Solar hot water systems work by capturing the thermal energy present in sunlight and turning it into usable energy in your home.

When solar hot water collectors (either flat plate or evacuated tube style) on your roof are warmer than your water tank, a differential temperature controller activates a pump. This pump then circulates a nontoxic antifreeze mix up to the solar collectors on your roof, where the fluid heats up.

This solar-heated fluid is then pumped to a super-insulated storage tank, where it gives off its heat as it passes through a heat exchanger. It is then pumped back to the roof, continuing the cycle as long as the sun is out.

Should there be unexpectedly high solar thermal gain (or should you be away), the system is designed to automatically go into an overheat protection mode called “Steamback.” Once a certain temperature threshold is hit, the solar circulating pump stops circulating fluid and the fluid present in the collectors turns to steam and fills an expansion tank. The copper elements of the solar hot water collectors are not harmed by high heat and will reach high temperatures until the end of the day. After the system has cooled overnight, the differential temperature controller will reset and allow the system to start running normally the following day.

From May to September, all of the hot water a household uses for washing dishes, clothes or taking showers will be provided by the sun, even on cloudy days. In the darker months of winter, the sun will still help out, but a backup unit – such as a boiler or electric element – will run to ensure a steady supply of hot water. Instead of running all summer, a home’s boiler will only fire up as a backup unit to heat the second coil in the upper part of your tank.

Overall a typical system will produce about 80% of a home’s water heating needs.

This recent project in Madbury, New Hampshire features both a flat plate solar hot water array, and 5kw of solar electric production

The dizzying drop in cost of photovoltaic (PV) panels is great news that’s making headlines, as well as shifting the way architects, green builders, and even solar installers think about venerable solar thermal technology.

With high-efficiency electric heating equipment available, and PV at record-low prices, doesn’t it make more sense to install an electric water heating system and then use PV to generate electricity to offset that electric load? Isn’t that “Solar hot water?”

Well… not so fast. While electric water heating backed up by PV works great for certain cases, the full story is a bit more nuanced. For homes occupied year-round with moderate (60GPD+) hot water consumption, solar thermal remains a clear winner for domestic hot water loads (the game is changing in solar space heating, but we’ll address that another time).

Solar Thermal: A Use it or Lose It Solution

For solar thermal to make sense you should occupy your home during the summertime, so as to take advantage of free hot water during peak production months. Unlike photovoltaic systems, which are able to bank excess summertime production for the winter by selling power to the grid, solar hot water systems are not good at providing storage for more than a few days at a time.

When properly sized, a solar hot water system will provide around 76% of year-round hot water consumption: 100% of the hot water a household consumes from May to September (and not too much excess), and ~50% of hot water use in the off-season.

Clearly, a home needs to be occupied in the summertime for solar thermal to make sense:

A Tale of Two Houses

For an active family, solar thermal is the least expensive way to meet year-round domestic hot water loads.

Solar thermal technology is much more efficient at converting sunshine to heat than photovoltaic panels are at converting sunshine to electricity (~60% vs. ~19%, respectively). This means that more hot water is available, at a lower cost, than a comparable electric-powered system. The differences are minor for a house with small hot water loads (<30gal/day) but as a household's hot water needs increase, solar thermal outpaces an all-electric system pretty quickly.

Let's compare the options for two houses. The former is a single person moving into a high-efficiency home. She uses a meager 20gal/day of hot water. Another is a family of 4 in a conventional home, which uses around 60gal/day.

Using a new, efficient electric tank, the single person will use around 1,600 kWh of electricity each year to heat water. To offset that with PV, she will need around a 1.3kw system. We will assume she has already maxed out state rebates on a previous PV system for "plug loads" (i.e. all of their other power needs) but still qualifies for the 30% federal tax credit. At a rate of $4.5/installed watt, the 1.3kw system + nice electric heater will come out to around $5,095. That's indeed less expensive than a solar thermal system, which will cost about $6,700 after state and federal rebates, and still require a bit (~$200) of electricity each year for water heating.

Things change quickly with a family that uses 3 times that amount of hot water (common, even conservative in many cases). In the 4 person household, we'll say the home uses 4,700 kWh/year for water heating. A 3.7kw PV is needed to offset the larger load. Again, assuming a federal credit but no state rebate, the PV system + nice water heater comes out to around $11,500. Now the solar hot water system, at $6,700 net, is the obvious choice (even if this home has not used up state PV rebates). Even adding a bit of PV to cover backup water heating (1kw will cover that $200/year nicely), the whole system comes out to around $9,200 total.

In short, the choice between one option or another depends a great deal on the behavior of a home's occupants and the size of a home. Seasoned solar designers will look at the overall picture of a home rather than assuming that "one size fits all."

Solar Hot Water Versus Heat Pump Water Heaters

Schematic of a Stiebel Eltron heat pump water heater

The introduction of heat pump water heaters makes the the solar hot water vs. electric water heating debate more complicated. For a cost of around $3,000 installed, these high-efficiency electric water heaters offer 2.5x more heating per kWh than conventional water heaters.

We recently calculated a demand of around 1289kWh/yr to run one of these units for a new construction project, or about the same annual production as installing 1kw of PV panels.

Based on an installed price of $4.5/watt for PV, this hypothetical hot water system results in an installed cost of around $7,500 prior to incentives, which are worth $1,950 (note that most PV systems are a minimum of 1.88kw so more likely the PV system would be sized a bit larger and meet some part of other household loads as well).

While the heat pump and PV solution hold a slight cost premium (around $1,150 less) than solar hot water in this case, it's not quite a clear case against solar hot water:

• If you are already investing in a PV project, adding solar thermal opens up additional state rebate money you would not other tap into. At $1,000 or $1,500 (ME and NH, respectively), the state rebates for solar hot water are more generous than for heat pump water heaters.
• Heat pumps are noisier typically than a solar hot water tank. This matters most in new, efficient buildings where mechanical rooms may be close to living quarters.
• Heat pumps achieve their higher efficiency by using ambient air temperature to heat water - in the wintertime this means using heated interior space for water heating, which adds to the overall heating load of the building, thus more energy is consumed to keep the building at a comfortable temperature.
• Warranties on heat pumps and solar hot water equipment are similar (10 years), though we feel that the long-term maintenance costs of solar hot water will be less than service costs of heat pump systems.
• Solar hot water collectors will take up less space to produce a household's hot water than a comparable PV array offsetting an electric device. This is important for households that want to make the deepest cut into their electric bill as possible and have limited south-facing roofspace.

Solar Thermal Stays Strong

The dramatically shifting landscape of solar electricity makes for exciting times. It's never been better to invest in a photovoltaic system, particularly while both Maine and New Hampshire have generous state rebates.

That being said, solar hot water systems still make great financial sense in many cases. Here's the takeaway:

Good Candidates for Solar Thermal Water Heating:

• Will replace an inefficient, high-mass boiler or older propane/electric tank for water heating
• Occupy the home year-round
• Use a fair amount of hot water
• Have adequate space to install a 80-100 gallon storage tank

Good Candidates for Solar Water Heating with Electric + PV:

• Vacation or seasonal homes
• Already have an efficient water heating system (on demand or efficient electric tank)
• Meager hot water needs
• Not a lot of space available for mechanical systems

A recently installed solar hot water system in Gardiner, Maine

Great news for Mainers ready to start kicking their oil habits – Biddeford Savings Bank is now offering a 7.99% APR loan program for financing solar energy projects and other efficiency upgrades.

With home heating oil averaging $3.67/gallon in Maine, financing a solar hot water system makes it possible for many homeowners to save more money in fuel costs than they spend on the loan note.

Why Financing Solar Hot Water Makes Fiscal Sense

More than 400,000 homes in Maine heat with oil, and many of these homes get their domestic hot water supply off of a high mass oil boiler. While this solution makes some sense in the wintertime, when the boiler is already running for space heat, in the summertime boiler efficiency is terrible – as low as 20% – leading to unnecessary waste from poor boiler efficiency and standby losses.

Solar hot water systems allow these homes to heat their water nearly entirely with solar energy from May through September (yes, even in Maine). With the boiler now largely dormant in these months, these homes will see a fuel oil savings of roughly 350 gallons of oil a year. That’s a savings of $1,284 a year and set to increase as oil prices continue climbing.

Now, with a solar hot water system averaging $12,500 gross, and around $7,750 after state and federal incentives, by paying cash the payback is less than 7 years. With financing that payback can be nearly instantaneous.

By putting down not quite half of the cost of the system, which is largely reimbursed by federal and state rebates, a homeowner can make a cash flow positive investment in solar hot water starting year one.

Financing Solar Hot Water Systems by the Numbers

Below posits a $6,500 loan with a $6,000 downpayment on a solar hot water system. The initial $6,000 outlay is largely reimbursed by state and federal incentives, and ultimately completely recovered by energy savings.

After the 10 year payback on the loan, you’ll experience many years of free hot water! Contact us with any questions about solar hot water system financials or for a complimentary site evaluation. You can call Biddeford Savings for loan details at: 207-284-5906.

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ReVision Energy co-founder Phil Coupe speaks at a press conference held to announce a new report on the efficacy of solar hot water in Maine.

In a press conference last Thursday Environment Maine released a report finding that Mainers can save millions of gallons of oil a year with solar hot water.

“We need to do everything we can to get Maine off oil, and installing solar hot water systems is one of the no-brainers. We have long had the technology and know-how to harness the zero-cost heat of the sun to produce hot water, while at the same time cutting pollution and putting people to work in our communities. And more than ever we have a workforce that is ready to install these affordable solar systems on roofs across the state,” said Environment Maine Field Associate Nathaniel Meyer, speaking in front of Senator Justin Alfond’s East End home, which has a rooftop solar hot water system.

ReVision Energy co-founder Phil Coupe continued. “Solar hot water is one of the safest investments you can make – with relatively small upfront investments, the financial and environmental return is guaranteed,” he said. “Maine is ripe for this technology. We get 33% more sun than Germany, the world leader in solar installations. On a sunny 20-degree day, a solar hot water system can generate water that’s 130 degrees – water that’s too hot to shower in.”

See video coverage from NECN:

Video also provided by WCSH:

And several news articles on this important story!

• Bloomberg Businessweek
• Bangor Daily News
• Portland Press Herald
• Solar Thermal Magazine
• Forbes

From our photo gallery:

Terry McIlveen recently reported that he is “amazed on sub zero days the temp on the collectors will hit 130-140 … It is high enough to offset demand for radiant heating.”

The idea of heating your home with solar may sound like a dream in the tough climate of New England, but with smart design, a well-insulated home, and reasonable expectations, solar can indeed carry a portion of your winter heating load.

A recent Mother Earth News article profiled one of our solar space heating customers, Terry McIlveen, and asked several questions of resident engineer and company co-founder Fortunat Mueller.

Scott Gibson writes:

When Terry McIlveen built his home in Maine in 1997, he made the unusual choice to install radiant floor heating. “People thought I was nuts,” he says. However, since that time, radiant floor heat has become increasingly popular, and it’s easy to understand why. This type of heating system works by pumping hot water — or water and propylene glycol (antifreeze) — through a system of tubing in the floor. That means houses using this heating system get warmer from the floor up. In the winter, there are no cold floors underfoot — instead the floor is the warmest part of the home.

McIlveen soon discovered an additional benefit — radiant floor heat is a great match with solar hot water. If you already heat your home with hot water, it’s just one more step to heat that water with solar energy. In the spring of 2010, McIlveen hired ReVision Energy, a southern Maine solar company, to install rooftop solar collectors to help heat his home and produce his domestic hot water, thereby cutting his fuel oil consumption by up to 25 percent each year.

The system cost $20,000, so McIlveen chose to finance it through a loan. His exact savings on fuel each year will depend on how much he has to run the heater in the winter, as well as the ever-changing price of fuel oil — but he knows that if he saves just two fuel tanks a year, he can cover his loan payments.

Using solar hot water for space heating won’t supply 100 percent of your heating needs. You’ll still need a supplemental heat source, so you won’t be able to scrap your furnace or boiler. But in new construction, that supplemental heat source can be much less expensive (a smaller system, for example). However, under the right conditions, a solar thermal system can replace a significant portion of conventional energy sources for both space heating and domestic hot water.

Read more: http://www.motherearthnews.com/renewable-energy/solar-hot-water-zm0z11zphe.aspx#ixzz1ByEFseL2

You can also see Terry McIlveen’s home when it was featured on WMTW in mid-2010.

With winter weather starting to arrive in Maine, many customers are wondering how their solar energy systems will perform. The answer is usually – better than you think it will!

See below:

Buried in snow, these collectors hit 81 degrees. Shortly afterwards, with the snow melting, we got a follow-up report: 19 degrees outside and the collectors hit 125.4.

Keep it sunny!

The new home of Susan Turner and Karl Karnaky will be net-zero, meaning it generates as much energy as it consumes during the course of a year. It was constructed using the “rammed earth” method and features numerous recycled/repurposed materials.

Although Maine is still the most oil dependent state in the U.S., a Bar Harbor couple is proving that people can live comfortably year round at our latitude with virtually no fossil fuel energy.

Rammed earth involves packing a mixture of sand, gravel, and cement into a form, which then that solidifies into walls. This dense material is an excellent thermal mass for passive solar applications, and practically soundproof.

The home also features numerous recycled/repurposed elements including a recycled aluminum and steel roof, a soapstone farm sink from a cabin found on the property, and plank flooring and beams recovered from an 1836 house in Dexter.

Going Solar

Turner and Karnaky’s sustainable mindset applied to their choice of mechanical systems, as well.

“Deciding to go with solar was the easy part,” Susan Turner writes. “We found that we could get a system that would cover the entire winter’s [radiant floor] heat. Knowing that the panels are creating our electricity leads me to consciously decide how I will use electricity and to be aware of not wasting it.”

Turner and Karnaky were so impressed with the installation that they opened their home up this past October to be on NESEA’s Green Building Open House Tour. Approximately 50 people visited to see the “striking” house which the MDI Village Soup said combines “the earthen feel of an ancient construction technique, the intriguing attractions of vintage elements reanimated by new use and the aesthetics of a contemporary sensibility.”

Enjoying the Sunshine

The new home should be “net zero,” meaning that it will generate as much energy as it consumes throughout the course of the year, required no fossil fuel inputs!

The 5kw grid tied photovoltaic (PV) system will produce roughly 6,000 kilowatt hours of clean, renewable electricity annually. Electricity will be used for normal household loads as well as a Thermolec electric boiler which supplies radiant heat, and a backup element to the solar hot water system.

From May through September, the evacuated tube solar hot water collectors will provide nearly 100% of the home’s domestic hot water supply. Combined, the two systems will offset roughly 11,600 lbs of C02 emissions each year.

“We hope our home will inspire more folks to decide that solar is the way to go,” Turner says, “We are excited about ‘free’ heat without using non-renewable energy, and we love the hot showers provided by the sun!”

Dan Clapp, ReVision’s New Hampshire branch manager, explains how solar energy systems work to a group of 7th and 8th graders.

7th and 8th graders at the Hillsboro-Deering Middle School learned a bit about how to capture energy from the sun this week thanks to ReVision Energy.

Dan Clapp, Branch Manager of our New Hampshire office, spent two hours with the class of Linda Carson explaining solar energy systems. His presentation covered essentials such as how photovoltaic cells work, how sun-generated DC energy is converted into grid-compliant AC, and how that electricity is then either used inside a building or sent back out to the grid.

For solar hot water, he explained how systems are different – how instead of generating electricity, the solar hot water collectors instead capture the sun’s heat in liquid, which is then pumped through a tank, heating the water used in a house. Demo units of the systems came along so students could see the technology up close.

Since a renewable energy investment is really an investment in the future, we’re grateful for the opportunity to get out into the community and talk to students. We hope to be back to Hillsboro-Deering sometime soon!