You already have a solar water heater on your roof and are thinking of adding solar panels. Can the two systems coexist? In many cases, yes, provided the available space and the design allow both systems to operate and be maintained properly.
The key question is not just how many panels will fit around the water heater. It is whether the new layout creates shading, preserves access to the existing equipment, and whether moving it would bring a genuine benefit. Ask for both systems to be assessed on the same roof plan before finalizing the quote.
Two systems that make different use of the sun
Solar panels generate electricity. A solar water heater uses heat from its collectors to warm water. Adding solar panels does not, by itself, mean that a working solar water heater must be removed.
If you are advised to remove the solar water heater to make room for more panels, ask for an assessment of how hot water will be produced afterward. The comparison should include your new energy use, the time of year you use the system, and the cost of the necessary work—not just the additional panel capacity.
For an initial assessment, gather photos, the water heater’s model and dimensions, its age, and any available instructions. Let the designer know if you are planning to replace it soon or if it has had operating problems.
Shading must be checked in both directions
In common thermosiphon solar water heaters, the water tank is positioned higher than the collectors. The design needs to assess the tank height, its supports, and the collector surface separately, because each can obstruct nearby solar panels.
Likewise, a row of solar panels can shade the thermal collectors. So it is not enough to protect only the new panels. Suitable conditions must also be maintained for the existing hot-water system.
The Nobel Classic solar water heater technical manual includes instructions for avoiding shading and choosing an installation location. This is an example of documentation for a specific product range; ask for the corresponding instructions for your own equipment.
A photo taken at midday in summer is not enough for an annual assessment. The sun’s position and the extent of shadows change throughout the day and from season to season. Ask for an assessment of the hours and periods when each layout is significantly affected.
Don’t rely on a general production map alone
Production estimation tools are useful, but you need to know exactly what they take into account. The European Commission explains that the calculated horizon in PVGIS does not include shading from very nearby objects, such as neighboring buildings or trees.
So don’t assume that a general production printout has automatically accounted for your water heater, parapet, or stairwell structure. Ask whether nearby obstacles have been mapped and how their impact was assessed for the specific proposal.
Ask to see the location and height of obstacles on the plan. The phrase “there’s a little more space” does not show whether adding another row will be worthwhile over the course of a year.
Three options to consider before deciding whether to move the solar water heater
It is worth considering alternatives that can be compared using clear assumptions:
- Keep the solar water heater in its current location. The solar panel layout is adapted around the space it occupies and the shade it casts.
- Choose a different layout or fewer panels. Assess whether a less dense installation can meet your needs while providing better access and fewer obstacles.
- Move the solar water heater, if there is evidence to support doing so. Compare the additional benefit from the solar panels with the work involved and the impact on the hot-water system.
The water heater’s qualified technician needs to assess any relocation. The pipework, supports, and routing positions may change. Don’t simply agree that “it will be moved a little farther over” without clarifying the new location, the scope of work, and the checks that will follow.
The specific manufacturer’s instructions and the engineer’s assessment determine the requirements. There is no single distance in meters that works for every case involving both systems, regardless of height, orientation, and geometry.
Allow space for the next service visit, too
The shared roof plan should show how the water heater can be accessed for scheduled work and how it could be replaced in the future. Ask specifically whether the solar panels would need to be temporarily removed first to take the tank away.
Also ask that the total load and support locations for both systems be taken into account. Do not assume that an old solar water heater proves the roof can support every additional installation. Roof protection is covered separately in our guide to waterproofing and solar panels.
Compare savings without double-counting
If the Your existing solar water heater already covers part of your hot water needs, and this is reflected in the consumption measured by your utility bill. It would not be correct to count again as a new benefit of photovoltaics the electricity you already save thanks to the water heater.
Conversely, if its removal and electric water heating are being considered, the new demand must be included in the assessment. Request a comparison of the overall energy impact, with clear assumptions about hot water use. In the Solar Home design guide we explain why we start with your property's actual consumption.
What to request before approving the layout
- A floor plan with dimensions, the water heater's location, and the panel layout.
- An explanation of the shading analysis for both systems.
- Access for maintenance and possible future replacement of the solar water heater.
- A separate description and cost breakdown if relocating it is proposed.
- A consumption calculation that takes into account whether the solar water heater is retained or removed.
At Solar Home, we assess your property's available space, shading, and energy needs to design your photovoltaic system. Tell us from the outset about your existing water heater and request a study and quote, so the proposal can account for how the systems will actually coexist on your roof.
