Sizing the generator is the first and ultimately most
important step, since it has a profound influence on
fuel storage requirements.

The minimum size is whatever's needed to start the 
largest single load on your system. That might be
an air conditioner, it might be a toaster. The 
resulting no-load fuel consumption will set a
baseline, along with the endurance desired, for
how much fuel you need to store. 

Suppose you want to run  a five ton air conditioner. 
That will have a running draw of about five kilowatts.
The starting surge will be around ten kilowatts, so 
let's assume you buy a ten kilowatt generator to have 
an adequate reserve (consumer generators have a very
slender starting power margin compared to ratings).

A ten kW generator will burn about 1 gallon per hour
_at_no_load_. Closer to 2 GPH at rated load. About
1.5 GPH at 5 kW load. Even if the A/C is used only
a few hours per day, tens of gallons of fuel must
be stored. If you want to use the genny for other
loads with longer use cycles (well or sump pumps,
kitchen appliances, lighting and communications),
the fuel storage burden will be too great for most
urban fire codes if you want to run for a week.

In contrast, a 2 kW generator will run most 
countertop kitchen appliances. That will burn
around a quart of fuel per hour at no load,
half a gallon at full load. For the same run
time that's a quarter as much fuel, a quantity
that _might_ be ok with the local fire chief. 

The point here is that load management is very
critical. We're used to having tens of kilowatts
at the flip of a switch. Backup power generally
needs to be managed much more carefully.  

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