Frequently Asked Questions About Solar Systems: Return on Investment, Subsidies, Permits and Connection
Frequently Asked Questions About Solar Systems: Return on Investment, Subsidies, Permits and Connection
When a customer is deciding on a solar system, technical parameters are only one part of the story. Equally important – and in practice often much trickier – are questions that don't relate directly to the collector or the storage tank, but to what comes afterward: how much the whole thing costs and how long it takes to pay back, whether a building permit is needed, how to connect it to an existing boiler, and where to even find a subsidy. From experience, we know that these exact questions keep people from deciding for months, sometimes even years – not because the answers aren't available, but because they're scattered across dozens of websites, authorities, and technical manuals. This article brings them together in one place, factually and without any unnecessary "solar will save the planet" marketing rhetoric.
Return on Investment for a Solar System: Real Numbers, Not Marketing Phrases
Return on investment is the first question we get from almost every customer. And it's the right question – it's just that the answer depends on so many variables that a single "7 years" or "12 years" figure is more marketing fiction than a real calculation.
What Affects the Payback Period
The basic input parameters for calculating payback are: the price of the system including installation, the annual energy cost savings (hot water), the price of the energy being replaced, and any subsidy that reduces the initial price. A secondary factor is location – Bratislava receives approximately 1,200–1,350 kWh of solar energy per year per square meter of collector, while northern Slovakia (e.g. around Žilina or Poprad) fares worse, roughly 1,050–1,150 kWh/m². This difference can shift the payback period by 1–2 years.
A solar system for a typical family house (4 people, 2 flat collectors with a total area of approx. 4–5 m², a 200–250 liter storage tank) covers 90–100% of hot water consumption in the summer months and, on average over the year, 55–70% of this consumption. Annual hot water consumption for 4 people is approximately 2,000–2,500 kWh of heat. The solar system therefore provides roughly 1,100–1,700 kWh per year.
At a natural gas price of €0.08–0.10/kWh (including distribution, a typical price for households in 2024–2025) and a gas boiler efficiency of 90%, the annual savings come out to approximately €95–190. For electricity (€0.20–0.25/kWh, boiler heating), it's €220–425 per year. For a heat pump with a COP of 3 and an electricity price of €0.22/kWh, the savings fall somewhere in between, roughly €80–190 per year – here solar loses its competitive advantage, since a heat pump is already very efficient on its own.
The price of a complete solar system with installation (2 collectors, 250 l storage tank, controls, expansion vessel, installation) in 2024–2025 ranges between €3,500–6,500, depending on the brand, type of installation (pitched vs. flat roof), and local labor costs. Premium systems from Vaillant or Protherm are at the higher end of this range, but offer a better service network and longer warranties.
After deducting the subsidy (see below) and with savings of ~€180/year (electricity), the payback period at a total cost of €4,500 and a subsidy of €1,500 (net investment €3,000) comes out to roughly 16–17 years. With electricity and a subsidy, it can be as low as 10–12 years. A quality solar system has a lifespan of 20–30 years, so financially it's a worthwhile investment – just not the dramatically fast kind that customers are used to from advertisements.
A Real-World Example: A Family House in Trenčín
A specific example: a customer with a 4-member family in Trenčín, combined heating – a Vaillant gas boiler plus an electric storage tank as backup heating. They installed a Vaillant auroSTEP VSL S 250/2 T on a pitched roof facing south with a 38° incline. The total investment including installation was €5,100. After the subsidy from the Green Households scheme, €3,570 remained to be paid. The measured annual gas savings (measured by the boiler control) in the first year was 1,150 kWh, which at a price of €0.095/kWh amounts to approximately €109 per year. The payback period therefore comes out to about 32 years without any rise in energy prices – longer than the commonly quoted 12 years. However, if we take into account an expected 3% annual increase in gas prices, the payback drops to roughly 22–24 years – which is realistic within the system's lifespan.
Conclusion: a solar system is a long-term investment with a real payback, but don't expect a "miraculous" 7 years. Those replacing electric heating come out significantly better than those replacing gas. For a more comprehensive calculation, we also recommend the article What Solar System Output Do I Need: A Calculator by Number of People and Hot Water Consumption in this Knowledge Center.
Subsidies for Solar Systems: Current Overview, Conditions, Procedure
This is a topic that changes faster than collector technical documentation, so always verify the current status on the relevant website. Here, however, we describe the schemes that have long been in operation in Slovakia and are relevant for family houses.
Green Households (SIEA)
The Green Households program, administered by the Slovak Innovation and Energy Agency (SIEA), is the main subsidy tool for family houses. In past rounds it covered 30–50% of eligible costs for solar collectors and storage tanks. The maximum voucher amount for solar thermal collectors (not photovoltaics) in recent rounds was set at €1,800–2,500 per family house.
Conditions (typical for recent rounds, always verify current information at siea.sk):
- The family house must be legally built, permanently inhabited, and owned by the applicant.
- Installation must be carried out by a contracted supplier registered in the SIEA system.
- The equipment must meet technical criteria (collectors with a Solar Keymark certificate or equivalent).
- The voucher is issued BEFORE installation – first the application, then the installation, then reimbursement.
- The installation must be completed and documented within the specified deadline (usually 3–6 months).
The procedure, simplified: 1) register on the SIEA portal, 2) submit an application and select equipment from the list of eligible devices, 3) receive the voucher, 4) choose a supplier from the list of SIEA-contracted installers, 5) after completion, submit the invoice and protocol, 6) SIEA reimburses the voucher directly to the supplier or to you.
An important note from practice: rounds open irregularly and vouchers run out quickly – sometimes within hours of opening. It's worth following the SIEA newsletter and being ready to submit your application immediately after a round opens.
Recovery Plan and Other EU Funds
Under Slovakia's Recovery and Resilience Plan, funds have also been allocated for renovating family houses, including the installation of renewable energy sources. Photovoltaic-related components receive a larger share of publicity, but thermal solar systems are also an eligible expense – especially in combination with comprehensive house renovation (insulation + renewable sources). In 2024–2025, a call is running through SLSP/SZRB for family houses, with advisory support via Enviro funds. Details change, so consult directly with SIEA or with a SlovSEFF-approved bank.
Tax Deductions and Other Benefits
In Slovakia there is no direct tax deduction for installing solar collectors for individuals (unlike in some EU countries). Some banks offer "green mortgages" or "eco loans" with lower interest rates for houses with renewable energy sources – this can reduce financing costs and thus indirectly improve the project's economics.
Permits: When You Need a Building Permit and When a Notification Is Enough
This topic causes customers the most confusion – and no wonder, since the answer depends on several factors at once: type of building, type of installation, location, local legislation, and, last but not least, the current interpretation of the building authority.
Basic Rule Under Slovak Building Law
Act No. 50/1976 Coll. (Building Act) as amended, and the 2021 amendment (Act No. 150/2021 Coll.), distinguish several categories of construction interventions. Solar thermal collectors are in most cases classified as an "alteration of a building" – specifically as the installation of technical equipment on an existing structure.
For a family house, the following applies in simplified terms:
- Integrated roof installation (replacing roofing with solar elements) – usually requires a building permit, since it involves an intervention in the load-bearing structure and changes the roof's appearance.
- On-roof installation on a pitched roof (collectors placed on existing roofing using mounting hooks) – usually a notification of a minor structure to the relevant building authority is sufficient; in some cases the authority may decide that not even a notification is required.
- Installation on a flat roof on a mounting structure (legs, frames) – the situation is similar to a pitched roof with on-roof mounting, usually just a notification.
- Installation on a facade – may require a permit, especially in a heritage-protected zone.
The new Building Act No. 200/2022 Coll. (effective from 1 April 2024 within a transitional period) introduces new categories and simplifies some processes, but at most authorities both old and new rules are currently applied in parallel. Practical approach: contact your local municipal/city building authority before ordering the system and verify the current requirements for your specific situation.
Heritage Protection and Protected Landscape Areas
If your house is in a heritage zone, heritage reserve, or is itself a registered cultural monument, the situation is significantly more complicated. The Monuments Board of the Slovak Republic and regional monuments offices have veto power over any changes to external appearance. In practice, this can mean a ban on installing visible collectors, or the need to use specially aesthetically integrated solutions (e.g. roofing with integrated collectors). Collectors on garden buildings or on the rear (street-invisible) side of the roof are usually more acceptable.
In protected landscape areas, conditions vary – depending on the protection zone and the local administrator. Most installations in villages within protected landscape areas are unproblematic, as long as the house is not in the first protection zone.
What a Minor Structure Notification Includes
If your authority requires a notification, you will typically need: a completed notification form, proof of property ownership (title deed, no older than 3 months), a simple site plan (roof layout showing the location of the collectors), a technical description of the equipment (manufacturer's accompanying documentation is sufficient), and in some cases a neighbor's consent if the collectors extend into the protective zone of the property boundary. The authority has 30 days to respond – if there is no objection within 30 days, installation may begin.
Connecting a Solar System: Technical Requirements and Procedure
Thermal solar systems (for water heating) are not connected to the electrical grid in the sense of "energy supply" – they are a closed hydraulic circuit with a heat exchanger. Connection is therefore not the same as with photovoltaics, which has a broader regulatory framework. Nevertheless, there are several important technical and legal aspects.
Hydraulic Connection: Principle and Requirements
A solar system is essentially a dual-circuit system: the primary circuit (collectors → pump station → solar heat exchanger in the storage tank → back to the collectors) and the secondary circuit (potable water in the storage tank, which is heated via the exchanger and flows into the distribution system). The primary circuit contains antifreeze (glycol), a pump, an expansion vessel, a pressure gauge, a safety valve, and a controller. The secondary circuit is a standard potable water installation.
Installation of the primary circuit must be carried out by a person with the appropriate professional qualification – this involves working with pressurized equipment (typical maximum operating pressure 6 bar). This is not a "weekend job," although some preparatory work (installing roof brackets, running piping in the ceiling) can technically be prepared in advance by a handy homeowner. You can find more about what is and isn't suitable to do yourself in the article Installing a Solar System Step by Step: What You Can Do Yourself and What Must Be Done by a Professional.
Electrical Wiring: What's Needed for a Solar System
A solar system needs electrical power for the pump station (the pump consumes 25–60 W, i.e. approximately 30–70 kWh per year) and for the controller (a few watts). The total electrical input is negligible. Connection is made to the existing household wiring, via a fused 230 V / 10 A circuit. This is not a dedicated electrical installation under Decree No. 508/2009 Coll. (low-voltage electrical equipment above 1 kW must undergo inspection, but for certified CE-marked systems the procedure is simplified). We recommend having an inspection done anyway – both for insurance purposes in case of a fault, and to meet the manufacturer's warranty conditions.
Connecting to a Boiler or Heat Pump
Most modern solar systems are designed with a bivalent storage tank – i.e. a tank with two heat exchangers or with electric backup heating. The lower exchanger is for solar (primary circuit), the upper one is connected to the boiler or has an electric heating element. The controller ensures that the boiler (or heat pump) only switches on backup heating when solar output is insufficient to maintain the required temperature (adjustable, typically 45–55 °C). This is a key aspect of proper setup – too low a set temperature (below 60 °C) risks Legionella growth, while too high a temperature unnecessarily increases the load on the boiler and reduces solar efficiency.
For example, the Protherm HelioSet FES2 250 BM includes a bivalent storage tank with a flanged electric heating element built directly into the set, which simplifies integration – you don't need an additional boiler, the system is self-contained. On the other hand, the Protherm HelioSet 2.250C HT is designed to work together with an existing Protherm gas boiler and includes a tank with two heat exchangers – in this case correct hydraulic connection to the boiler is critical and must match the boiler manufacturer's specifications. You can find more on combining a solar system with a boiler in the article Combining a Solar System with a Boiler or Heat Pump: How to Properly Connect the Systems.
Choosing the Right System Based on Roof Type and Needs
An incorrectly chosen system is one of the most common causes of unmet expectations. Before purchasing, three things need to be decided: roof type (pitched / flat), number of people in the household, and primary backup energy source. A more detailed guide can be found in the article How to Choose a Solar System for a Family House: Output, Number of Collectors and Storage Tank Volume; here we provide a basic overview for the context of this page.
For a pitched roof facing south to southwest with a 30–50° incline: the Vaillant auroSTEP VSL S 250/2 T is a premium solution with proven reliability and a well-developed service network in Slovakia. It includes 2 flat collectors and a 250-liter bivalent storage tank – ideally sized for a 3–5 person household.
For a flat roof or terrace installation: the Vaillant auroSTEP VSL S 250/2 F has special mounting brackets for flat roofs with adjustable frame inclination of 25–35°, which optimizes solar capture for horizontal installation. A detailed comparison of installation variants can be found in the article Solar System on a Pitched vs. Flat Roof: Which to Choose and What It Means for Installation.
For budget-conscious customers with a smaller household (2–3 people) who prefer a compact solution, the Solar System No. I S is an interesting choice – a simpler system with 1 collector, suitable as a supplement to an electric storage tank in a smaller house or cottage.
Practical Scenarios: What Happens with Common Orders
From experience, we see a few recurring patterns that surprise customers:
Scenario 1: The customer has a 150 l electric boiler and wants to save money. This is one of the best cases for a solar system. Replacing electric heating with a solar system plus backup heating brings the highest financial savings per kWh. We recommend a storage tank of at least 200–250 l (to provide a sufficient energy buffer for cloudy days) and collector output of 3–4 m². With a subsidy, the payback period can be as low as 8–10 years.
Scenario 2: The customer has a modern condensing gas boiler. Here the situation is different. A modern condensing boiler has an efficiency of 95–109%, so replacing the heating that the boiler performs with a solar system brings relatively small gas savings. The economics of the project depend heavily on the subsidy. If the customer's primary motivation is ecological and financial concerns are secondary, it makes sense. If the primary motivation is financial, this needs to be calculated realistically.
Scenario 3: A house with a heat pump. Combining solar thermal collectors with a heat pump is technically possible, but economically the least advantageous – a heat pump is already a very efficient source of heat for water heating on its own. If the customer has a heat pump, a better investment is photovoltaic panels (to cover the electricity consumption of the heat pump), not thermal collectors.
Scenario 4: A cottage or cabin without gas. An excellent case for a solar system. A cottage with an electric boiler and a solar system will save practically 100% of hot water heating costs during the summer season. The payback can be very fast, especially if the cottage has no other heating alternative.
Scenario 5: The customer wants to combine it with a solar pool mat. A thermal solar system for domestic hot water and a solar tube pool mat are two different systems with different primary circuits and controls. They can coexist in one house, but they are not one system. Combination is possible, but the sizing and hydraulics must be designed from the start for both applications.
Frequently Asked Questions (FAQ)
Do I need a project from a design engineer for a solar system?
For a family house with a standard system (2 collectors, 1 storage tank), usually not – manufacturers provide project documentation as part of the system's technical guide, and this is sufficient for the building authority when filing a notification. If it's a larger system (more than 3 collectors), an atypical installation, or a combination with a more complex heating system, a project from a licensed engineer is recommended, also to ensure proper hydraulic design and correct sizing of the expansion vessel.
Can I install a solar system myself and still get a subsidy?
No – a condition of the Green Households subsidy is installation by a contracted supplier registered in the SIEA system. Self-installation automatically excludes you from the subsidy. In addition, installing the pressurized primary circuit requires professional qualification (a gas fitter with the appropriate authorization or an installer certified for solar systems). You need an installer – you can still get the subsidy if you follow the correct procedure.
What happens to a solar system in winter – will it freeze?
The primary circuit is filled with antifreeze (a propylene glycol water solution certified for solar systems), which is resistant to temperatures down to -28 °C to -35 °C (depending on concentration). The collectors don't produce heat in winter when covered with snow, but on sunny frosty days they can contribute surprisingly significantly. Freezing is not a risk with a properly designed system. More details can be found in the article A Solar System in Winter: How It Works at Low Temperatures and How to Prevent the Circuit from Freezing.
How long does installing a solar system take?
A standard installation of 2 collectors on a pitched roof, with connection to the storage tank and controls, takes 1–2 days for an experienced two-person installation team. Add to that the time needed for filling and bleeding the system, setting up the controller, and initial start-up (another half day). With more complicated piping routing (a long distance between the roof and the utility room, wall penetrations), installation can take up to 3 days. The completion date depends on the company's workload – in summer, installation companies are often booked out months in advance, so it's best to order the work in autumn or winter for a spring installation date.
Is annual inspection or servicing required?
Manufacturers recommend an annual visual inspection (condition of collectors, piping, insulation) and checking the pressure in the primary circuit. Professional servicing (checking glycol concentration, pump inspection, expansion vessel check, controller function test) is recommended every 2–3 years or after a fault. The antifreeze concentration should be checked at least every 3–4 years – glycol degrades over time and loses its protective properties. More information can be found in the article Maintenance and Servicing of a Solar System: What to Check Every Year and When to Call a Technician.
Will a solar system affect my home insurance?
Yes – always inform your insurance company about installing a solar system. Solar collectors must be insured as part of the property (or equipment), since they are permanently installed. Most insurance companies automatically include them in property insurance once notified; some require an adjustment to the insured amount. Without notification, you risk the insurance company refusing to pay out in the event of a claim (storm, hail damage) on the grounds that you changed the insured property without their knowledge.
Conclusion: Yes to a Solar System – But With a Realistic Outlook
Thermal solar systems are a proven, reliable technology with a lifespan of 20–30 years. They are not "miracle machines" that pay for themselves in 5 years and halve your energy costs – those days belonged to different energy prices and different technology prices. Today it's a long-term investment that makes the most sense when replacing electric heating, when properly using a subsidy, and for families who plan to live in the house for a long time.
The key to a successful installation is correct sizing (neither undersized nor oversized), professional installation, correct control settings, and regular – but not costly – maintenance. If you meet these conditions, the solar system will be quiet, reliable, and will contribute to lower water heating costs every year for decades.
To choose a specific system, we recommend browsing the category of solar systems on atria.sk, where you'll find proven systems from Vaillant, Protherm, and other manufacturers – including technical data sheets, complete documentation, and current prices. A comparison of the two most popular lines can be found in the article Comparing Solar Systems: Vaillant auroSTEP vs. Protherm HelioSet – Differences and Suitability of Use.
Do You Have a Question on This Topic?
Not sure what to decide, or dealing with a specific situation in your household? Write to us - we'll be happy to help.