Does a solar system pay off even in cloudy weather or winter
Does a solar system pay off even in cloudy weather or during winter?
This is by far the most common question I hear from people considering the installation of a solar system for water heating. Usually, it stems from a legitimate concern: Slovakia is not Spain. We have long gray winters, many cloudy days, sometimes even weeks without sunshine. Is it worth investing in a solar system at all, given that half the year the sun either doesn't shine or is so low on the horizon that it's not worth the effort?
Short answer: yes, it does pay off. But not naively and not without context. The long answer follows below – and I believe it's really worth reading, because most people who fear that collectors "don't work in winter" are working with incorrect assumptions about exactly what a solar system does and what its performance depends on.
What a solar system actually uses – and it's not just "sunlight"
The first thing to understand: a solar thermal collector does not work on the principle of photovoltaics. Nothing is converted into electricity. It absorbs electromagnetic radiation – specifically solar radiation – and directly converts it into heat in a liquid that circulates in the collector. And here is a key technical fact that most laypeople don't know:
Solar radiation reaches the Earth's surface even under dense cloud cover. Not in full intensity, of course – clouds reflect and absorb part of it – but diffuse (scattered) radiation still passes through. In practice, this means that even on a cloudy day, the collector collects energy. How much? It depends on the type of collector, the thickness of the cloud cover, and other factors. But it's definitely not zero.
Solar radiation consists of direct radiation, which comes directly from the sun, and diffuse radiation, which comes from the entire sky after scattering in the atmosphere. In Slovakia, diffuse radiation makes up an average of about 40–55% of the total incident radiation annually. In winter months, the share of diffuse radiation can be even higher – up to 60–70% in December. And it is precisely this radiation that collectors process even without direct sunlight.
From the graph, it is clear that winter months bring less total energy, but not zero. The diffuse component remains relevant throughout the year. And that is the basis for why solar systems work even under Central European weather conditions.
How much energy does a solar system actually produce in winter in Slovakia?
It is always better to talk about specific numbers than to speak in generalities. Let's look at real values of global solar radiation for typical Slovak locations. Engineers use the unit kWh/m²/month – that is, how many kilowatt-hours of energy fall on one square meter of horizontal surface per month.
For the central part of Slovakia (e.g., Banská Bystrica, Zvolen), typical values are roughly as follows:
- January: 25–35 kWh/m²
- February: 45–60 kWh/m²
- March: 90–110 kWh/m²
- April: 120–145 kWh/m²
- May: 155–175 kWh/m²
- June: 160–180 kWh/m²
- July: 165–185 kWh/m²
- August: 145–165 kWh/m²
- September: 105–125 kWh/m²
- October: 65–85 kWh/m²
- November: 25–40 kWh/m²
- December: 18–28 kWh/m²
What follows from this? Summer months (May to August) produce five to six times more energy than December. That's a huge difference. But December still produces several times more than zero – and that is essential. A collector with an area of 4 m² can still obtain 70–110 kWh of heat in December, which, at the performance of a typical electric water heater, represents 5–8 weeks of operation. In practice, it won't replace the entire consumption, but it is a measurable and real contribution.
Moreover: it is important to realize that collectors are usually mounted at an angle of 35–45 degrees from the horizontal. This tilted surface "catches" the winter sun much better than a horizontal surface, because the sun is low on the horizon in winter. A collector tilted at 45 degrees in January receives significantly more energy per m² than a horizontal reference surface, from which the above figures are calculated. The real winter performance of collectors is thus somewhat better than the figures for a horizontal surface suggest.
Flat plate collectors vs. tubular collectors in winter – a significant difference in performance
Not all collectors behave the same in winter. This is one of the most important practical aspects that determines whether an investment in a solar system pays off even outside the summer season. I deal with this topic in detail in the article Flat vs. tubular solar collectors – which type pays off more. Here, I will mention the most important connections.
Flat (plate) collectors work on the principle that the absorber surface is enclosed in an insulated box with a glazed cover. They are cheaper, more robust, and in summer they deliver excellent results. Their Achilles' heel is precisely winter: heat losses through the flat glazed surface are high when there is a large temperature difference between the collector and the outside air. The collector operates efficiently when it is, for example, -5 °C outside and the medium needs to be heated to 55 °C – but heat losses increase quadratically with the temperature difference.
Tubular (vacuum) collectors elegantly solve this problem: each tube is essentially a thermos flask. Vacuum insulation between two glass walls almost completely eliminates heat losses by convection and conduction. The only losses are radiative, and these are very small with high-quality absorbers with a selective surface. Result: a tubular collector can operate efficiently even at an outside temperature of -20 °C, while a flat collector either does not work at all at such a temperature or works with losses that may exceed the gains.
From the graph it is clear that at low outside temperatures the difference between the types of collectors is dramatic. A flat collector practically stops providing benefit at -15 °C, while a tubular collector maintains a decent efficiency. This is key information for anyone who wants to get the maximum out of a solar system even in winter.
If your goal is to maximize annual production and you want a real contribution from November to February, a tubular collector is the clear choice. If your main concern is heating a swimming pool in summer or preparing hot water from April to October, a flat collector is a cheaper and equally efficient alternative.
Solar system on cloudy days – how it works in practice
Cloud cover is not a single state. It is a spectrum: from thin white clouds (which hardly reduce radiation at all) to thick grey blankets of heavy cloud cover (which reduce it to 10–20 % of normal levels). In practice, the system reacts proportionally to the intensity of the radiation.
A concrete example from practice: a customer in Trnava with two flat collectors of 2.5 m² (total aperture area 5 m²) and a 300-liter storage tank. On a typical sunny summer day the system heats the tank to 70–80 °C. On a typical partly cloudy summer day it reaches 45–55 °C. On a typical overcast summer day (light, white cloud cover) it reaches 30–40 °C – which is not enough for hot water by itself, but the tank is "preheated" and the auxiliary heating system (boiler, electric element) consumes significantly less energy. On a typical overcast spring or autumn day preheating may amount to 15–25 °C, which still saves dozens of percent of energy for auxiliary heating.
This is an important mental shift: a solar system does not have to be the only source of heat. It is sufficient that it is a valuable contribution to the energy balance of the house. And it is in the vast majority of cases, when it is properly dimensioned. More on dimensioning can be found in the article What solar collector capacity do I need for my house?.
Real annual balance – how much energy a solar system actually saves
Let’s take a look at a specific annual balance, because individual days tell only part of the story. What an investor is interested in is the total annual energy saving and the return on investment.
A typical solar system for a four-person household in Slovakia consists of 2–4 collectors with a total aperture area of 4–8 m² and a storage tank of 200–400 liters. Such a system covers:
- Hot water heating: 50–70 % of the annual energy consumption for hot water preparation
- Heating support (solar heating support): with a proper system solution, an additional 10–20 % of the heating energy
- Monthly coverage: July–August: 80–100 %, May–June and September: 60–80 %, April and October: 40–60 %, February, March and November: 15–30 %, January and December: 5–15 %
In numerical terms: an average four-person household consumes about 3,000–4,500 kWh per year for hot water preparation (depending on appliances, cold water temperature, and habits). A solar system with 5 m² of tubular collectors in Slovakia produces 2,000–2,800 kWh of heat annually. This is a 60–70 % energy saving for water heating, which at the current price of gas or electricity represents 200–450 € in annual savings depending on the energy carrier and its current price.
The investment in a solar system typically ranges from 1,800 to 4,500 € including installation. With a saving of 300 € per year, the payback period is 6–15 years. The lifespan of a quality system is 20–30 years. The economics therefore make sense, and that is even without state subsidies (if available, they only shorten the payback period).
Winter is not a problem – the problem is improper sizing
From my experience: when a customer says "my solar system did not work in winter," it is almost always one of the following cases:
1. The system was oversized and poorly regulated. Paradoxically, too large a collector area in summer causes system overheating, but in winter the same area cannot reach the operating temperature due to losses. Proper sizing is an art of compromise.
2. The storage tank was undersized. A small tank overheated to 90 °C in summer and the regulation shut off the pump – the system nominally "did not work," although it produced energy. In winter, the tank was too small to accumulate the weaker heat flow during the shorter day.
3. A flat collector was used instead of a tubular one. For year-round operation in the climatic conditions of Slovakia, this is a handicap that the customer feels precisely in the months when the system is needed most – in the transitional seasons.
4. The system was not properly connected to the backup heat source. A solar system by itself cannot survive winter without a backup heat source. This is not a fault of the solar system – it is a mistake of the designer who did not explain it to the customer. Details on the correct combination can be found in the article Solar system in combination with a boiler or heat pump.
5. The system was improperly set up or had a regulation fault. The differential controller, which compares the temperature of the collector with the temperature in the tank, is the brain of the whole system. If it is improperly set, the pump may turn off too early or too late. More about the setup can be found in the article How to set up and commission a solar system.
Concrete scenarios from practice
Scenario 1: Family house in Banská Bystrica, tubular collectors, combined tank
Customer with three vacuum tubular collectors (total aperture area 5.7 m²), combined tank of 400 liters connected to a gas condensing boiler. Installation seven years ago. Annual gas savings: 280–320 m³ (depending on the year), which at the average gas price represents 220–280 €. In January, solar covers about 15–20 % of the energy for water heating, in July practically 100 %. The customer is satisfied – the system paid for itself after 9 years, and it is still fully functional.
Scenario 2: Cottage in the High Tatras, flat collectors
Cottage with two flat collectors, 200-liter tank. Operating season: April to October (the cottage is not used outside the season and the system is put into winter mode). In this case, a flat collector is perfectly suitable – there is no need to operate the system in winter, so the loss of the advantage of a tubular collector in winter mode is irrelevant. Summer season: coverage 70–90 %. Result: full performance for 6 months of operation, no problems.
Scenario 3: Panel building, apartment block, 120-liter tank
One flat collector 2 m², small tank. System for a two-person household. Result in summer excellent, in the transitional period satisfactory. In winter, the contribution is minimal, but not zero – preheating of water from 10 °C to 20–25 °C saves energy for reheating even in January. The system paid for itself in 11 years – less attractive return on investment, but the customer was satisfied, as energy prices had risen significantly in the meantime.
What to do with a solar system in extreme winter conditions?
A few practical notes for situations when winter really intensifies:
Frost and freezing: The primary circuit of the solar system is filled with a non-freezing mixture – typically propylene glycol and water in a 1:1 ratio. This mixture can withstand temperatures down to -28 °C and sometimes even lower. Freezing of the collector or piping is not a risk with proper system function. However, it is necessary to regularly check the concentration of the non-freezing mixture – it can decrease over time, especially if the system has been topped up with water. Details on what and when to check can be found in the article Maintenance and service of solar collectors – what and when to check.
Snow on collectors: Flat collectors usually shed snow quickly – the surface is warm and smooth. Tubular collectors behave differently: each tube is cold on the surface (vacuum insulation prevents heat loss), so snow can stay on them longer. In practice, this is usually not a problem for more than one or two days, as snow slides off the inclined tubes even without thermal effect. Only in very exceptional situations (wet snow, horizontal mounting) may manual removal be necessary.
Overheating in summer vs. frost in winter: A paradox is that summer months are technically more demanding for the system than winter. Stagnation (when the tank is full, the system does not need more heat and the pump stops) causes the temperature in the collector to rise to 180–200 °C. This load is more demanding for materials than winter frost. A quality system can handle it without problems, but it is a reason why component quality matters in selection.
Impact of collector position and orientation on winter performance
In winter, the orientation and tilt of collectors play a much greater role than in summer, when the sun is high in the sky and the system works well even with small deviations from the ideal. A few specific rules:
Orientation: The ideal is due south (azimuth 180°). Deviations up to ±15° cause a loss of less than 5 % of annual production. Deviations of 15–30° to southeast or southwest cause a loss of 5–10 %. In winter, this can mean the difference between whether the collector reaches operating temperature before sunset. Collectors oriented to the southeast have an advantage in the morning hours (when hot water consumption is typically higher in households), while southwest orientation produces more in the afternoon.
Tilt: In summer, an ideal tilt is 20–30°, in winter 50–60°. For year-round operation, a compromise of 35–45° is recommended. If you have a roof with a slope of 30° and you mount the collectors directly on the roof covering, the system will work, but winter performance will be lower than if the collector were tilted more steeply. Installation on a flat roof with adjustable frames allows for optimizing the tilt, which is an advantage.
Shading: In winter, the sun is low and shadows from a chimney, dormers, or neighboring buildings can affect the collectors for entire hours. What causes no problem in summer (short shadow) can eliminate the entire daily production of the collector in winter. Therefore, it is essential to perform a shading analysis before installation, ideally using software that simulates the movement of the sun in December.
Economics in the long-term perspective – why winter is only one part of the equation
Customers who focus only on winter months and their low performance make a mistake by looking at the whole through a small detail. A solar system pays off or does not pay off in a month. It pays off or does not pay off in a whole year – and in that annual total, winter months (December, January, February) account for less than 10–15 % of the system's annual energy production.
If the system saves 1,800 kWh in 9 summer and transitional months and another 200 kWh in 3 winter months, the total saving is 2,000 kWh. The question is not "does it pay off in winter," but "does it pay off in a year" – and the answer is yes.
In the long-term perspective, you should also consider the rising cost of energy. A historically confirmed trend shows that the prices of gas and electricity increase faster than inflation. A solar system, whose investment is fixed (you pay once), becomes relatively more profitable every year.
If you are considering choosing the right system for your family home, I recommend reading the article How to choose a solar system for water heating in a family home, where you will find a comprehensive view of what to consider when choosing.
Most frequently asked questions (FAQ)
Does a solar collector work even at negative outdoor temperatures?
Yes, it does – as long as it is sunny or at least diffusely lit. The primary circuit is filled with an antifreeze mixture, so freezing is not a risk. A tubular collector works efficiently even at -15 °C outside, when the sun is shining, because the vacuum insulation prevents heat loss from the absorber. A flat collector works with lower efficiency or may not work at all in extreme cold, if the outside temperature drops below -10 °C and solar radiation is weak.
What percentage of annual energy consumption for water heating can solar cover in Slovakia?
With a properly dimensioned system (4–6 m² of collectors for a 4-person household) and a 200–400 liter storage tank, you can achieve coverage of 50–70 % of annual energy consumption for hot water preparation. The exact number depends on the location, orientation of the collectors, tilt, type of collector, and household consumption habits. A rough calculation can be made in a day or two, but an accurate forecast requires simulation software (for example T*SOL or Polysun).
Do I need to turn off the system or "winterize" it in winter?
A properly installed solar system designed for year-round operation does not require any winterization or turning off. The antifreeze mixture and proper regulation ensure safe operation. An exception are seasonal systems (for example for pool heating), which are dimensioned only for summer months and may have problems with stagnation or an undersized expansion tank in winter operation. These systems are shut down and drained before winter. If you are unsure which category your system belongs to, read the article Common solar system faults and how to fix them or contact the installation company.
Is a tubular or flat collector better for Slovak conditions?
For year-round operation with an emphasis on maximum annual production, a tubular (vacuum) collector is better. Its advantage is significant especially in winter and transitional months, in cloudy weather and at low outdoor temperatures. A flat collector is cheaper, more robust, and in summer months achieves comparable or even better performance (at high radiation intensity). For seasonal use (spring–summer–autumn) or for applications where the primary goal is summer pool heating, a flat collector is a valid choice. A detailed overview can be found in the article Flat vs. tubular solar collectors – which type is more profitable.
What happens if there is several weeks of overcast weather in a row?
Several weeks of overcast weather, which realistically occurs in Slovakia (typically January–February in lower altitudes), means that the solar system covers only a small part or almost nothing of the heat needed for water heating. This is a normal situation and a properly designed system takes it into account. A backup heat source (boiler, electric heating element, heat pump) takes over the entire load during such a period. A storage tank with an electric backup element or a boiler heat exchanger ensures that the household will never be without hot water – regardless of the weather. A solar system is a supplement, not a replacement for the backup source.
Does altitude affect the performance of a solar system?
Yes, and surprisingly in a positive way: at higher altitudes, the atmosphere is thinner, so more direct sunlight reaches the ground. In addition, longer sunshine hours (less fog and inversion layers) occur at higher altitudes. The downside is lower temperatures, which increase the requirements for thermal insulation of the collector. Practical result: in high-altitude areas (for example, mountains above 600 m), tubular collectors can work more efficiently in winter than in foggy lowlands (for example, Záhorská nížina in January). On the other hand, in Záhorie, there is more sunshine in summer than in the mountains. In the annual total, the differences between locations are smaller than they might seem.
Conclusion: Winter reality of solar systems in Slovakia
A solar system pays off even in cloudy weather and in winter – but you need to have realistic expectations. It is not a device that replaces a boiler in January. It is a device that saves you 50–70 % of energy for water heating throughout the year, doing almost all of it by itself in summer and contributing a smaller, but measurable amount in winter. A properly dimensioned, properly oriented, and properly connected system with a backup heat source is an economically meaningful investment in Slovakia with a real payback period of 6–15 years and a lifespan of 20–30 years.
The key is in three things: choosing the right type of collector (prefer tubular for year-round use), proper dimensioning of the storage tank and solar area (not too large, not too small), and proper integration with a backup heat source. If you haven't dealt with this yet, start with the article How to choose a solar system for water heating in a family home or check out the offer of solar systems on atria.sk – you will find solutions suitable for different types of households and operational requirements.
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