+1 to what Tom wrote. Tell us a bit more about your needs.
If it is for backup power, the question you have to ask yourself, is "what can I live without, and for how long?" I have a 6/1-ST5 genset good for right around 3KW sustained power output. I am all electric, but use a pellet stove for heat which when running pulls about 300 watts to power blower and auger. Mine is used as a backup power plant, and I can run all the basic necessities which include the following. Reefer, Freezer, Well pump, pellet stove, entertainment center, dishwasher on air dry only and any/all lighting. I can even manage the occasional 1KW space heater in kitchen or bathroom as my typical average load is well under 2KW(lights entertainment and pellet with reefers and freezers cycling as needed). Cooking is a bit of a juggle as we shutdown power to well pump and reefer/freezer to allow range cooktop use(2 small or one large burner) and microwave to be used. The oven is a stretch and would probably have to shutoff the pellet stove as well to maintain the load, but again mine is for backup power, and we can live for a good long time without a conventional oven. I can basically manage all the normal items except the electric hot water heater(4.5KW) and electric dryer(11KW).
I have a heat exchanger instead of a radiator on the engine and it delivers 120F water out of the secondary loop which will keep the hot water tank topped with hot water. My avitar is a diagram of my system. You can do a lot with a full tank of hot water. 2 loads of laundry, 2 quick showers, wash dishes ect... In fact with the ammount of water my washer uses, I think the heat input would keep up with output so I could probably do laundry all day long. I just couldn't dry it that fast:) At a 2KW load, I get about 1/2 GPM of 120F water, so a 50 gallon HW tank will take about 100 minutes to recharge. Since my average load is less than 2KW, a fully charged tank of water will take over two hours. Once the tank is full of hot water the water exiting the bottom of the tank starts to get hot. This water returns to the heatex via a fan-coil unit(radiator). During the winter this will cool the water going back to the gen and put that heat into the house. During the summer when I don't need heat, the fan-coil gets connected in the loop outside to direct the excess heat outside. Ironically I can harvest about the same ammount of waste heat from the generator that the pellet stove puts out, so when running the generator the pellet stove can be shut off. Powering the pellet stove and the well pump was the prime reason for building the gen in the first place:)
So I can live quite comfortably on a 3KW generator. If I averaged a load of 2KW, I will consume right at 0.25 gallons per hour or 4 hours per gallon. Running say 16 hours a day, a 55 gallon drum of diesel would last me 14 days... But since I average less than 2KW(even less with the pellet stove shutoff), I am forcasting closer to 20 days for a 55 gallon barrel of fuel, which I always have at least one of on hand...