Photovoltaic efficiency keeps improving. SunPower recently achieved 24 percent efficiency with its panels, delivering 70% more electricity than conventional solar panels from the same space over 25 years.
So, you are thinking of building a new home, and you want it to be as energy efficient as possible. You’ve heard that the cost of solar panels has dropped significantly. But you aren’t sure of the next steps. First the good news — you heard right! The cost of photovoltaics has dropped precipitously in recent years, thanks to a rush of imports, improved solar efficiencies, and bigger economies of manufacturing scale. The installed cost of a residential system has fallen to a tempting $2.70 a watt, making it very competitive with purchasing power from the utility.
That’s led to a big increase in residential installations – they nearly doubled from 2014 to 2015 alone. “Things are really looking good for solar,” said Ron Fergle, of Solart Inc., speaking to a packed room of architects at this year’s American Institute of Architects conference. It’s reached the point where producing solar electricity is cheaper in the long run than buying it from a utility, as long as you are willing to make an upfront investment. “Photovoltaics are cost competitive with fossil fuels,” says Jason Jewhurst, a senior associate with Bruner/Cott Architects & Planners, which specializes in urban housing projects.
How much would it cost to buy a system for your new home? Installers say the size of the typical photovoltaic installation is about 5 kilowatts (kW). At $3 per watt, that means the system only costs $15,000, half of what it cost five years ago. A super energy efficient new home, on the other hand, may only need a 2kW system. That would run about $6000. And that’s before tax breaks. The first is a federal solar tax credit of 30 percent. States may offer tax inducements.
The economics of photovoltaics are so compelling that some people design net-positive homes that produce more power than they consume. In more than 35 states, the excess power can be sold to the local utility at full retail rather than wholesale rates. And you can still buy electricity from the utility when you might need it. Photovoltaics are typically only one part – arguably the most important one – of a system that powers your home. Most photovoltaic panels produce direct-current (DC) power that must be converted to alternating current (AC) with a stand-alone inverter. Some newer panels now come with built-in inverters.
The missing piece of this equation has been a reliable battery for storing electricity for use on a cloudy day. Inefficient lead-acid batteries used to be the only choice. In the last two years a new generation of deep-charging batteries – some work with lithium-ion technology, others with salt water – have hit the market that can hold four times the electricity and last three times longer. Acquion, whose battery modules are shown above, employs environmentally benign salt-water technology to produce a deep-charging battery that’s compatible with the new generation of lithium-ion batteries that have hit the market.
Many solar systems also come with software that tracks your energy consumption and use. You can come home in the evening, find out how much electricity your home produced during the day, and adjust your nighttime use accordingly. Real-time data may remind you to close doors and windows so that conditioned air doesn’t escape. Some systems tell you how much electricity different home appliances consume.
If you think you are located in a part of the country where photovoltaics won’t work well due to clouds or industrial pollution, think again. One of the toughest places to run a solar system in the United States is Seattle, where storm clouds cover rooftops for much of the year, Fergle said. But Germany has roughly the same solar insolation levels (or solar exposure) as Seattle, and it was the world leader in electricity produced by PVs until it was recently passed by China. The United States is in a way better position to use photovoltaics than Germany. “The United States has huge untapped potential,” said Fergle.
Where Should I Put the Panels? To make optimal use of PVs, your home must have a clear view of the sun for most or all of the day — unobstructed by trees, roof gables, chimneys, and buildings. The best location for a PV system is usually on a south-facing roof, since in the U.S. the sun is always in the southern half of the sky. That said, a roof that faces east or west may also be acceptable. If you can’t use the roof, you could mount the panels on a pole or a trellis in the yard, as shown below at a Deltec demonstration house in Asheville, SC.
Roofs with composition shingles are the easiest to work with; slate the most difficult. PV panels can often be integrated into the roof itself. Some modules are actually designed as three-tab shingles or metal-roof sections. Using PV panels as an awning can provide both electricity and shade.
What Type of Solar Should I Use? There are two basic kinds of photovoltaics to consider. The first employ Crystalline Silicon cells. They are the most efficient and command 80 percent of the market. If your home will have good solar exposure – a big roof facing south in a sunny climate — they are often the best choice. They may also live longer. They commonly come with a 25-year warranty. The actual efficiency of the panels ranges from 15% to as high as 24%, depending on the manufacturer. SunPower achieved a record 24.1 percent efficiency with its X-Series panel.
There are two types of crystalline silicon cells — monocrystalline and polycrystalline. Monocrystalline are more expensive but yield better results.
Thin film photovoltaics, on the other hand, may perform better at lower light levels, and at high temperatures. Thin film may be a better choice with low light levels — where the lot is heavily treed, or if the home isn’t oriented to have much south-facing roof space. Thin film photovoltaics are made of thin layers of PV material deposited by gas on backing material. The film is basically rolled on roofs, louvers, or walls.
(Here thin film is part of the roof in an early version of the Future Farmstead project in Georgia.) The film is flexible, lending itself to application on a wide range of surfaces – not just the roof. Thin film also may make more sense where space is not an issue – like on a farm or in a rural setting. Homogenous in appearance, thin film isn’t as efficient as crystalline silicon at converting sunlight to electricity, with efficiency ranging between 7 and 13 percent. It also degrades faster and comes with a shorter warranty.