Energy 101: Solar Thermal

The sun’s energy is absorbed by a south-facing collector that heats a fluid (water or antifreeze). In warm climates, this heated water can be used directly. In cold climates, the fluid transfers its heat to potable water stored in a tank. While some areas are sunnier than others (e.g., Miami, Florida vs. Seattle, Washington), all U.S. states receive enough sunlight to operate solar hot water systems. In 2008, over 20,000 solar hot water systems were installed in the U.S. Even in cold climates, these systems can provide up to 80% of the hot water needed, with the remainder supplied by a tankless water heater or a conventional storage tank system.

Today’s solar hot water systems are reliable, efficient, adaptable, and affordable. The purchase and installation cost for a residential system ranges from $4,000 to $10,000. Considering operating costs, solar hot water systems can be especially cost-effective compared to electric and propane water heaters, often reducing water heating bills by 50 to 80 percent. These systems can pay for themselves quickly, with payback time decreasing as fuel costs rise. Government and utility tax incentives and rebates can further reduce the final system cost, decreasing payback time. A properly designed and installed system can last up to 40 years. While solar hot water systems can also be used for radiant floor heating and swimming pools, the following factsheets focus on agricultural buildings and processes, as well as residential systems for heating water in bathrooms and kitchens.

Use the information is this article to determine if a solar hot water system is suitable for you. The information provided is basic and will help you discuss solar hot water systems knowledgeably with a company or installer. Each of the following sections can be used individually or together for a step-by-step decision-making process:

• Building & Site Assessment: Maryland has ample sunlight for solar hot water systems, but building and site conditions must be considered.
• Conservation & Efficiency: Efficient use of water and energy allows for a smaller, more efficient, and affordable system.
• System Options: Several systems work well in Maryland’s climate and do not freeze in the winter.
• System Sizing: Proper sizing is crucial for a resource-efficient and cost-effective system.
• Costs: System and operation costs vary. Rebates and incentives can lower the final purchase and installation cost.
• Installation: Considerations for DIY installation or hiring a contractor.
• Operation & Maintenance: Routine inspections and maintenance ensure efficient and long-lasting systems.
• Solar Hot Water Collector Sizing Guide.

Answering these questions will help determine if a solar hot water system is suitable for your building or site:

1. Do you have a south-facing roof? In Maryland, solar hot water collectors need to face south for optimal performance, as this placement maximizes exposure to the sun’s path. A slightly southeast or southwest orientation will not significantly affect performance. Consider adding a south-facing roof extension or another structure that can safely support collectors. Collectors can also be mounted on a wall or the ground, but be mindful of potential shading, snow drifts, lawn care, and vandalism. Mounting collectors on east- or west-facing roofs to face south is possible but may be visually unappealing. Architects and builders can design “solar-ready” buildings to integrate solar technology seamlessly.

• Yes: Proceed to Question #2.
• No: A solar hot water system may not be ideal.
   

2. Does your roof have enough space for collectors? For a residential system, you need 20 square feet of collector area per person for the first two people in a household. Add 12-14 square feet for each additional hot water user.

• Yes: Proceed to Question #3.
• No: Consider a smaller system, extending your roof, or mounting collectors on a wall or the ground.
   

3. Is your roof unshaded? Solar hot water systems are most efficient when collectors receive direct sunlight and are not shaded. Consider potential shading from nearby buildings and the mature height of landscape plants.

• Yes: Proceed to Question #4.
• No: If the shade is from landscaping, consider removing the plants. If a structure will shade the system, check local and state codes for “solar access” rights. Refer to Factsheet 6: Pre-Installation Considerations.
   

4. What’s the angle of your roof? Ideally, use your latitude (39 degrees for northernmost Maryland; 38 degrees for southernmost Maryland) as the collector tilt angle to maximize annual sun energy collection. Collectors can be installed at various angles depending on your hot water needs. Installers can mount collectors directly on an existing roof or tilt them to shed snow more easily or produce more hot water in a particular season. On flat roofs, collectors should be angled, not placed horizontally, to ensure sufficient sun exposure and snow shedding.

• Yes: Proceed to Question #5.
• No: Adjust the angle as needed for optimal performance.
   

5. Is your roof in good condition? Most roofs can support the weight of collectors (about 160 pounds for two residential-scale collectors). While innovative roof flashing can make collector removal easy, it’s less expensive and labor-intensive to make roof repairs before installation.

• Yes: Proceed to What's Next.
• No: Complete any necessary roof repairs or replacements first. If considering a new roof, consult a solar hot water company for recommendations that might simplify installation.

What’s Next? If you answered yes to every question or can make adjustments where needed, your building or site may be a good candidate for a solar hot water system. A system supplier or installer can provide a detailed assessment. But first, consider how conservation and efficiency measures can result in a more efficient and affordable system.

Solar hot water systems can be designed for various climates and typically include a solar collector, pump, controller, piping, and a backup source, usually the existing conventional water heater. Collectors absorb the sun’s energy to heat water or antifreeze fluid. In non-freezing climates, the heated water can be used directly. In cold climates, the heated fluid transfers its heat to potable water in a storage tank. The cooled fluid is then pumped back to the collector to be reheated. Several system types prevent collector fluids from freezing, making them suitable for cold climates like Montana and Wyoming. These options apply to small systems for homes; consult a qualified installer for larger systems.

Collectors:

• Flat Plate Collectors: Made of copper flow tubes connected to a dark absorber plate within an insulated, weatherproof box covered with hail-resistant tempered glass or plastic. They can heat water or antifreeze.
• Evacuated Tube Collectors: Composed of rows of parallel, clear glass tubes with an inner tube that absorbs solar energy. The vacuum inside the glass tubes reduces heat loss. They work well in cloudy conditions but are less hail-resistant than flat plate collectors.

Complete System:

• Active Systems: Use pumps and electronic equipment to move fluids and operate the system. They can be: a) Open Loop (Direct): Collectors directly heat the water. Hard or acidic water can cause scale and corrosion. A recirculation system pumps warm storage tank water to the collector during freezing weather, best for mild climates; b) Closed Loop (Indirect): Collectors indirectly heat water using heat-transfer fluids and a heat exchanger.
• Passive Systems: Have no moving parts and rely on gravity or convection to move fluids, typically used in warmer climates.

Back-Up Water Heater Systems: 

Solar hot water systems can generate a large portion of your hot water, especially in warmer months, but typically not 100% in colder climates. Back-up systems activate during prolonged cloudy days or when hot water demand exceeds the solar system’s capacity. For existing homes or buildings, the conventional hot water system can serve as the backup. For new constructions, a backup system is installed alongside the solar hot water system. Many homeowners and businesses are now considering tankless water heaters as their primary water heating system, which also serve as excellent backups for solar hot water systems.

Tankless/On-Demand Water Heaters: 

Traditional water heaters use storage tanks to keep water heated continuously, accounting for 15% or more of a water heating bill. Tankless water heaters, which have no storage tanks, only use energy to heat water when needed, conserving energy and reducing utility bills. They work by detecting when the hot water faucet is turned on or when an appliance requires hot water. For gas-powered systems, the gas valve opens, and the burner ignites, heating the water as it flows through the system. These systems can supply unlimited hot water but have flow rate limits, typically between two to five gallons per minute.

• Gas-Powered Units: Gas-powered tankless units provide higher flow rates than electric units. If considering a gas-powered model, ask the manufacturer about the gas consumption of the pilot light. Models with a standing pilot light can be turned off when not in use to save energy. An intermittent ignition device is another energy-saving option. If your home or building uses a lot of hot water simultaneously, you might need more than one unit.
• Conventional Tank Water Heaters: While less expensive to purchase, conventional storage tank water heaters are more costly to fuel and maintain. They vary in efficiency and typically last about 15 years. In contrast, tankless systems are very efficient (up to 96%) and can last 20 or more years. Note that not all tankless water heaters are compatible with solar hot water systems, so be sure to inquire. Some manufacturers offer special models designed to work with solar systems. Additionally, if considering a gas-powered tankless water heater, ensure your existing gas line can handle the increased demand.

Ensuring your solar hot water system is properly sized for your needs is crucial. An oversized system wastes money by producing more hot water than necessary, while an undersized system increases reliance on a backup heater powered by natural gas, electricity, or propane. The right size and number of collectors, storage tank size, and system type will result in a more resource-efficient and cost-effective setup.

Collector Sizing / Roof Space:

• Flat Plate Collectors: Range from 20 to 48 square feet and weigh 80 to 150 pounds, with dimensions from 3x6 feet to 4x10 feet and a 3-inch thickness.
• Evacuated Tube Collectors: Size depends on the number of tubes. A 20-tube collector (about 6x7 feet) can provide enough hot water for one to three people and weighs more than flat plate collectors.

For a quick estimate, a south-facing roof/surface needs at least 20 square feet of collector area for each of the first two people in the home. Add 12 to 14 square feet for each additional person. For example, 52 square feet of south-facing roof area is needed to provide 100% of the hot water for three people. Conserving water and using water-efficient fixtures can reduce the required collector size and overall system.

Solar Storage Tank Sizing:

• Some companies offer one-tank systems that store solar-heated water and serve as a backup.
• Most building owners keep the existing hot water heater as a backup and purchase a new storage tank for solar-heated water.
• The rule of thumb for storage tank size is 20 gallons of hot water per person per day.

For active systems, typically used in Maryland, a more accurate estimate is 1.5 gallons of storage tank capacity per square foot of collector area. This prevents system overheating when hot water demand is low. For example, 52 square feet of collector area x 1.5 = 78 gallons of storage tank volume, so an 80-gallon tank would be used.

System Efficiency: The efficiency of a solar hot water system is crucial for quicker payback. Two key efficiency metrics are:

• Solar Energy Factor (SEF): Ranges from 1.0 to 11. The higher the number, the more efficient the system. SEF is determined by the energy delivered to the system divided by the electrical or gas energy input.
• Solar Fraction (SF): Ranges from 0 to 1.0. This is the fraction of your water heating energy requirement provided by the sun. A higher SF means greater efficiency and less reliance on a backup heater.

Refer to the Solar Rating and Certification Corporation’s (SRCC) website to compare SEF and SF ratings for complete systems (under the OG 300 Directory). These ratings cover all system components: collector, tank, pumps, motors, valves, piping, etc. The OG 100 Directory provides individual collector ratings. While rules of thumb and sizing worksheets offer a general idea of collector and storage tank sizes, solar hot water system companies and installers can provide a more precise assessment based on your water use, roof tilt, latitude, solar resource, and seasonal temperature variations. Use online tools like Solar Estimate at www.Solar-Estimate.org to input your information and get an idea of system size and cost.

Pre-Installation Considerations: Before installing a solar hot water system, check local building codes, subdivision covenants, and zoning ordinances. Contact your local zoning and building department representatives to determine requirements. If you live in a subdivision with a homeowner’s association (HOA), review the covenants and contact the HOA Board or management company. You may need a building permit for installation on an existing home. If you are the first in your HOA or town to install a renewable energy system, you may need to educate building code officials and local representatives. Consider potential installation issues such as historic district guidelines and future shading from trees or nearby structures. Some jurisdictions have solar access zoning regulations to prevent sun blockage. 

Who Will Install Your System? Proper installation of a solar hot water system involves numerous considerations and safety measures (roof work, electrical hookups, etc.), so hiring a qualified solar hot water company or contractor is recommended. Some manufacturers extend warranties if their trained contractors install the system, and some utility rebates are only available if installed by a trained professional. Ensure you hire a qualified professional by asking about their experience, licensing, certifications, and customer service.

• Experience: Does the company or contractor have experience installing and maintaining the system you want? Ask for references and, if possible, visit other installations to learn about the system and customer service.
• Licenses: Some states require solar hot water system installers to have a plumber’s or solar contractor’s license. Confirm licensing with your state’s licensing boards.
• Insurance: Verify liability and worker’s compensation insurance coverage.
• Certification: Many installers undergo specialized training and exams to receive certifications. The North American Board of Certified Energy Practitioners (NABCEP) provides training and certification for solar hot water and other renewable energy system professionals. The NABCEP website lists certified professionals by state (www.nabcep.org).

Finding an Installer: The Solar Energy Industries Association and its state chapters provide lists of solar energy system companies by state or city. You can also search the phone book. Obtain bids from multiple companies and compare them. Request bids for Solar Rating and Certification Corporation (SRCC)-certified systems and ensure the bid specifies the system type, size, energy output, and maintenance requirements, in addition to the cost. The bid should include installation, initial setup (pressurizing the system, etc.), all hardware, permits, sales tax, and warranties. Some companies will handle the paperwork for federal, state, local, and utility incentives. Ask if incentive deductions are included in the cost estimates, but be aware that incentives may arrive later (rebates, etc.), and you will typically need to pay the full initial cost upfront. If you decide to install the system yourself, educate yourself by attending classes, workshops, or training sessions with qualified instructors. Whether you hire a contractor or install the system yourself, ensure it is done correctly and safely.

Regular inspections and maintenance will keep your solar hot water system running efficiently for a long time. While the overall system can last over 40 years with proper care, active systems have electrical and mechanical parts that may need repair or replacement sooner. Active systems also require more frequent maintenance than passive systems due to components like pumps, heat exchangers, and temperature sensors. Passive systems are not recommended in cold climates. This article provides sources for finding qualified companies and contractors who can conduct system inspections and perform maintenance tasks. Ask your system provider about maintenance requirements and read the owner’s manual. Hiring a licensed and/or certified contractor is recommended, but if you plan to do the work yourself, the following inspection list (from the U.S. Department of Energy’s Energy Savers website) can guide you:

• Collector Shading: Check for shading of the collectors during mid-morning, noon, and mid-afternoon annually. Any shading can affect performance.
• Collector Glazing and Seals: Look for cracks in the collector glazing and ensure seals are in good condition. Replace plastic glazing if it is excessively yellowed.
• Plumbing and Wiring Connections: Check for fluid leaks at pipe connections and ensure all wiring connections are tight.
• Piping, Duct, and Wiring Insulation: Inspect for damage or degradation of insulation covering pipes and wiring.
• Roof Penetrations: Ensure flashing and sealant around roof penetrations are in good condition.
• Support Structures: Check all nuts and bolts attaching the collectors to support structures for tightness.
• Pressure Relief Valve: Ensure the valve is not stuck open or closed.
• Pumps: Verify that distribution pumps are operating. Listen for the pump when the sun is shining on the collectors after mid-morning. If you can’t hear the pump, either the controller or pump may have malfunctioned.
• Heat transfer fluids: Antifreeze solutions in liquid solar heating collectors need to be replaced periodically. It's a task best conducted by a qualified technician. If water with a high mineral content (i.e., hard water) is circulated in the collectors, mineral buildup in the piping may need to be removed by adding a de-scaling or mild acidic solution to the water every few years.
• Storage systems: Check storage tanks for cracks, leaks, rust, or other signs of corrosion.

According to the National Renewable Energy Laboratory and RS Means “Green Building Cost Data” from 2011, small solar water heating systems for homes cost between $187 and $199 per square foot of collector area. Larger, central systems for agricultural purposes average about $60 per square foot. For example, a residential system with 52 square feet of collector area at $187 per square foot totals $9,724. This is a rough estimate, and rebates or tax incentives can reduce the final cost. Once you know the system option that will work best for you, determine the purchase and installation costs of several systems you are considering. The “second price tag” takes back-up system fuel costs and operation and maintenance costs into consideration. After calculating yearly operating costs for each system, you can make a complete cost comparison.

Calculating Yearly Operating Costs: To estimate yearly operating costs, gather the following information:

• Solar Energy Factor (SEF)
• Cost and fuel type for the backup system

Gas Backup: Determine the fuel cost in therms or British thermal units (Btu). Use the formula: 356 × (0.4105 ÷ SEF) × fuel cost = estimated yearly cost of operation.

Electric Backup: Determine the cost per kWh of electricity. Use the formula: 365 × (12.03 ÷ SEF) × electricity cost (kWh) = estimated yearly cost of operation.

Backup system costs and maintenance costs contribute to yearly operation costs. Solar hot water system maintenance typically costs $25-$30 every 3-5 years. Once you know the purchase and installation costs and have calculated yearly operating costs, you can compare systems and determine the simple payback period.

Cost Comparison and Payback Period: The additional purchase price of the more efficient (higher SEF) System B is $500 ($10,000 - $9,500). However, System B’s yearly operating cost is $50 less ($170 - $120). The more efficient system will pay for itself in 10 years or less. Simple payback is calculated by dividing the price difference by the yearly savings: $500 ÷ $50 = 10 years. Note that payback periods decrease as fuel costs rise, and government and utility rebates and tax incentives can further reduce the final system cost, decreasing the payback period.

When an installer visits your home or building, they will consider how the solar hot water system components will integrate with your home’s structure and existing hot water system. Installation costs can vary based on the location of collectors and the piping distance to the storage tank(s). Various federal, state, and local government and utility incentives are available for energy efficiency and renewable energy. These incentives vary by state and duration. The Department of Energy’s Database of State Incentives for Renewables and Efficiency (DSIRE) tracks available tax credits, rebates, and other incentives to reduce your system’s final cost.

Federal Income Tax Credit: 30% of the system cost (after any utility rebates): $9,724 x 0.30 = $2,917 tax credit. Note: To qualify, the solar water heating property must be certified by SRCC or a comparable entity endorsed by the state where the system is installed. At least half of the energy used to heat the dwelling’s water must be from solar.