When Drew and Lily Derrick decided they wanted to add central air conditioning to their 200-year-old Catonsville home, they turned to an unexpected source: the ground.

The couple installed a geothermal system that uses the constant 55-degree below-ground temperature as a means for both cooling and heating. The system — which required drilling three 430-foot-deep wells in their front yard, plus piping and ductwork — was completed in July. The next month, they cranked up the air conditioning.

Even with the addition of central air to cool the entire house, their utility bill was only $20 higher than the month before — when they had window units cooling only parts of their home. The new system is far more efficient, they say.

Not far away, Dave Sinclair and his wife, Angela, chose to have solar panels installed at their Catonsville home in spring 2015. In 2014, their average utility bill was $165 a month. The solar-panel system has since cut their bill by more than half, to the point they are sometimes paying a minimum $22 fee on their monthly bill.

Across the region, more homeowners are looking at alternative energy sources, spurred by a new generation of efficient, cheaper systems, as well as tax credits, rebates and incentives — as well as the desire to reduce the use of fossil fuels.

“Resources are finite. Fossil fuels are finite,” said Lily Derrick.

Since 2008, more than 21,000 of Baltimore Gas and Electric Co.'s 938,000 residential electric customers have added solar systems, the utility said. More than 1,800 geothermal units have been installed since 2012 by BGE residential customers.

While state and county agencies don't actively track installations, in the past two years, Baltimore County has done an average of 150 solar system inspections a month, up from 15 a month in 2013.

The Solar Energy Industries Association, a national trade group, said $410 million was invested in solar installation projects in Maryland last year, an 86 percent increase over 2014.

“It's becoming more of an everyday thing,” said John Hencken, vice president of SolarGaines, a Baltimore solar power system installer who has two solar arrays on his Catonsville house.

Tax credits, rebates and other incentives can lower costs for such systems, but prices can still be steep: An average geothermal system in Maryland runs around $25,000 and purchasing a solar system can be about $30,000.

Industry officials say the savings on utility costs over a five- to 10-year period can offset those upfront costs.

“It's not some sort of off-the-wall thing, where people are hoping it would pay them back one day in the future,” Hencken said. “It's much more clear.”

For the Derricks, the upfront cost to install a geothermal system and related interior work was about $70,000. After applying rebates and tax incentives, Derrick said the cost dropped to about $40,000.

The state makes solar projects eligible for a $1,000 credit and geothermal heating and cooling systems are eligible for a $3,000 credit. BGE offers a $1,620 rebate on geothermal heat pumps through its BGE Smart Energy Savers program.

Home solar systems can qualify for a federal tax credit equal to 30 percent of the purchase price. The federal government also has offered a 30 percent credit for residential geothermal heating and cooling installations, but it's set to expire at the end of 2016.

Baltimore County provides a property tax credit of up to $5,000 for solar or geothermal heating devices, and $1,500 for solar hot-water heaters. The program has been so popular there's a waiting list until 2021; the county funds the program at $750,000 a year, officials said — not enough to keep pace with applications.

The Sinclairs are on the wait list for the county credit.

The Derricks estimate it will take about six years for the savings from his geothermal system to cover the price difference between geothermal and a more conventional system.

“People may have known about geothermal in the past or heard about it, but never considered it a real option for them,” said Alex Kougianos, owner of Supreme Air, the Baltimore air conditioning, heating and geothermal company that installed the Derricks' system. “Then incentives were created, and it got the word out.”

Supreme Air's geothermal sales have been on the rise each year, Kougianos said. Last year, the company's geothermal division installed about 170 units — about $3.8 million in revenue, he said. This year, the company is projected to do about $4 million in geothermal work.

While system prices vary, largely based on drilling fees, the installation cost for a typical house in Maryland is in the mid-$20,000 range, in line with the national average, according to Douglas Dougherty, president and CEO of the Geothermal Exchange Organization, a nonprofit trade group.

The typical payback period for a geothermal system — as lower energy bills compensate over time for the installation costs — is four to seven years, and can be shorter with rebates, according to the trade group.

SolarGaines' Hencken said solar panel purchases gained momentum after 2009, when the federal government sweetened tax credit incentives, removing a $2,000 cap that was in place. He said the cost of materials has come down in recent years, making systems more affordable.

“That's what made it an industry,” Hencken said.

SolarGaines, opened in 2009, has had business volume double every year and now has about 400 customers, he said.

When it comes to solar-power systems, customers can buy them, lease them or sign what's known as a purchase power agreement, or PPA. With such an agreement, a company installs, owns and maintains the system and sells electricity to the homeowner at a price that is typically below market rates.

Lease and PPA contracts generally are in force for 10 to 25 years, according to trade associations, though terms can vary.

The Sinclairs spent between $30,000 and $40,000 to install a 36-panel solar system. They decided to buy the panels outright instead of leasing, so they could take advantage of tax credits and incentives. With rebates and credits, Dave Sinclair believes they'll recoup costs within 10 years.

“I think plugging into things like solar energy ... is a great opportunity,” Sinclair said. “It's a good way to supplement and offset some of the dirty power generated for our use.”

Drew Derrick has recommended geothermal systems to others but acknowledges that the costs can be prohibitive without incentives. He's concerned about the federal credit expiring at the end of this year.

“It's going to be harder and harder to warrant people getting these systems installed,” he said. “The cost is twice as much, and without that 30 percent off and those other rebates, it's just not going to be affordable to an average person.”