Caveat: I work for Honda Generators.
The preceding posts were all spot-on, especially about using a transfer switch to isolate a generator from the power company's lines to use your home's wiring/outlets. With "backfeeding," you run the risk of fire, or worse, electrocuting power company employees trying to get your real power restored.
Next, the comments about engine speed...most engine-driven generators are direct drive, so engine rpm = generator rpm. In order for such a generator to make 60 Hz power (the US standard) the engine/generator must run about 3,600 rpm. However, some new generators (still direct-drive) use an inverter circuit to make power, and can do so from idle to full-throttle, depending on the load. Inverter-type generators are more expensive, but run quieter (due to lower speeds) and use less fuel.
And about fuel. For home stand-by power, unless you're in an earthquake zone, natural gas is probably the best overall choice. Gasoline goes stale, and can be hard to find/purchase if the power is out everywhere. Propane ain't bad, as it won't rot, but it's about triple the cost of gasoline for the same amount of power. Still have to go fill up large, "tall boy" tanks or have it delivered, both of which may be difficult if the power loss is widespread.
For home stand-by power, I'd go with a permanent-installed, natural-gas unit and automatic transfer switch. Ironically, Honda doe not make either. So, I get by with a portable gasoline unit and some extension cords.
