EmailChoosing 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, which is 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. Some brands put the maximum wattage or surge wattage in the model name, which can be misleading.
As you think about 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 on a boat or to a campsite. At the top end, these can keep a few appliances running simultaneously during a power outage, such as a refrigerator, several lights and a computer or TV. For electronics, the safest bet is an inverter generator, which keeps power consistent 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 feel are essential to run at the same time.
- 10,000 to 17,500 watts -- for providing backup power to larger homes or all-electric 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. So if you want to run two 30-amp circuits plus two 20-amp circuits, for total of 100 amps, that's 1,200 running (rated) watts. For individual appliances or tools, you can check the labels to find the starting and running wattages. If the label lists voltage and amps instead of watts, multiply the two figures together to get the wattage.
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), so if you can afford it, buy a bit more than you think you'll need.
Another approach is to buy the size generator you can afford and rotate usage among essential devices as you need to. 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. You can also buy a conversion 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. For example, the 10,500-watt NorthStar Trifuel 165939 (*Est. $3,000) produces only 9,600 watts on propane and 8,400 on 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.
While we found some online directions for installing transfer switches, most experts recommend professional installation by a licensed electrician. Understandably, most localities require an inspection, too.
Shopping tips
Most portable generators now 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.
In 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 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