Retrofit a heating element with solar power

Anyone who regularly feeds surplus solar power into the grid at midday but still buys additional energy for hot water in the evening is missing out on a real opportunity. That’s exactly why retrofitting a heating element with solar power is so appealing to many homeowners: with the right control system, surplus solar power is converted into hot water—automatically, efficiently, and without disrupting daily life. This surplus can be put to targeted use, especially when combined with the SMARTFOX Pro Heater.

Why a heating element often pays for itself quickly in PV systems

A heating element is not a new invention. What is new is the way it is integrated into a smart energy system today. Instead of generating hot water on a fixed schedule or using expensive grid electricity, it uses energy specifically when the solar panel system is producing more than the home is currently consuming.

This is particularly interesting in the case of typical single-family homes. During the day, only basic loads are usually running, while the solar system generates significantly more power when the sun is out. Without suitable appliances to use it, this surplus flows into the grid—usually at a rate that is significantly lower than the price you’ll later pay for electricity. Using surplus solar power to heat water improves this economic leverage.

There is also a practical advantage: hot water is an ideal form of energy storage. The boiler essentially acts as a thermal storage unit. Instead of simply feeding electricity into the grid, you use it directly in your own home. This increases self-consumption, lowers electricity costs, and improves your level of self-sufficiency.

Retrofitting a heating element with solar power – how the system works

Technically, the principle is simple. The heating element is located inside the hot water tank and converts electrical energy into heat. The key factor is not the heating element itself, but the smart control system.

Without a control system, the heating element would either run at full power or not at all. This can be problematic because solar power surpluses fluctuate. One moment there might be 700 watts available, ten minutes later 2,500 watts, and then significantly less again. A good solution detects these surpluses in real time and adjusts the output so that as much solar power as possible stays in the home.

This is exactly where a simple DIY solution differs from a truly efficient retrofit system. Simply installing a heating element does not optimize self-consumption. It is the combination of energy metering, smart control, and compatible loads that transforms a PV system into an active energy system.

What requirements your home should meet

Whether a heating element can be retrofitted with photovoltaic technology depends primarily on the existing hot water system. The ideal setup is a storage tank or boiler with an opening suitable for a screw-in heating element. Many existing systems already offer this option, while for others it must be determined whether retrofitting makes technical and economic sense.

The electrical aspects are also important. A heating element draws power, and this must be properly integrated. This requires professional installation, appropriate circuit protection, and a control system that works in tandem with your solar power system and the other electrical loads in the house.

It’s also worth taking a look at your energy usage profile. Retrofitting is particularly worthwhile if you regularly generate surplus energy during the day, if there are several people in your household, or if hot water accounts for a significant portion of your energy needs. If you already own a battery storage system, you should clearly define your priorities: Should the system charge first, or should surplus energy be used for hot water? The best answer depends on your household’s daily routine.

Not just a component, but a strategy

Many people underestimate the fact that a heating element should not be treated as an isolated, standalone component. In practice, the key question is always which appliance should take priority and when. Hot water systems, wallboxes, heat pumps, and battery storage units often compete for the same surplus PV energy.

A smart control system determines in real time where the energy will be most useful at any given moment. Perhaps the storage unit should be charged first thing in the morning, the water heater at noon, and the electric car in the afternoon. Or the system might heat water only to a set temperature so that there are still reserves available for other appliances afterward.

This is particularly important for homeowners who want to gradually get more out of their solar power system. A properly integrated heating element is often an excellent first step toward sector coupling—but it delivers the greatest benefits when it is part of a well-designed overall system.

Which advantages really matter in practice

The most obvious advantage is the cost savings. Using your own solar power to heat water is usually more economical than feeding it into the grid at a low rate and later purchasing expensive grid electricity or other energy sources. This improves the profitability of the entire PV system.

Equally important is the greater independence this provides. By using more energy on-site, households become slightly less dependent on electricity prices and external suppliers. For many households, this is a major motivator—not as an abstract concept, but as a tangible gain in control.

Then there’s the convenience. If the control system is set up properly, hot water production runs automatically in the background. You don’t have to switch anything on manually, interpret weather forecasts, or coordinate individual devices. The system takes care of this for you and manages production and consumption as intelligently as possible.

Where the boundaries lie

As appealing as retrofitting may be, it isn’t always the perfect solution. While an electric heating element is simple and reliable, it isn’t always the most energy-efficient way to generate heat. A heat pump delivers significantly more heat per kilowatt-hour of electricity used. So if you’re planning a major upgrade to your home’s heating system, you should carefully compare both options.

Storage capacity is also a factor. If the hot water tank is small, it can only store a limited amount of energy. In that case, the boiler reaches the desired temperature quickly, and any excess PV energy must still be used for other purposes or fed into the grid.

Furthermore, a heating element does not automatically replace smart load management. If several large loads are active at the same time, this can lead to unnecessary power consumption from the grid without coordinated control. This is precisely why it’s worth looking not just at the hardware, but at the entire energy management system.

Should I retrofit a heating element with solar power, or think bigger from the start?

For many households, an electric heating element is a practical solution that offers good value for money. The investment is manageable, the impact on the existing system is often minimal, and the effect on self-consumption can be significant. This makes it particularly attractive if a usable hot water tank is already in place.

However, if you’re planning a more extensive renovation anyway, you should take a broader view. In that case, it may make sense to consider hot water heating as part of an overall concept that includes a heat pump, storage tank, wallbox, and PV control system. This way, you can avoid standalone solutions that will need to be adapted later on.

In practice, therefore, there is no single right answer. There is a solution that’s right for your home, your energy needs, and your goals. Is your main goal to increase self-consumption? Are you looking for a quick retrofit with manageable costs? Or would you like to centrally control your energy system from the start? Only once these questions have been answered can a technical idea become an economically sound decision.

What to Look for When Upgrading

Compatibility is key. The heating element must be compatible with the storage tank, the control system with the inverter or metering system, and the entire system with your other electrical appliances. Manufacturer-independent solutions are often a major advantage here because they offer greater flexibility when integrating existing equipment.

Equally important is the quality of control. A continuously variable or finely tuned power control system makes much better use of excess PV energy than simply turning the system on and off. This has a direct impact on daily life, as less energy is fed into the grid unused and less electricity is drawn from the grid.

Usability should not be underestimated either. A good system clearly shows how much power the solar PV system is generating, how much of that is being used for hot water, and which priorities are currently active. When you have a clear overview at all times, energy optimization becomes a manageable part of your home rather than a mystery.

This is precisely the approach taken by smart energy management solutions like those from SMARTFOX: not just switching things on and off, but actively coordinating energy flows in the home.

Who is the retrofit particularly suitable for?

This solution is particularly well-suited for households with a typical daily surplus from their solar PV system, for families with a constant demand for hot water, and for buildings that do not require major heating system upgrades. An immersion heater can also be a very attractive option as a supplement to existing systems if the goal is to increase self-consumption without major renovations.

It is less suitable in situations where there is hardly any surplus solar power, hot water demand is very low, or a complete system overhaul is planned in the near future anyway. In such cases, it may be more cost-effective to direct investments directly toward a more comprehensive solution.

Anyone looking to retrofit a heating element with solar power today shouldn’t just ask whether it’s technically feasible. The better question is: How can I turn my solar power system into a setup that uses electricity where it’s most effective? If you plan it carefully, surplus energy won’t just become heat—it will be a real step toward greater energy independence in your own home.