Backup Options for Solar Hot Water Systems

Backup Options for Solar Hot Water Systems

A solar water heater can provide the majority of a household’s or facility’s hot water needs, but the sun does not shine 24 hours a day. Cloudy weather, seasonal changes, and periods of high demand can outstrip the heat collected by the solar array. For this reason, nearly every solar thermal installation requires a backup heating system. The backup ensures that hot water remains available whenever solar gain is insufficient, without compromising comfort, hygiene, or operational requirements.

Choosing the correct backup option is just as important as selecting the solar collectors and storage tank. The backup must integrate seamlessly with the solar loop, operate efficiently, comply with safety codes, and match the local energy costs and infrastructure. This guide examines every major backup option for solar hot water systems, including electric resistance, natural gas, propane, heat pumps, boilers, oil-fired units, biomass, and even the decision to operate without backup in specific scenarios.

1. Electric Resistance Backup

Electric resistance is the most common and straightforward backup for solar water heaters. It uses an electric heating element—similar to the one in a standard electric water heater—immersed directly in the storage tank or installed in a secondary tank.

How It Works

In a typical solar storage tank, the electric element is positioned in the upper portion of the tank. The solar heat exchanger (coil or external plate) usually heats the lower and middle sections. When the solar-heated water in the upper part of the tank falls below the delivery setpoint, the thermostat activates the element. Once the water reaches the target temperature, the element shuts off. This stratification ensures that solar energy is always used first, and the electric backup only tops up the final temperature.

Configurations

• Single-tank integrated:​ The solar tank includes a built-in electric element. This is common in residential systems where space is limited.
• Two-tank setup:​ The solar preheat tank feeds a standard electric water heater. The electric unit acts as both backup and final-stage heater.
• Immersion diverter with PV:​ In some hybrid installations, excess solar electricity from a photovoltaic array powers an immersion element in the solar tank, though this is less efficient than dedicated thermal collection.

Advantages

• Simple installation and control logic.
• Low upfront equipment cost compared to gas or heat pump alternatives.
• No venting, combustion air, or fuel lines required.
• Highly reliable and easy to service.
• Can be paired with time-of-use electricity tariffs to heat water during off-peak hours.

Disadvantages

• Higher operating cost in regions with expensive electricity.
• Slower recovery rate compared to gas burners or heat pumps.
• Increases electrical load, which may require panel upgrades in older homes.
• Not the most environmentally friendly if the grid relies heavily on fossil fuels.

Best For

Homes with low to moderate hot water demand, areas where electricity is cheap or generated on-site via PV, and installations where gas infrastructure is unavailable.

2. Natural Gas Backup

Natural gas remains one of the most popular backup fuels for solar water heating, particularly in urban and suburban areas with existing gas mains.

How It Works

Gas backup can be integrated as a tank-style water heater or a tankless (on-demand) unit. In a two-tank configuration, the solar preheat tank supplies warmed water to the gas heater, which raises it to the final setpoint. In a combi-boiler setup, a single gas boiler provides both space heating and domestic hot water, with solar preheating the feedwater.

Tank-Style Gas Backup

A standard gas storage water heater receives preheated water from the solar system. The gas burner activates when the outlet temperature drops below the thermostat setting. This is a simple and effective arrangement, though it requires floor space and proper venting.

Tankless Gas Backup

A tankless gas water heater can serve as an on-demand backup. It heats water only when a tap is opened, using solar-preheated water as the inlet. This reduces gas consumption and provides unlimited hot water. However, tankless units require a minimum flow rate to activate, which can conflict with low-flow fixtures.

Advantages

• Fast recovery and high heat output.
• Lower operating cost than electric resistance in many markets.
• Compatible with existing gas infrastructure.
• Tankless options save space.

Disadvantages

• Requires gas line and venting (chimney or power vent).
• Combustion produces emissions.
• Tankless units may have minimum flow requirements.
• More complex installation than electric elements.

Best For

Households with high hot water demand, areas with affordable natural gas, and homes already using gas for space heating or cooking.

3. Propane Backup

Propane is the rural equivalent of natural gas. It is stored in an on-site tank and delivered by truck, making it ideal for off-grid or remote locations.

How It Works

Propane backup operates identically to natural gas backup, using either a storage tank or tankless heater. The solar preheat tank feeds the propane unit, which finishes the heating process.

Advantages

• Available off-grid without reliance on utility infrastructure.
• High energy density and fast recovery.
• Cleaner burning than oil or wood.

Disadvantages

• Fuel must be purchased and stored.
• Prices can fluctuate seasonally.
• Requires regular tank refills and monitoring.
• Venting and combustion air requirements similar to natural gas.

Best For

Rural homes, farms, cabins, and any location without access to natural gas or grid electricity.

4. Heat Pump Water Heater Backup

Heat pump water heaters (HPWH) are gaining popularity as an energy-efficient backup for solar thermal systems. They extract heat from the surrounding air and transfer it to the water, using electricity far more efficiently than resistance elements.

How It Works

In a solar + heat pump configuration, the solar system preheats the water, and the heat pump provides the final temperature boost. Because the heat pump works best with warm inlet water, solar preheating actually improves its efficiency. The heat pump can be integrated into a single tank with a solar coil or configured as a two-tank system.

Advantages

• Extremely high efficiency—up to 2-3 times more efficient than electric resistance.
• Reduces overall electrical load compared to standard electric backup.
• Can cool and dehumidify the surrounding space, useful in warm climates or mechanical rooms.
• Pairs well with on-site solar PV for a fully renewable setup.

Disadvantages

• Higher upfront cost than electric resistance or gas tanks.
• Requires a minimum ambient air temperature (typically above 40–50°F / 4–10°C) to operate efficiently.
• Generates cool air as a byproduct, which may be undesirable in cold spaces.
• Slower recovery than gas burners.

Best For

Energy-conscious homeowners, mild to warm climates, homes with existing solar PV, and locations where gas is unavailable or expensive.

5. Boiler and Oil-Fired Backup

In colder climates, many homes and commercial buildings use boilers for space heating. These boilers can also serve as backup for solar domestic hot water.

How It Works

A combi-boiler or indirect water heater uses the boiler’s heated water or steam to warm domestic water via a heat exchanger. Solar preheating reduces the boiler’s workload. Oil-fired boilers or water heaters operate similarly but burn heating oil instead of gas.

Advantages

• Ideal for cold climates where space heating and water heating loads overlap.
• High output and reliable performance in freezing conditions.
• Can serve large volumes for commercial or multi-family use.

Disadvantages

• High upfront cost for boiler infrastructure.
• Oil storage and delivery logistics.
• Combustion emissions and maintenance requirements.
• Overkill for small residential systems.

Best For

Northern climates, large homes, apartment buildings, and facilities with existing boiler plants.

6. Biomass and Wood-Fired Backup

Biomass—including wood, wood pellets, and agricultural waste—can provide backup heating in off-grid or rural settings where other fuels are expensive or unavailable.

How It Works

A wood-fired boiler or water heater heats water that circulates through a heat exchanger, either directly or via a buffer tank. Solar preheats the domestic water, and the wood system tops it up. This often requires manual feeding and monitoring.

Advantages

• Renewable and carbon-neutral if sustainably sourced.
• Low fuel cost in wood-rich areas.
• Independent of grid electricity and fossil fuel supply chains.

Disadvantages

• Labor-intensive (loading fuel, removing ash).
• Requires significant storage space for fuel.
• Emissions and air quality regulations may restrict use.
• Slower response and harder to automate than gas or electric.

Best For

Off-grid homesteads, farms, and rural properties with abundant biomass resources.

7. No Backup or Seasonal Use

Some solar water heating systems are designed without any backup. These are typically used for seasonal applications, preheating, or non-critical uses.

How It Works

The system relies entirely on solar gain. If the water is not hot enough, the user waits, mixes with cold water, or uses an alternative source. This is common in pool heating, summer cabins, or agricultural preheating where a complete lack of hot water is not a critical failure.

Advantages

• Lowest cost and simplest design.
• No backup fuel consumption or emissions.
• Easy to install and maintain.

Disadvantages

• Unreliable for year-round domestic use.
• Unsuitable for hygiene-critical applications (hospitals, care homes, restaurants).
• User must manage expectations and adapt usage patterns.

Best For

Swimming pools, seasonal cottages, frost-free agricultural preheating, and situations where occasional cold water is acceptable.

8. Control Logic and Integration

The backup system is only as good as its control strategy. Poor control can cause the backup to reheat water that the sun has already warmed, wasting energy and money.

Priority Settings

Solar should always have priority. The controller should allow the solar loop to heat the tank to its maximum setpoint before the backup activates. Once the solar pump stops (due to collector cooling or tank saturation), the backup monitors the top of the tank and engages only if the delivery temperature is insufficient.

Differential Controllers

Modern differential controllers manage both the solar pump and the backup. They use sensors on the collector, tank bottom, and tank top to make decisions. For example, the backup might be locked out entirely during daylight hours or when the tank bottom exceeds a certain temperature.

Anti-Legionella Control

In potable systems, especially those with large storage tanks, there is a risk of Legionella bacteria growth if water stays in the 68–113°F (20–45°C) range for extended periods. Backup controls should include a periodic thermal disinfection routine, raising the entire tank to 140°F (60°C) or higher, then tempering the outlet to a safe delivery temperature.

Avoiding Energy Waste

• Setback modes:​ Reduce backup operation during vacations or low-use periods.
• Dual sensors:​ Prevent backup from firing if solar is still active.
• Smart grid integration:​ In some regions, backup can be programmed to heat only during off-peak electricity hours.

9. Choosing the Right Backup: A Scenario-Based Guide

10. Frequently Asked Questions

Is a backup heater always necessary for solar hot water?

For year-round domestic use in most climates, yes. Without backup, you will experience periods of insufficient hot water during cloudy weather, winter, or high demand. Exceptions include pools, seasonal cabins, and preheat-only systems.

Can I use my existing water heater as a backup?

Often, yes. If you have a conventional electric or gas water heater, it can be repurposed as a backup in a two-tank configuration. The solar preheat tank feeds the existing heater, which tops up the temperature. This is one of the most cost-effective retrofit strategies.

Will the backup heater run all the time?

No, not if properly controlled. A correctly configured system uses solar energy first. The backup only activates when the solar tank cannot meet the setpoint. On sunny days, the backup may not run at all.

How do I prevent the backup from wasting energy?

Ensure the controller prioritizes solar, set the backup thermostat to a reasonable delivery temperature (around 120°F / 49°C for most homes), and use a tempering valve to mix down from a higher storage temperature if needed for disinfection. Regular maintenance of sensors and controls also prevents malfunctions.

Can I have more than one backup option?

Yes. Some systems include a primary backup (e.g., heat pump) and a secondary emergency backup (e.g., electric element). This provides redundancy but increases complexity and cost.

Does the backup system affect solar fraction?

Yes. A well-matched backup that only supplements when necessary preserves a high solar fraction. An oversized or poorly controlled backup can reduce solar fraction by reheating water unnecessarily.

What size should the backup heater be?

The backup should be sized to meet peak demand on a completely sunless day. For a family of four, this might be a 40–50 gallon gas tank or a 4500–5500 watt electric element. Larger households or commercial facilities require proportionally larger backup capacity.

Is it safe to combine solar thermal with an electric tank?

Yes, it is very common and safe when installed according to plumbing and electrical codes. The solar coil or external heat exchanger must be properly isolated from the potable water, and all safety devices (T&P relief valve, expansion tank, backflow prevention) must be in place.

Can I turn off the backup in summer?

If your solar system consistently provides sufficiently hot water in summer, you can disable the backup manually or via controller settings. However, be mindful of Legionella risk if water sits in the tank for extended periods without reaching disinfection temperatures.

How much does a backup system add to the total cost?

A backup element in a solar tank adds relatively little to the equipment cost. A separate gas tank or heat pump adds more, but often the backup is simply an existing water heater repurposed. Installation costs vary based on fuel type, venting, and electrical work.

Conclusion

Selecting the right backup for a solar water heater is a balance of climate, fuel availability, demand patterns, and budget. Electric resistance offers simplicity and low upfront cost. Natural gas and propane provide fast recovery and familiar operation. Heat pumps deliver outstanding efficiency and synergy with solar preheating. Boilers and biomass serve specialized or high-demand applications. And in some seasonal or non-critical uses, no backup at all may be the most practical choice.

The key to a successful installation is control: the backup must complement the solar system, not compete with it. With proper integration, your solar water heater will deliver reliable, low-cost hot water in every season, while the backup stands ready to ensure comfort and safety whenever the sun is not enough.