Choosing generator wattage

Once you decide to buy a generator, the first decision to make is how much wattage you'll need. Reviews recommend comparing portable generators by rated or running wattage -- the watts the generator can produce on a continuous basis. Maximum or surge wattage shows how much extra power the generator can provide for a short time, to start motors or large appliances like air conditioners, for example. Be advised: Some brands put the maximum wattage or surge wattage in the model name, which can be misleading.

As you consider your wattage needs, it's helpful to know that portable generators fall roughly into three size groups:

  • 750 to 3,500 watts: small enough to carry onto a boat or to a campsite. At the top end, these can keep a few appliances running simultaneously during a power outage, including a refrigerator, several lights and a computer or TV. For electronics, the safest bet is an inverter generator, which maintains consistent power and prevents electrical surges that can damage computers and televisions.
  • 4,000 to 8,000 watts: enough for extended emergency use for a 1,200- to 3,000-square-foot house. The exact size needed will depend on how many appliances you think are essential to run simultaneously.
  • 10,000 to 17,500 watts: for providing backup power to larger homes.

For household backup, the simplest way to estimate the wattage you need is to look at your circuit box. Add the amps of the circuits you think you would want to use simultaneously, and multiply the total amps by 120. If you want to run two 30-amp circuits plus two 20-amp circuits, for a total of 100 amps, that's 12,000 running (rated) watts. For individual appliances or tools, you can check the labels to find the starting and running wattages.

See the Useful Links section for information to help you calculate the wattage you need, as well as links to several online charts that list typical wattages required by various devices. Experts say generators last longer if used at about 75 to 80 percent of their rated wattage (rather than maxed out), and if you can afford it, buy more output power than you think you'll need.

Another approach is to buy the size generator you can afford and rotate usage among essential devices as needed. Reviews note that you can rotate appliances as frequently as is necessary -- run the refrigerator for 10 minutes to an hour, then run the freezer, then the water pump, etc.

Choosing fuel

Generators can run on a number of fuels -- gasoline, propane (LP), natural gas or diesel -- even on solar energy. However, most portable generators are powered by gasoline. Multifuel or dual-fuel portable generators allow you to switch among fuel types, but we found no reliable reviews of such models. You can also buy a kit (Est. $160 to $400) to convert just about any portable generator to a multifuel or propane model.

The wattage you need may help you determine the best fuel to use, because big portable generators use more gasoline than you may wish to store. Another consideration is that gasoline may become unavailable during a widespread, prolonged power outage.

Bear in mind that the fuel you burn affects the wattage produced, because portable generators produce less wattage from propane than from gasoline, and still less from natural gas. If you plan to buy a multifuel portable generator or convert one to propane or multifuel, be sure to allow for this.

Transfer switches

Most portable generators build in several outlets -- at least two 120-volt AC outlets and often a DC outlet for charging a car battery. You can plug devices directly into the generator's outlets or use heavy-duty extension cords to bring the electricity into a building. For more convenience, and to run any device that doesn't plug into an outlet, you can install a transfer switch (Est. $250 and up). When you use the transfer switch, the circuits of your choice are transferred from your main breaker box to the transfer circuit-breaker box installed next to it.

When you use a transfer switch with a portable generator, you still have to start the generator first. Then you can run just one cord from the generator to the transfer switch -- or for even more luxury, have a power inlet box (Est. $50 and up) installed on the outside wall. Then you can plug the generator into that. You'll still need to go outside to refuel the generator periodically and change the oil.

Although we found some online directions for installing transfer switches, most experts recommend professional installation by a licensed electrician. Most localities require an inspection, too.

Shopping tips

Most portable generators use fuel-efficient, four-cycle overhead valve (OHV) engines that are easier to start, quieter to run and better for the environment than two-cycle engines. You can tell it's a two-cycle engine if you have to mix oil with the gasoline. Most new portable generators also provide circuit breakers, automatic voltage regulation and some type of weather protection for outlets and controls.

When choosing a portable generator, you should consider these additional factors: 

  • The rated wattage can be optimistic. Wattage ratings are calculated under ideal conditions; lower temperatures and/or higher elevations will lower the amount of electricity produced. Experts recommend allowing about a 20 percent margin.
  • CARB compliance means lower emissions. Portable generator engines that meet California Air Resources Board (CARB) requirements, sometimes called 50-state compliant models, are the most environmentally friendly.
  • Acceptable sound levels depend on the situation. Portable generators' noise levels range from 47 to 79 decibels, with an average of about 75 decibels. For camping, note that the National Park Service requires sound levels no higher than 60 decibels at a distance of 50 feet.
  • Full lubrication with a spin-on oil filter prolongs engine life and reduces maintenance. Reviews say portable generator engines with this feature cost more, but last up to three times longer.
  • Automatic low-oil shutdown can save the generator. Portable generators with this feature shut themselves down if the oil gets too low, which can save the motor.
  • A fuel gauge adds convenience. The fuel gauge also reminds you to check the oil, because that should be done every time you add fuel.
  • Idle control lets you save fuel and minimize noise by adjusting the engine speed to one-fourth, half or three-fourths of the maximum, according to the electrical load. A generator with a smart-throttle mode or economy mode adjusts the engine speed automatically. However, adjusting the engine speed also limits the wattage available for starting big motors, and you may not be able to use one of these features all the time.
  • A spark arrester approved by the U.S. Department of Agriculture or U.S. Forest Service is especially important for portable generators used for camping and to help prevent fires in general.
  • For household use, look for at least four AC outlets, preferably lock-on with a ground fault circuit interrupter (GFCI) and one DC outlet. Locking outlets are safest, as are OSHA-approved GFCI outlets, because portable generators are often used in wet conditions. 

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