Depending on the season of the year, both in single-family homes and buildings, solar thermal energy installations usually provide between 30% and 100% of the hot water demanded, with annual averages around 40-50%, which is why they need the support of conventional hot water production systems.

Used for heating, they are only indicated for low temperature systems, such as underfloor heating, where they are used to preheat the boiler water. According to the different studies consulted, the reduction in consumption obtained is estimated between 25-45%, although in practice it is not usually economically profitable to size the installation for consumption reductions greater than 30%, unless the installation is subsidized. The problem with using it for heating is that on days when heating needs are higher, the collection and performance of the collectors are lower, while when the panels are more efficient, heating needs are lower.

To heat spaces, air can also be circulated through panels specially designed for this purpose, providing direct heating without the operational risks that water presents (although with less efficiency due to the lower heat-carrying capacity of the air).

The use of thermal solar panels is particularly suitable for the air conditioning of swimming pools, since the low working temperature required even allows types of collectors without protective glass, which greatly reduces both the costs and the environmental impact of the installation. In addition, they do not need an accumulator since it is the pool water itself that acts as such.

The use of collectors for cooling with absorption machines is being developed, since unlike in heating, the greater cooling demand coincides with the better performance of the collectors.

It is estimated that 80% of a home's energy consumption occurs in the form of low-temperature hot water (heating and domestic hot water). Of this consumption, approximately 70% is used for heating. Heating is therefore one of the great workhorses of energy saving.

Flat solar collectors are not technologically complex, so their scope for evolution is very limited. However, they currently manage to capture around 80% of the energy received from the sun. (Compare to 15-20% for common PV solar panels).

For example, in Mexico City, 15 MJ/day/m² are obtained in summer, and 8-10 MJ/day/m² in winter.

Although until the end of 2006 their use in heating was economically debatable and their viability depended on state subsidies, today and due mainly to the increase in the price of oil, they constitute an interesting investment.

However, the main obstacle that this technology has to overcome is its limited use throughout the year: the annual demand for heating, unlike hot water, is not distributed homogeneously, but is concentrated in the coldest months, which also coincide with those with less sunlight. For this reason, heating panels remain inactive most of the year, making it difficult to pay for themselves over time. The massive use of thermal solar panels will therefore depend on our ability to provide them with use in summer, for example for cooling. Other minor improvements would include what to do with the excess heat in the months in which, even if they are available for cooling, the collectors are not used (such as in spring or autumn), since if it is not dissipated properly, the excess heat can destroy the collectors, so they must be provided with prevention systems such as small external radiators, which increase the cost of the installation.

Finally, its use for heating is being threatened by the growing popularization of geothermal heating.

The EN 12975" standard is the one used for testing solar collectors.[2]​.

Flat solar collectors work by taking advantage of the greenhouse effect - the same principle that can be experienced when entering a car parked in the sun in summer.

Glass acts as a filter for certain wavelengths of sunlight: it mainly allows visible light to pass through, and is less transparent with lower energy level infrared waves.

The sun hits the glass of the collector, which is transparent to the wavelength of visible radiation, allowing most of the energy to pass through. This then heats the collecting plate which, in turn, becomes an emitter of long-wave radiation (infrared). But since glass is very opaque for those wavelengths, more energy enters than leaves, and the enclosure of the box heats up above the outside temperature.

The collector plate has ducts through which a heat transfer fluid circulates that heats up, and transports that thermal energy to where it is necessary.

The performance of collectors depends on several factors. Naturally, the insolation (hours of sunshine and geographical location, more or less sunshine) and the good placement of the collector (orientation, inclination and absence of elements that shade them); Although the box is thermally insulated, it has losses towards the outside environment, losses that increase the colder the ambient air is and the higher the temperature of the heat carrier (greater temperature difference with the outside) so performance improves the lower the working temperature. Also, the higher the temperature of the collector plate, the more energetic its radiation will be, and the more transparency the glass will have in front of it, therefore decreasing the efficiency of the collector.

Depending on the season of the year, both in single-family homes and buildings, solar thermal energy installations usually provide between 30% and 100% of the hot water demanded, with annual averages around 40-50%, which is why they need the support of conventional hot water production systems.

Used for heating, they are only indicated for low temperature systems, such as underfloor heating, where they are used to preheat the boiler water. According to the different studies consulted, the reduction in consumption obtained is estimated between 25-45%, although in practice it is not usually economically profitable to size the installation for consumption reductions greater than 30%, unless the installation is subsidized. The problem with using it for heating is that on days when heating needs are higher, the collection and performance of the collectors are lower, while when the panels are more efficient, heating needs are lower.

To heat spaces, air can also be circulated through panels specially designed for this purpose, providing direct heating without the operational risks that water presents (although with less efficiency due to the lower heat-carrying capacity of the air).

The use of thermal solar panels is particularly suitable for the air conditioning of swimming pools, since the low working temperature required even allows types of collectors without protective glass, which greatly reduces both the costs and the environmental impact of the installation. In addition, they do not need an accumulator since it is the pool water itself that acts as such.

The use of collectors for cooling with absorption machines is being developed, since unlike in heating, the greater cooling demand coincides with the better performance of the collectors.

It is estimated that 80% of a home's energy consumption occurs in the form of low-temperature hot water (heating and domestic hot water). Of this consumption, approximately 70% is used for heating. Heating is therefore one of the great workhorses of energy saving.

Flat solar collectors are not technologically complex, so their scope for evolution is very limited. However, they currently manage to capture around 80% of the energy received from the sun. (Compare to 15-20% for common PV solar panels).

For example, in Mexico City, 15 MJ/day/m² are obtained in summer, and 8-10 MJ/day/m² in winter.

Although until the end of 2006 their use in heating was economically debatable and their viability depended on state subsidies, today and due mainly to the increase in the price of oil, they constitute an interesting investment.

However, the main obstacle that this technology has to overcome is its limited use throughout the year: the annual demand for heating, unlike hot water, is not distributed homogeneously, but is concentrated in the coldest months, which also coincide with those with less sunlight. For this reason, heating panels remain inactive most of the year, making it difficult to pay for themselves over time. The massive use of thermal solar panels will therefore depend on our ability to provide them with use in summer, for example for cooling. Other minor improvements would include what to do with the excess heat in the months in which, even if they are available for cooling, the collectors are not used (such as in spring or autumn), since if it is not dissipated properly, the excess heat can destroy the collectors, so they must be provided with prevention systems such as small external radiators, which increase the cost of the installation.

Finally, its use for heating is being threatened by the growing popularization of geothermal heating.

The EN 12975" standard is the one used for testing solar collectors.[2]​.