators normally use ½ gallon of gas per hour!
What exactly do you want to provide power for?
A typical 5,000 watt generator:
Based on the above you now know you may need about 8 hours per day of run time during a grid down or 240 hours of generator run time for a 30 day off grid survival period. That’s 240 hours of generator run time burning ½ gallon per hour of gasoline equals a whopping 120 GALLONS OF GAS!
For comparison a Honda 2000i generator:
My tested results show I actually use 1 gallon of gas for every 8 hours of run time or 30 gallons per month.
If you are thinking about using 55 gallon drums keep in mind that a full drum weigh’s nearly 350 pounds. Getting them off a pick-up truck bed can be a chore. Also don’t forget you will need a transfer pump to get the gas from the drum to the generator tank. For safety do not store the drums in the garage or anywhere near the house. This is no time for a fire and this fire would be catastrophic in size.