When your electricity goes out, virtually nothing works.

Water still runs from faucets and toilets flush, but furnaces and air conditioners are kaput. Sump pumps stop. If the power failed because of a storm with flooding, your basement may begin taking on water.

Your refrigerator, microwave, your electric garage door opener, even the power behind oxygen tanks needed for medical reasons — everything stops.

That’s when a backup generator would be really handy.

There are two kinds: portable models that use an internal combustion engine, usually powered by gasoline, and standby units that are professionally installed, permanent pieces of equipment powered by natural gas or liquid propane. They sit outside your home on a concrete pad, much like an air conditioner.

Depending upon their size and corresponding cost, the standby units can power everything in your house.

Quad-City area businesses that sell standby generators say sales are steadily increasing.

“We started doing installations in 2005 and used to get about one inquiry a month,” Jeff Lanum of Russell Electric in Bettendorf said. “Now we get at least an inquiry a day.”

Not everyone who inquires ends up actually buying a system, but interest definitely is up, he said.

The typical cost range for units his company installs is from $5,000 to $10,000, he said. So far in 2013, the company has installed 12, he said.

Matt Hines, the owner of Doug’s Heating & Air Conditioning in Moline, agreed that demand is “steadily increasing. People are asking about them more and more.”

The units are going into new construction as well as existing homes.

Craig Windmiller of Windmiller Development and Design said that one in 10 new, mostly upper-end, homes might be built with a standby generator. The cost would be $7,000 to $12,000, he said.

With standby generators, size and installation are handled by licensed professionals.

Portables come with many cautions

The option of a portable generator — either purchased or rented — comes with a lot more caveats.

As Larry Smith of K&K Hardware in Bettendorf said in a bit of understatement, "You kind of have to know what you’re doing. It all sounds nice until you actually start digging into it."

First, and critically, if you want the generator to power your furnace, you must have a properly sized transfer switch installed by an electrician so the furnace is off your home's wiring. Without a switch, the electricity generated may result in harmful back feed onto power lines, putting you, utility workers and your home at risk.

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You cannot go out the day of the emergency and expect to plug the generator into your furnace.

On the other hand, a free-standing appliance — one that is able to be unplugged from your whole-house wiring — CAN be powered by a generator on the spot. Plug the appliance into an extension cord and then plug the cord into the generator.

With a portable, you will need to know how much wattage is required to operate the things you want, so you have to do your homework with a wattage calculator.

 "You have to add it all up and make sure you have enough (wattage)," Smith said.

You also will need at least three or four gallons of gasoline on hand, and you will need to keep purchasing more since the generators can use eight to 22 gallons per day, according to Consumer Reports. (Propane units are also available.) And because combustion emits deadly carbon monoxide, the generators should never be used in an enclosed space.

Stores that sell portable generators recommend that you start the equipment at least once a year to make sure it works. Because if you let it sit until you need it, chances are it won’t work.

Finally, they can be noisy — "louder than a lawn mower," Smith said.

While standby generators run into the thousands of dollars, the cost of a good portable can also go into the thousands, according to Consumer Reports.

K&K rents them for $40 to $70 per day, depending upon the size.