Best Solar Water Heaters in the USA

That makes choosing the best solar water heater less about finding a single tank and more about choosing the right solar thermal system architecture for the home’s climate, hot-water demand, roof, available sunlight and backup requirements.

For the broader comparison of all major water-heating technologies, see Best Water Heaters in the USA.

Quick Answer: Best Solar Water Heaters in the USA

Our overall choice: the Rheem SolPak RS80-40BP because it is a complete active solar thermal package rather than merely a collector or storage tank. The package includes the storage tank, collectors, controller, adjustable-speed pump, mixing valve and other system components. Rheem lists SRCC OG-100 certification for its collectors and OG-300 certification for the complete system.

What Is a Solar Water Heater?

A solar water heater, also called a solar domestic hot-water system, uses solar radiation as the primary heat source for domestic water.

The basic energy path is:

Sunlight → solar collector → heat-transfer fluid or water → heat exchanger/storage tank → household hot water

Solar thermal systems can be active or passive. Active systems use pumps and controls to circulate water or heat-transfer fluid. Passive systems rely primarily on natural convection.

Solar Water Heater vs Solar PV

Solar thermal and photovoltaic systems both use sunlight, but they produce different forms of energy.

A solar thermal collector directly captures heat, whereas PV produces electricity that can subsequently power an electric or heat-pump water heater.

This distinction is important when comparing a dedicated solar thermal system with a home that already has photovoltaic panels.

How a Solar Thermal Water Heater Works

A typical active indirect system operates as follows:

1. The solar collector absorbs sunlight.
2. A heat-transfer fluid inside the collector becomes warmer.
3. A controller detects suitable collector temperature.
4. A circulation pump moves the fluid toward the storage tank.
5. A heat exchanger transfers heat into domestic water.
6. The cooled heat-transfer fluid returns to the collector.
7. The cycle repeats while useful solar heat is available.
8. A backup heater supplies heat when solar energy is insufficient.

Stiebel Eltron describes its systems in essentially this configuration: collectors heat a transfer fluid, a pump circulates it to the domestic hot-water tank, and a heat exchanger transfers the heat into stored household water.

Active vs Passive Solar Water Heaters

DOE identifies active direct-circulation and indirect systems as well as passive thermosiphon and integral collector-storage systems.

Direct vs Indirect Solar Water Heating

Direct Circulation

In a direct system, household water itself circulates through the solar collectors.

This can be simple and efficient, but freezing and water-quality conditions can make the configuration unsuitable in some climates.

Indirect Circulation

An indirect system uses a separate heat-transfer fluid, commonly a water/glycol mixture, in the collector loop.

The fluid transfers heat to domestic water through a heat exchanger.

This architecture is particularly useful where freezing conditions must be considered.

Rheem’s SolPak closed-loop systems use glycol in the collector loop and include a heat exchanger, expansion tank and associated components.

Why Closed-Loop Solar Systems Matter

In a cold climate, simply circulating household water through a rooftop collector can expose the plumbing loop to freezing conditions.

A closed-loop system separates the solar collector circuit from the potable-water circuit.

The basic architecture becomes:

Collector loop → heat-transfer fluid → heat exchanger → domestic-water tank

This adds components but provides much greater flexibility for cold-weather installations.

Flat-Plate vs Evacuated-Tube Collectors

DOE identifies glazed flat-plate and evacuated-tube collectors as two important residential solar thermal collector types. Evacuated-tube collectors can perform efficiently at higher temperatures and can be used in cold climates with appropriate system design.

Why Collector Area Matters More Than Tank Size

A common solar-water-heater mistake is to focus almost entirely on the storage tank.

The tank determines how much heated water can be stored, but the collector determines how much solar heat can be harvested.

The approximate useful solar energy captured is related to:

Q_solar ≈ Collector area × Solar irradiance × Collector efficiency × System efficiency

Increasing the tank size without providing enough collector area can produce a large tank that simply does not receive enough solar energy to remain adequately heated.

How Many Solar Collectors Do You Need?

Collector count depends on:

• Number of occupants
• Daily hot-water consumption
• Climate
• Solar resource
• Collector orientation
• Collector tilt
• Shading
• Desired solar fraction
• Storage volume
• Backup-heater strategy

Stiebel Eltron’s current SOLkit packages are offered in one-, two- and three-collector configurations, with different storage capacities and configurations intended to match different domestic hot-water requirements.

Solar Fraction

Solar fraction is the portion of a home’s hot-water energy supplied by solar energy rather than the backup heater.

A simplified expression is:

Solar Fraction = Solar Energy Supplied ÷ Total Hot-Water Energy Required

For example, if solar supplies 60% of annual water-heating energy, the solar fraction is approximately 60%.

The objective is generally not to eliminate the backup heater completely. Instead, the solar system is designed to provide a substantial portion of annual hot-water energy while the backup system handles periods of inadequate solar availability.

DOE’s solar hot-water sizing guidance notes that optimal system size can often correspond to a substantial fraction of the hot-water load, depending on solar resource and economics.

Why Solar Water Heaters Need Backup

Solar availability varies from hour to hour and season to season.

A system can produce abundant heat on a clear summer day but substantially less energy during prolonged cloudy weather or winter conditions.

Therefore, a residential solar water-heating system normally includes a backup heating source.

Possible backup systems include:

• Electric resistance element
• Heat-pump water heater
• Gas water heater
• Gas tankless heater
• Existing central hot-water system

Rheem’s SolPak with Electric Backup incorporates a backup electric element, while Stiebel Eltron’s SOLkit systems use storage tanks with integral electric backup heating.

Solar Water Heater Storage Tank Sizing

Solar systems generally use larger storage tanks than conventional water heaters because solar heat availability does not necessarily coincide with household hot-water demand.

For example, solar energy may be strongest around midday while a family may consume much of its hot water in the morning and evening.

The storage tank acts as a thermal battery.

These are starting ranges rather than universal sizing rules. The correct tank and collector combination should be determined from local solar resource and domestic hot-water demand.

Why Solar Tanks Are Often Larger

Consider a conventional gas storage water heater. The burner can operate whenever the thermostat calls for heat.

A solar collector cannot do that.

The sun may be unavailable when the family needs hot water.

Therefore:

More storage allows solar energy collected earlier to remain available later.

This is why solar systems commonly use larger tanks than conventional water heaters serving similar households.

Solar Water Heaters in Cold Climates

Solar water heating can work in cold climates, but system design becomes much more important.

The major concerns are:

• Freezing
• Reduced winter solar availability
• Heat loss from exposed piping
• Collector efficiency at low ambient temperature
• Snow accumulation
• Roof orientation
• Backup heating requirements

Indirect glycol systems or drainback systems can protect the collector loop from freezing.

DOE documentation describes drainback systems in which collector-loop fluid drains into a protected reservoir when the pump shuts down, helping protect the system from freezing and overheating.

See Best Water Heaters for Cold Climates.

Solar Water Heaters in Warm Climates

Warm climates can be particularly favorable for solar thermal water heating because freezing protection is less demanding and solar resources can be strong.

Passive thermosiphon systems can also become attractive in suitable warm climates.

DOE specifically describes thermosiphon systems as passive systems appropriate for warmer coastal climates.

However, warm climates can create another issue: overheating.

A well-designed solar thermal system must account for periods when the collectors produce more heat than the household can use.

Solar Collector Orientation

Collector orientation is critical.

For a typical northern-hemisphere installation, a south-facing orientation is generally favorable, although the optimal orientation depends on the site’s latitude, roof geometry, shading and desired seasonal performance.

DOE recommends positioning collectors to maximize daily and seasonal solar energy, with true south generally being the optimum orientation in the northern hemisphere.

Roof Shading

Even a high-quality collector cannot compensate for severe shading.

Evaluate:

• Trees
• Neighboring buildings
• Chimneys
• Roof structures
• Parapets
• Seasonal shadows

A roof that appears sunny at noon in summer may experience significant winter shading.

Roof Structural Considerations

Solar thermal collectors add weight and wind loading to the roof.

The installation also creates roof penetrations and mounting requirements.

DOE specifically emphasizes that collector mounting must avoid water intrusion and protect the roof structure.

Therefore, roof condition should be evaluated before installing a solar thermal system.

Solar Thermal vs Solar PV + Electric Water Heater

This is one of the most important decisions for a new solar installation.

A homeowner should compare a dedicated solar thermal system against installing additional PV capacity and using a highly efficient heat-pump water heater.

Solar Thermal vs Heat Pump Water Heater

For many homes, the heat-pump alternative deserves serious consideration because it avoids the rooftop hydraulic system entirely.

See Best Heat Pump Water Heaters.

Solar Thermal vs Gas Water Heating

Solar thermal can substantially reduce gas consumption, but the system usually retains a backup heater.

A useful design is therefore:

Solar thermal → primary heating

Gas → backup heating

This allows solar energy to displace much of the annual gas consumption while preserving reliable hot-water availability.

Solar Thermal vs Electric Water Heating

A solar thermal system can reduce electricity consumption substantially when it supplies most of the water-heating load.

However, conventional electric resistance heating can be simpler and cheaper to install.

The economic question is therefore not simply:

“Does solar use less electricity?”

It is:

“Will lifetime energy savings justify the additional solar-system installation cost?”

Solar Water Heater Efficiency

Solar thermal systems do not use UEF in exactly the same way as conventional electric and gas storage water heaters.

Solar water-heating performance can involve metrics such as solar energy factor and system solar contribution, depending on the applicable certification and rating system.

Federal guidance lists solar water heaters separately and specifies solar uniform energy factor requirements depending on the backup configuration.

For homeowners, however, the more useful practical questions are:

• How much hot-water energy does the system supply annually?
• What solar fraction is expected?
• How much backup energy will be required?
• What is the installed cost?
• What maintenance is required?

SRCC Certification

The Solar Rating & Certification Corporation (SRCC) is important when evaluating solar thermal equipment in the USA.

There is an important distinction between:

• Collector certification
• Complete-system certification

Rheem’s SolPak system illustrates this distinction: its collectors are OG-100 certified while the complete system is OG-300 certified.

What Is OG-100?

OG-100 certification applies to solar thermal collectors.

It helps establish standardized collector performance and certification requirements.

What Is OG-300?

OG-300 applies to complete solar water-heating systems.

This distinction is important because a high-performing collector does not automatically mean that the complete installation will deliver equivalent real-world performance.

Solar Water Heater Pumps

Active systems require a circulation pump.

The pump must move the heat-transfer fluid through the collector loop at an appropriate flow rate.

Too little flow can increase collector temperature and reduce useful heat transfer under some conditions.

Too much flow can increase pumping energy unnecessarily.

Modern systems therefore use controllers to determine when and how the pump operates.

Solar Differential Controller

A solar controller can compare collector and storage-tank temperatures.

A simplified control strategy is:

• If collector temperature is sufficiently higher than tank temperature → pump ON.
• If the temperature difference becomes too small → pump OFF.

This prevents the system from circulating fluid when the collectors cannot provide useful heat.

Heat-Transfer Fluid

Closed-loop systems often use a water/glycol mixture as the heat-transfer fluid.

The fluid must be appropriate for the system’s operating temperatures and manufacturer requirements.

Stiebel Eltron specifies propylene-glycol heat-transfer fluid for its solar thermal systems and describes it as providing freeze protection for cold nights.

Solar Thermal Expansion Tank

Solar-loop fluid expands as it heats.

A closed solar loop therefore needs appropriate expansion management.

Rheem’s SolPak closed-loop configuration includes a thermal expansion tank for the glycol loop.

Mixing Valve

Solar collectors can produce water at temperatures substantially above normal domestic hot-water delivery temperatures.

A thermostatic mixing valve can blend hot storage water with cold water before distribution.

This provides temperature control and reduces scalding risk.

Solar Water Heater Overheating

A solar collector can continue absorbing energy even when the household is not consuming hot water.

This creates an overheating problem if the system is poorly designed.

Possible solutions include:

• Proper storage sizing
• Controller-based heat management
• Drainback systems
• Heat dissipation strategies
• Collector stagnation design
• Appropriate expansion management

Solar Water Heater Freeze Protection

Freeze protection is essential outside warm climates.

Common approaches include:

• Closed-loop glycol systems
• Drainback systems
• Freeze-resistant system architectures
• Appropriate insulation

Do not assume that a solar water heater advertised for one climate can be installed unchanged in another climate.

Solar Water Heater Maintenance

Collector Inspection

Inspect collectors for damage, shading, dirt and mounting problems.

Heat-Transfer Fluid

Closed-loop systems require periodic inspection of the heat-transfer fluid and its condition.

Pump

Check pump operation and listen for unusual noise.

Controller

Verify that temperature sensors and controller functions operate correctly.

Expansion Tank

Inspect the expansion-management components according to the manufacturer’s maintenance requirements.

Storage Tank

The domestic storage tank still requires the same basic corrosion and plumbing attention as other storage water heaters.

Roof Connections

Inspect roof penetrations and mounting points for signs of water intrusion.

DOE notes that solar thermal systems can require relatively infrequent maintenance when properly designed, but professional inspection remains appropriate for many installations.

Solar Water Heater Installation Cost

Solar thermal systems generally have a higher installation cost than simply replacing an existing electric or gas water heater.

The system may require:

• Roof mounting
• Collectors
• Storage tank
• Pump station
• Controller
• Heat exchanger
• Insulated piping
• Heat-transfer fluid
• Expansion tank
• Backup heater
• Electrical work
• Roof penetrations

Therefore, solar thermal should be evaluated as a complete mechanical project rather than as a replacement water-heater appliance.

Solar Water Heater Incentives

Federal, state and local incentives can materially change the economics of a solar thermal installation.

Incentive eligibility can depend on equipment certification, installation date, system configuration, location and current tax rules.

Do not calculate payback using an assumed incentive. Verify the incentive that actually applies to the property and system before purchasing.

Solar Water Heaters for New Construction

Solar thermal is particularly interesting during new construction because the house can be designed around:

• Collector orientation
• Roof structure
• Pipe routing
• Storage location
• Drainage
• Backup heater
• Mechanical-room layout

This can substantially reduce installation compromises.

See Best Water Heaters for New Construction.

Solar Water Heaters for Existing Homes

Retrofitting an existing home is possible, but the feasibility assessment should include:

• Roof condition
• Roof orientation
• Shading
• Structural capacity
• Distance between roof and tank
• Pipe routing
• Existing water heater
• Backup strategy
• Local permitting

See Best Water Heaters for Existing Homes.

Solar Water Heaters for Large Families

Large households need more than simply a large collector array.

The system must balance:

• Collector area
• Storage capacity
• Solar fraction
• Backup capacity
• Peak hot-water demand
• Seasonal solar availability

A 119-gallon storage system can provide considerably more thermal storage than an 80-gallon system, but the larger tank only becomes useful if sufficient solar energy is available to charge it.

See Best Water Heaters for Large Families.

Solar Water Heaters for Vacation Homes

Vacation homes create a special problem: the property may be unoccupied for long periods.

Solar thermal systems must therefore be designed around:

• Freeze protection
• Stagnation protection
• Remote monitoring
• Vacation periods
• System shutdown procedures
• Backup-heater control

See Best Water Heaters for Vacation Homes.

Solar Water Heaters for Off-Grid Homes

Solar thermal can be attractive for off-grid homes because the primary energy source does not depend on the electrical grid.

However, active systems still require electricity for pumps and controls.

An off-grid installation therefore needs to decide whether those electrical loads will be supplied by:

• PV panels
• Battery storage
• Generator backup
• Another local power source

Passive solar thermal systems can reduce electrical requirements but have narrower installation and climate suitability.

See Best Off Grid Water Heaters.

Our Recommended Solar Water Heater Systems

1. Rheem SolPak RS80-40BP — Best Overall

The Rheem SolPak RS80-40BP is our overall solar-water-heating choice because it is a complete active package rather than a standalone collector.

Why We Selected It

• 80-gallon storage
• Active solar thermal system
• Electric backup
• Flat-plate collectors
• Closed-loop configuration available
• Adjustable-speed pump
• Controller
• Mixing valve
• Heat exchanger
• SRCC OG-100 collector certification
• SRCC OG-300 system certification
• ENERGY STAR qualified
• 2.4 listed energy factor

Rheem lists 80-gallon storage, three collector-size options, an electric backup element, closed-loop capability, OG-100 collector certification and OG-300 system certification for the SolPak platform.

Relevant Certification / Standards

The collectors are SRCC OG-100 certified and the complete system is SRCC OG-300 certified. Rheem also identifies the system as ENERGY STAR qualified.

Important Limitations

• Requires significant installation planning.
• Roof space is required.
• Collector orientation and shading affect performance.
• Active systems require pump and control components.
• Solar thermal does not eliminate the need for backup heating.

Who Should Buy It

Homeowners with good solar exposure who want a complete solar thermal package and are prepared for professional installation.

Who Shouldn’t Buy It

Homes with heavily shaded roofs, poor roof condition or very limited solar exposure should investigate other technologies first.

Maintenance Considerations

Inspect collectors, pump, controller, heat-transfer fluid, expansion equipment and roof penetrations according to the manufacturer’s requirements.

Product Information

Verify the current Rheem model configuration and installation requirements before purchase.

For the broader comparison, see Best Water Heaters in the USA.

2. Rheem SolPak RS80-48BP — Best Higher-Output 80-Gallon Solar System

The RS80-48BP keeps the 80-gallon storage format while using a larger collector configuration than the RS80-40BP.

Why We Selected It

• 80-gallon storage
• Active solar thermal system
• Electric backup
• Flat-plate collectors
• Closed-loop capability
• Adjustable-speed pump
• Controller
• Mixing valve
• OG-100 collector certification
• OG-300 system certification
• Listed energy factor of 2.6

Rheem lists the RS80-48BP at 2.6 energy factor, compared with 2.4 for the RS80-40BP, while retaining the 80-gallon system format.

Important Limitations

• Requires adequate roof space.
• Higher collector area increases installation requirements.
• Solar resource still determines real-world output.

Who Should Buy It

Households with higher hot-water demand and sufficient roof area that want more collector capacity without moving to the larger 119-gallon storage configuration.

Who Shouldn’t Buy It

Small households with limited hot-water demand may obtain better economics with a smaller solar system.

Maintenance Considerations

Maintain the collector loop, pump, controller, fluid and storage tank according to Rheem’s requirements.

Product Information

Confirm current availability and system configuration through Rheem or an authorized solar installer.

3. Rheem SolPak RS120-64BP — Best Large-Storage Solar Water Heater

The RS120-64BP moves into the large-storage category and is designed for applications where substantial solar thermal storage is appropriate.

Why We Selected It

• 119-gallon storage
• Active solar thermal system
• Flat-plate collectors
• Electric backup architecture
• Closed-loop configuration
• System-level SRCC certification
• Listed energy factor of 3.2

Rheem’s SolPak specifications list the RS120-64BP as a 119-gallon system with a listed 3.2 energy factor and SRCC certification.

Important Limitations

• Requires substantially more installation space.
• Large storage is unnecessary for many small households.
• Collector and roof requirements must be carefully evaluated.

Who Should Buy It

Large households or applications where high thermal storage is justified by domestic hot-water demand and available solar resource.

Who Shouldn’t Buy It

Small households should not install a large solar storage system simply because larger capacity sounds better.

Maintenance Considerations

Large systems require careful inspection of the solar loop, storage tank, controls, expansion equipment and collectors.

Product Information

Professional solar-system sizing should be performed before selecting this configuration.

4. Stiebel Eltron SOLkit 1 — Best Starter Solar Thermal Package

The Stiebel Eltron SOLkit 1 is a packaged solar thermal configuration built around one SOL 27 Premium collector and an 80-gallon SB 300 E storage tank.

Why We Selected It

• One SOL 27 Premium collector
• 80-gallon SB 300 E tank
• SE Flowstar pump station
• SOM 6 Plus controller
• 50-foot dual line set
• Integral electric backup
• Packaged-system architecture

Stiebel Eltron’s published SOLkit documentation identifies SOLkit 1 as a one-collector package using an 80-gallon SB 300 E tank, Flowstar pump station and SOM 6 Plus controller.

Important Limitations

• One collector may not be appropriate for high-demand households.
• System sizing must account for local solar resource.
• Professional installation is required for the complete system.

Who Should Buy It

Smaller households with suitable solar exposure and moderate domestic hot-water demand.

Who Shouldn’t Buy It

Large households should compare larger collector configurations before selecting a one-collector package.

Maintenance Considerations

Inspect the collector, pump station, controller, heat-transfer fluid and storage tank.

Product Information

Verify current kit configuration with Stiebel Eltron because package contents can change.

5. Stiebel Eltron SOLkit 2 — Best Two-Collector Solar Thermal Package

SOLkit 2 increases collector capacity while retaining the 80-gallon SB 300 E storage format.

Why We Selected It

• Two SOL 27 Premium S collectors
• 80-gallon SB 300 E storage tank
• SE Flowstar pump station
• SOM 6 Plus controller
• 50-foot dual line set
• Integral electric backup
• Complete packaged configuration

Stiebel Eltron lists SOLkit 2 with two SOL 27 Premium S collectors and an 80-gallon SB 300 E tank.

Important Limitations

• Requires sufficient roof area.
• Two collectors increase installation complexity.
• Actual solar contribution depends heavily on site conditions.

Who Should Buy It

Households with greater hot-water demand that want a packaged two-collector system rather than a single-collector installation.

Who Shouldn’t Buy It

Small households with limited hot-water consumption may not justify the additional collector capacity.

Maintenance Considerations

Maintain the complete collector loop, pump station, controller, storage tank and heat-transfer fluid.

Product Information

Confirm the current kit configuration and local distributor availability before purchase.