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Editors' pick: Originally published Jan. 20.

Here comes the sun. While just 30,000 homeowners tapped the orb for free electricity a decade ago, there are now about half a million homes equipped with solar panel installations. That number is expected to soar to as many as 4 million homes within four years.

It is easy to see the growing attraction for two reasons. Homeowners are increasingly interested in slashing electric bills that have been steadily increasing for decades, and they're finding that with solar panel prices plummeting, they have a way to do so without spending a fortune. They may even wind up paying nothing for utilities.

For example, over the past few decades, electric utility rates have tripled to a U.S. average of 12.5 cents a kilowatt hour. The average American uses more than 10,000 kilowatt hours annually, so that leaves a person with a bill of about $1,300 a year or more than a $100 a month.

Wiping out that budget drain keeps getting easier to do with solar energy. In the 1970s. it cost $100 per watt to install a system. Now it's roughly $3.50, and the price is expected to drop to a buck by 2020. As the typical system is five kilowatts, the average home solar buyer may soon be paying just $5,000.

In a few years, consumers could recoup their investment and slash their electricity utility bill by half or more. Install a somewhat bigger system, and the homeowner of an average-sized house could expect to knock it down to zero.

Still, prospective buyers need to do their homework. Outcomes may vary dramatically, and solar panels are not for everyone. There are many factors besides cost that homeowners must take into account, including financial incentives, a home's size, design and location, types of panels and whether buying or leasing works best.

The most important incentive is the 30% federal tax credit, which extends through 2023 and used at tax time following the purchase of a new solar installation. However, individual states may also offer significant incentives for reducing the cost of buying solar systems. These programs include rebates, tax refunds and access to utility grids, enabling homeowners to sell excess electricity back to power companies. The online Database of State Incentives for Renewable Energy offers a comprehensive state by state listing of what's available.

The location, design and size of a home also play an important role in determining how advantageous solar installations may be for a homeowner. How much sun a house gets versus shade also plays a big role.

A roof facing south provides the best exposure for a solar panel system. North is worst, while west and east are in between. A pitch of 25 degrees is good and typical for many moderate climes. The southwestern United States gets the most sun during the year, the Northwest the least. Yet almost any spot in the U.S. beats Europe, where home solar installations are even more popular.

The type of solar panel a homeowner buys may also be a factor for some homeowners. Panels cost much the same for the amount of wattage they deliver from the amount of sunlight they receive. However, the more efficient (and expensive) the panel the fewer a homeowner will have on the roof, which leads to less roof space used and possible lower installation costs. The world's most efficient solar panel is produced by SunPower (SPWR) . Panasonic (PCRFY) and Sanyo are competitive; all last 25 years or more.

For homeowners who think purchasing a solar installation requires too big an upfront cost and too much homework, or for those who aren't planning to stay long in their enough current home to make it worthwhile, there is often a way to have solar without buying it. Consumers may lease a system, which will deliver more modest savings on utilities, but without paying thousands of dollars for an installation. Among the leasing companies are SolarCity (SCTY) , the largest, as well as Sunrun (RUN) and Sungevity.

With more than 100 million homeowners in the U.S., there are many more millions who would benefit from having the sun as their most significant or only source of electricity.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.