Avoid storing fuel in your generator's tank. It will break down within a few weeks and form gums and varnish in your fuel system that will clog your carburetor. Always store gas someplace away from your generator. Before storing gas, add a fuel stabilizer to extend the storage life. Fuel stabilizers are sold in hardware, home supply, automotive, and outdoor equipment stores. With stabilizer, gasoline can last up to six months but preferably should be used within three months.
Amount of Fuel
How much gas you store will depend on the fuel consumption rate of your generator. The higher the wattage of your generator, the more gas it will burn per hour of operation. Figure on using your generator six to eight hours out of each day. For typical household generators rated around 2,500 to 3,500 watts, 25 gallons is enough to sustain your critical systems for three to four days. Some municipalities limit the amount of gas you can store at home, so check with your local fire department. It's a good idea to rotate your fuel stock, putting the older gas in your other outdoor power equipment, refilling that can and putting it at the back of your fuel stash.
Type of Container
Containers for your gasoline should be approved for flammable liquid storage. They should be of robust construction and designed to store your fuel in any weather, hot or cold. Never use milk jugs or other makeshift containers for your fuel store. Containers should be of a size and design that's relatively easy to handle and be able to dispense gasoline without splashing. Such cans typically hold 5 gallons. It's a good idea to have a small gas can of 1 or 2 gallons capacity that you fill from your large 5-gallon gas cans and then use to fill your generator's fuel tank.
Where to Store
Never store gasoline in your house. Gasoline is dangerous. Keep your fuel store in a shed, free-standing garage, or other secure, well-ventilated outbuilding away from the house. In event of a house fire or a natural disaster that damages your house, you don't want to have gallons and gallons of highly-flammable liquid down in your basement. Never smoke or have any open flame in your fuel storage structure, and keep a fire extinguisher for flammable liquids on hand.