Once you determine the size generator you need, there are several considerations to keep in mind about your generator purchase.

Fuel choices
Generators can run on several different types of fuel. Home generators almost always run on natural gas or propane. The primary advantage of this is that no fuel storage is required - the generator is simply connected to your utility lines and draws fuel when it needs to. The choice between gas and propane is based solely on which is available in your area.

Diesel fuel is popular for larger, commercial generators, but very rare for home models. They tend to be dependable and somewhat cheaper to operate than generators that run on natural gas or propane. However, they require deliveries of diesel fuel and a large holding tank, and while they've cleaned up their act considerably in recent years, diesel generators are still not as clean-burning as other types.

For portable generators, the most common choice for fuel is gasoline. However, the major drawback is that gasoline can't be stored for very long, so if you intend to keep a portable generator around for emergencies, you may want to consider propane instead. In no circumstances should you run a portable generator indoors: they produce dangerous fumes and cause serious health problems or even death.

Both portable and standby generators are sometimes available in dual- or tri-fuel configurations, where switching from natural gas to gasoline to propane is as easy as reconnecting a hose.

Motor speeds and cooling systems
Like any motor, a generator motor creates quite a bit of heat and needs a cooling system to prevent overheating. Standby generators can be either air-cooled or liquid-cooled. The major difference is that air-cooled systems are louder and not quite as effective. Liquid cooled systems are quieter and more dependable - and also more expensive to purchase and to maintain.

A related aspect of generator motors is the speed at which they run, as measured in RPMs. Generators made for the US market operate at one of two speeds: 1800 RPM or 3600 RPM. 1800 RPM motors will last longer and run quieter; 3600 RPM motors are smaller and lighter.

20 kW and larger generators are typically liquid-cooled, 1800 RPM units, which give the best combination of quiet operation and reliability. Portable generators are almost always air-cooled, 3600 RPM models. In some cases, you may be able to choose between the two RPM settings - go with 1800 RPM unless portability is important.

It's important to note that generator manufacturers don't actually build their own motors. They use engines from well-known names like Ford, GM, and Honda to power their generator heads. When choosing a generator, look for one that comes with a brand name motor: it will be much easier to find parts and service for a well-established brand than for some no-name model.

Additional features

• Safety features can prevent injuries as well as damage to your generator. Quality standby generators will shut down if they lose oil pressure, overheat, or are being worked too hard.
• Portable generators should have large fuel tanks for the best running time, a fuel indicator gauge, and enough receptacles to allow you to plug in all the devices you want to run.
• Standby generators should have solid steel or aluminum enclosures, good mufflers to reduce noise, and be compliant with all relevant emissions regulations.