You know you need a new air-conditioner, but with more options then ever, how can you be sure you've chosen the right one? And how much do you really need to spend on it?

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Summer is here, and our homes are beginning to heat up. But just the thought of looking for a new air-conditioner is enough to make you break out in a sweat. How do you know which one to buy, and how it will affect your energy bill? What if your apartment building doesn’t allow window units?

Not to worry: We’ve done the research for you.

The key to comfort and savings is finding the unit that fits your space. An air-conditioner that isn’t powerful enough won’t effectively cool your home and could end up inflating your power bill. One that is too powerful will work too quickly, shutting off before most of the humidity has been removed, leaving your place cool but clammy. And all that cycling on and off can stress the equipment and shorten its lifespan.

Energy efficiency is another factor: The energy efficiency ratio is a measure of how well a cooling system will operate when the temperature outside is 95 degrees. Look for an air-conditioner with an Energy Star label, indicating that it is more efficient than required by government standards. But remember that no matter how efficient your air-conditioner is, you won’t get the most for your money — or be as comfortable as you could be — if your home is not properly sealed and insulated.

With all of that in mind, here are some guidelines to help you find the right air-conditioner for your home.

Central air

Central air-conditioning is the most popular type of cooling system in the United States. More than 75 percent of households with a cooling system use central air, according to the latest data from the U.S. Energy Information Administration.

But New York City is one of the big outliers. The reason: Its building stock is older than that of most major U.S. cities, according to estimates by the Department of Buildings, with about three-quarters of the buildings constructed before 1960 — in other words, before central air-conditioning became popular. Retrofitting older apartments with central air means installing a condenser outside the home, a fan-and-coil system inside and ducts to distribute the cooled air. That’s not cheap, and it requires space, a resource lacking in many apartments.

“It is very tricky to retrofit a building that was built without air-conditioning,” says Andrew Gerringer, managing director for new business development at the Marketing Directors, a New York development, leasing and marketing company. “Buildings that have installed it have done so through great expense and aggravation.”

Even if you are willing to give up a closet to install a central air system in your prewar apartment, your building may not allow you to put a condenser on the roof. And retrofitting an apartment built without ductwork could be prohibitively expensive. In the best-case scenario — where you don’t have to run a pipe through your neighbor’s kitchen ceiling to get to your compressor — installing or replacing a whole-house system can cost, on average, from $2,650 to as much as $15,000, according to Fixr.com, a site that tracks home improvement costs.

Another consideration: Most central air systems lack room-by-room control, so if you want to cool the bedroom while you’re sleeping, you’ll have to cool the rest of your home, too. Still, because the ducts and mechanical components are built in, effectively hidden within the walls, floors or attic, central air is the most discreet option.

If you decide it’s right for you, check out Consumer Reports’ buying guide for reliability by brand and be sure to consider the system’s Seasonal Energy Efficiency Ratio, which measures how efficiently a central air-conditioner operates over an entire season (like the energy efficiency ratio, the higher the number, the better).

Bottom line: If you have space for the ductwork, central air is a quiet, convenient and design-friendly way to cool your home.

Ductless mini-split air-conditioner

If central air is not an option, a ductless mini-split system may be your best bet. Mounted on a wall and operated by remote control, these systems still require an outdoor compressor, but there is no bulky ductwork involved. Refrigerant is circulated through tubing that connects the indoor and outdoor units.

Though not as discreet as central air, ductless mini-split systems are highly efficient, as each unit can be controlled separately. These systems can also provide heating. The cost of outfitting a 2,000-square-foot home ranges from about $1,800 to $7,000, according to Fixr.com; the more wall units you need, the higher the cost. As with central air, you will need a professional installer and should budget for an annual maintenance check.

Window units

Inexpensive and easy to install, window units are among the most popular options for cooling individual rooms. Starting prices range from $129 for a small unit designed to cool a 150-square-foot room to $599 for a unit designed to cool a 1,600-square-foot space, according to Lowe’s.

To figure out what size you need, measure the room you want to cool and calculate the overall square footage. You can find the recommended cooling capacity (measured in British thermal units, or BTUs, per hour) for your room size at Energystar.gov/roomac.

Be sure to take into account sun exposure and how the room is used. If the room is heavily shaded, for example, the Energy Star chart recommends reducing capacity by 10 percent. If more than two people regularly occupy the room, add 600 BTUs for each additional person. In the kitchen, increase capacity by 4,000 BTUs.

Smart air-conditioners

If you want to control your window unit with your smartphone, most of your options have glitches. “A smart appliance is supposed to make your life easier,” says Liam McCabe, who covers appliances for The Sweethome, a product review site owned by The New York Times. “But so far, every smart A/C that we’ve tested has done the opposite.”

Even the best of the smart window unit air-conditioners tested last summer stopped consistently responding to commands from the app over time, he says.

For controlling whole-home systems, or central air, the sleek Nest Learning Thermostat ($245) is the best option for most people, according to The Wirecutter, another product review site owned by The New York Times. If you want to change the temperature without getting up from the couch, it allows you to do so using your smartphone or computer, or with your voice via Google or Amazon Echo.

Nest also learns the temperature you like and builds a schedule around when you’re home, which can help reduce energy bills.

Other options

Built-in air-conditioners. Don’t want to block the view with an unsightly window unit? Built-in air-conditioners, also known as through-wall units, are mounted in a metal sleeve that goes through an opening cut in the wall.

Portable air-conditioners. These free-standing units, most professional reviewers say, should be used only as a last resort. Portable air-conditioners are not only more expensive, less efficient and noisier than window units, but they sit entirely inside the home, taking up valuable floor space, and use unsightly venting tubes that look as if they belong on the back of a washing machine. In short, they are bulky, ugly and don’t work very well.