A solar storage tank is the heart of a solar water heating system. It holds the solar-heated water until your household needs it, and its size, insulation, and internal design directly affect how much of your hot water the sun can provide. This guide walks through the key features of solar storage tanks, using guidelines from the U.S. Department of Energy’s 2003 consumer guide and the Florida Solar Energy Center’s 2006 solar water and pool heating manual, so you can understand what to look for and what questions to ask a contractor.
What a solar storage tank does
In a solar water heating system, the storage tank receives heated water or heat-transfer fluid from the solar collectors and keeps it ready for use. According to the U.S. Department of Energy’s 2003 consumer guide, most solar water heaters require a well-insulated storage tank, and solar storage tanks have an additional outlet and inlet connected to and from the collector. Unlike a standard water heater tank that only connects to cold supply and hot delivery lines, a solar tank also interfaces with the collector loop, making it a central junction in the system.
For a broader look at how the tank fits alongside collectors, pumps, and pipes, see our article on solar water heater components explained. The storage tank category page at storage tanks also covers tank types in more detail.
Sizing the tank to your household and collectors
The DOE’s 2003 guide provides a sizing rule of thumb for active systems: the solar storage tank volume increases with collector size, typically 1.5 gallons per square foot of collector. The guide also lists rough household sizes: a small 66-gallon system usually serves one to three people, a medium 80-gallon system works for three or four people, and a large 120-gallon system suits four to six people. These numbers are from the 2003 DOE publication and reflect that era’s typical guidance; a contractor will calculate sizing based on your specific household demand and climate.
Collector area itself follows another DOE guideline from the same source: about 20 square feet of collector for each of the first two family members, plus 8 square feet per additional person in the Sun Belt, or 12 to 14 square feet per additional person in the northern United States. Because tank volume and collector area are linked, a properly sized tank avoids wasting collected heat or leaving the household short. For help estimating your own hot water needs, see household demand and the article on sizing a solar water heater for your household.
Single-tank versus two-tank arrangements
Solar storage can be configured in one tank or two. The DOE’s 2003 guide explains that in two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In a one-tank system, the backup heater is combined with the solar storage in one tank. The Florida Solar Energy Center’s 2006 manual describes the same distinction, noting that a solar tank can serve as the sole storage vessel with an integrated backup element, or as a dedicated preheat tank that feeds a separate auxiliary tank.
The choice depends on available space, existing plumbing, and climate. Our backup heating page discusses how backup integrates with storage.
Heat exchangers: internal, external, and double-wall
Indirect systems use a heat-transfer fluid that circulates through the collectors and then passes through a heat exchanger to warm the household water inside the tank. The FSEC manual from 2006 describes several common heat exchanger designs. A coil-in-tank heat exchanger is immersed in the lower section of the storage tank. A wraparound-tube heat exchanger uses tubes welded to the outside of the water vessel on the lower half of the tank. A wraparound-plate design, also called a double jacket, uses a separate plate welded around the lower half of the tank. External designs include tube-in-tube, shell-in-tube, and side-arm heat exchangers that sit outside the tank and allow thermosiphoning of water from the storage tank through the exchanger.
The FSEC manual also distinguishes single-wall and double-wall heat exchangers. Double-wall heat exchangers have two walls separating the heat-transfer fluid from the potable water, often with small passageways between the walls to provide leak detection. According to the FSEC manual, nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger, but ethylene glycol, a toxic fluid, should be used only with a double-walled heat exchanger. The manual notes that systems with heat exchangers can experience a 10 to 20 percent loss in total efficiency in the transfer of heat from the collection loop to the stored water, though economic savings and fail-safe freeze protection may offset that loss.
Insulation and overnight heat retention
A solar storage tank must keep water hot from the afternoon sun through to the next morning’s showers. The DOE’s 2003 guide states that most solar water heaters require a well-insulated storage tank. Heat lost from the tank is heat the backup has to replace. When evaluating a tank, ask about the insulation material and how pipe penetrations are sealed.
Safety components: T&P relief valve and expansion tank
Solar storage tanks must include safety devices that protect against overpressure and overtemperature. The FSEC manual explains that a temperature-pressure relief valve, also called a pressure-temperature relief valve or T&P valve, protects system components from excessive pressures and temperatures. It is always plumbed to the solar storage tank as well as the auxiliary tank. The manual notes that these valves are usually set by the manufacturer at 150 psi and 210 degrees Fahrenheit. In thermosiphon and integral collector-storage systems where the solar tanks are located on a roof, those tanks may also be equipped with a temperature-pressure relief valve since they are in some jurisdictions considered storage vessels. The FSEC manual adds that temperature-pressure relief valves located inside a building must drain to the outside, and to follow local code requirements if uncertain.
The Solar Oregon maintenance guide reinforces that pressure and temperature release valves and thermal expansion tanks protect the home and safety of the occupants. The FSEC manual describes the expansion tank in indirect systems: as the heat-transfer fluid temperature rises, it expands, and the expansion tank provides room for this change in volume. Inside the expansion tank, a flexible bladder maintains a constant pressure on the fluid while allowing it to expand and contract. The manual notes that some solar systems also have an expansion tank on the cold water supply line when a check valve or back-flow prevention device is on the cold-water line.
Anti-scald protection
During summer, solar-heated water can reach temperatures high enough to scald. The Solar Oregon guide states that an anti-scald valve, required by code, will be installed to mix super-hot water with cold water before being sent to the home’s faucets. The FSEC manual distinguishes between a mixing or tempering valve, which mixes cold water with the hottest water from the storage tank to deliver water at a preset temperature and is intended to conserve hot water, and an anti-scald valve, which also mixes cooler water with hot water but functions as a safety device, closing off the flow into the house if the hot or cold mixing supply fails.
Tank location, weight, and roof-mounted thermosiphon tanks
Where the tank sits affects plumbing complexity, freeze protection, and structural requirements. The DOE’s 2003 guide explains that in a thermosyphon passive system, the collector must be installed below the storage tank so that warm water will rise into the tank. The guide cautions that contractors must pay careful attention to roof design because the water in the storage tank is heavy. The FSEC manual similarly notes that in thermosiphon and ICS systems, solar tanks are located on a roof and may be considered storage vessels in some jurisdictions.
A roof-mounted tank adds significant weight, so a structural assessment is essential before installation. Ground-floor or basement tank locations avoid roof loading but may require a pump with more head pressure to lift water to roof-mounted collectors. For more on roof suitability, see roof and site suitability and is your roof suitable for a solar water heater.
Installation codes, permits, and professional help
Connecting a solar storage tank to a home’s pressurized drinking-water supply must follow local plumbing and building codes and usually requires a permit and a qualified, licensed installer. The DOE’s 2003 guide states that some cities and counties have ordinances or require permits for home improvement construction, including solar system installation. The FSEC manual repeatedly directs readers to follow local code requirements for relief valve drainage and tank installations. Homeowners should verify that the installer pulls the necessary permits and that the tank’s safety devices meet current code.
Quick-reference table: tank features, purpose, and questions to ask
| Tank feature | Purpose | What to ask a contractor |
|---|---|---|
| Storage volume | Matches collector area and household demand | How many gallons per square foot of collector are you sizing for? |
| Insulation | Reduces overnight heat loss | What insulation material and thickness does this tank use? |
| Heat exchanger type | Transfers heat from collector fluid to potable water in indirect systems | Is the exchanger internal or external, and single-wall or double-wall? |
| T&P relief valve | Protects against excessive temperature and pressure | Is it plumbed to drain outside per local code? |
| Expansion tank | Absorbs fluid expansion in closed-loop systems | Is the expansion tank sized for the total system fluid volume? |
| Anti-scald valve | Prevents overly hot water delivery to faucets | Is an anti-scald or tempering valve included and set correctly? |
| Tank location | Affects weight load, freeze exposure, and thermosiphon flow | Does the roof structure need reinforcement for this tank weight? |
Frequently asked questions
Can I use my existing water heater tank as the solar storage tank?
In most cases, a standard water heater tank lacks the additional inlet and outlet connections needed for a solar collector loop. The DOE’s 2003 guide notes that solar storage tanks have an additional outlet and inlet connected to and from the collector. Ask your installer about the options. In a two-tank system, the DOE guide describes the existing water heater as the backup tank downstream of a new solar preheat tank.
How do I know if the heat exchanger is double-wall and why it matters?
The FSEC manual explains that double-wall heat exchangers have two walls separating the heat-transfer fluid from potable water, often with leak-detection passageways between the walls. If your system uses a toxic heat-transfer fluid such as ethylene glycol, a double-wall exchanger is required. For nontoxic fluids like propylene glycol, a single-wall exchanger is acceptable. Ask your contractor which fluid the system uses and whether the heat exchanger matches that fluid’s toxicity level.
Does a solar storage tank need its own expansion tank?
In indirect systems that use a heat-transfer fluid other than water, the FSEC manual states that an expansion tank is incorporated to handle fluid expansion as temperatures rise. The expansion tank prevents damage from over-pressurization and eliminates the potential creation of a vacuum inside the collector loop. On the potable water side, some systems also have an expansion tank on the cold water supply line when a check valve or back-flow prevention device is present. Your installer should confirm whether one or both sides of the system require expansion tanks.
For more on choosing and comparing storage tanks and other components, visit storage tanks and the broader system components section.
