Solar Heat Gains
The solar energy quantity entering a building (kWh/m2a). A credit to heating in winter, but a cooling load if it arrives when heating is not needed.
What is a solar heat gain?
A solar heat gain is the quantity of solar energy entering a building through windows and absorbed by surfaces, measured in kilowatt-hours per square metre per year (kWh/m2a). It differs from solar heat gain coefficient (g-value), which is a window property (0 to 1); the gain is an actual energy quantity. In winter, gains are a heating credit; in summer, a cooling liability. This reversal occurs because a gain is useful only when the building needs heat.
What factors determine the size of a solar gain?
A gain is the product of irradiance times glazed area times g-value times frame fraction times fixed and movable shading factors times a utilisation factor. Each step reduces the energy reaching useful work.
| Factor | Range or Description | Meaning |
|---|---|---|
| Incident irradiance (tilt and orientation) | Slovakia: south 150–850 W/m2 seasonal; west 50–750 W/m2 | Solar power available, varying by season and direction |
| Glazed area (m2) | Actual window exposed to sun | Shading and obstructions reduce effective area |
| Solar heat gain coefficient | g-value, 0 to 1 | Fraction of incident energy transmitting through the window |
| Frame fraction | 0.70 to 0.85 | Glazed area minus frame; frame does not transmit |
| Fixed shading (overhang, terrain, neighbours) | 0 to 1.0 | Fraction of window still receiving sun after obstruction |
| Movable shading (blinds, louvers) | 0 to 0.8 | Additional reduction when deployed |
| Utilisation factor | 0.7–0.9 winter; 0.0–0.1 summer | Fraction of gain that offsets heating demand (rest is waste or cooling load) |
Slovak energy codes (STN 73 0540, STN 73 0605) multiply all these factors. A window with large overhang and high frame fraction might transmit less than 10 percent of nominal irradiance.
What is the utilisation factor and why does it matter?
The utilisation factor reveals a critical distinction: arriving energy is not useful energy. A gain is useful only if the building has a heating deficit to fill. In December at 5°C outdoors and 21°C indoors, a 5 kWh solar gain is almost entirely useful (utilisation 0.8–0.9). In July at 28°C and 21°C indoors, the same 5 kWh is waste; it must be expelled (utilisation near zero, gain becomes a cooling load). The same kWh carries opposite signs in the annual balance. Software conforming to EN ISO 13790 calculates utilisation from the ratio of gains to losses and the building's thermal mass and time constant.
How do direct gains differ from indirect gains?
Direct gains are radiation transmitted through glazing that immediately heats air and surfaces indoors. Indirect gains occur when radiation heats an opaque exterior surface (masonry wall, concrete floor), which then emits heat inward with a time lag. Indirect gains are slower; thermal mass in a wall can absorb midday sun and release it in evening and night, shifting the peak to when heating is needed. Both contribute to annual gain, but they interact differently with ventilation and occupancy.
How do solar heat gains differ from internal heat gains?
Solar gains come from the sun; internal gains come from people, appliances, and lighting (roughly 80–100 W per person). Both affect heating and cooling loads but follow different patterns. Internal gains are steady during occupancy; solar gains peak at midday and vanish at night. They are calculated separately because a passive design captures solar gains to offset heating but cannot capture internal gains the same way. Confusing the two is a common energy-budgeting error.
Why do solar heat gains matter more in well-insulated buildings?
Heating demand drops dramatically with insulation: a standard house needs 150 kWh/m2a, a passive solar design needs 30 kWh/m2a. If both receive 40 kWh/m2a of useful solar gain, it is trivial in the first case but overproduction in the second. A well-insulated building becomes exquisitely sensitive to solar orientation and shading. Unshaded west glazing, a minor nuisance in a poorly insulated building, becomes a major overheating risk in a well-insulated one.
How do solar heat gains appear in Slovak building energy calculations?
STN 73 0540 and STN 73 0605 require solar gain calculation in the annual energy balance, shown on the energy performance certificate as specific heat demand in kWh/m2a. A home with south-facing glazing, thermal mass, and strategic orientation shows lower demand than random window placement. The gain credit requires documented shading, mass, and orientation; north windows receive no credit but still lose heat.
How do seasonal variations affect the value of gains?
| Scenario | Winter Gain | Summer Gain | Design Implication |
|---|---|---|---|
| South facade (low sun 15–20° in winter, high 60–65° in summer) | 150–400 W/m2, valuable for heating | 700–850 W/m2, blocked by modest overhang | High g-value (0.65–0.75) justified; overhang dimensions critical |
| West facade (weak winter afternoon, intense summer low sun) | 50–150 W/m2, negligible benefit | 600–750 W/m2, extreme overheating risk | Low g-value (0.30–0.45) or external shading mandatory |
| Utilisation of same energy quantity | Utilisation 0.7–0.9; gain offsets heating | Utilisation 0.0–0.1; gain is waste heat | Calendar and thermodynamic clocks diverge; same kWh has opposite sign |
Thermal mass moderates diurnal peaks, absorbing midday gains and releasing them in evening and night when heating is needed. A massive building (concrete, masonry) can discharge this heat over 8–12 hours; a lightweight building (steel frame) equilibrates rapidly and benefits little from mass.
How does passive solar design capture useful gains while avoiding overheating?
Passive solar design combines south orientation, high-g south glazing with external shading (fixed overhangs, louvers), thermal mass, and night ventilation. The strategy captures winter gains while rejecting summer heat. A window capturing 30 kWh/m2a in winter but allowing 20 kWh/m2a of summer overheating is suboptimal; passive design balances both sides seasonally.
What happens when gains exceed the building's cooling capacity?
Overheating risk occurs when gains arrive faster than the building expels heat through ventilation. Slovak regulations (STN 73 0540) quantify this as hours exceeding 26°C in summer. West-facing unshaded glazing is the primary driver of overheating in Slovak homes. Solutions include lowering g-value, installing external shading, increasing thermal mass, and night ventilation. Energy calculations account for these measures, reducing overheating hours when in place.
Frequently asked questions
- Are solar heat gains the same as the g-value?
- No. The g-value is a window property (0 to 1) that tells you what fraction of incident solar energy a window transmits. Solar heat gains are the actual energy quantity (in kWh or watts) delivered into the building by that window. G-value is material; gains are the result.
- Why do solar heat gains matter more in well-insulated houses?
- In a well-insulated home, total heating demand is small, so solar gains become proportionally larger relative to what the building actually needs. A 2 kW solar gain that is negligible in a 50 kW heating load becomes significant in a 10 kW load. If that gain arrives when heating is off, it triggers overheating instead of saving fuel.
- What is the difference between direct and indirect solar gains?
- Direct gains enter through glazing as radiation passing straight into the space. Indirect gains arrive when solar radiation heats an opaque surface (a south-facing concrete wall or green roof) and then that surface emits heat inward. Indirect gains are slower and more diffuse; direct gains are immediate and concentrated.
- How do I distinguish solar heat gains from internal heat gains?
- Solar gains are energy from the sun entering via windows and absorbed by external surfaces. Internal gains come from people, appliances, lighting, and occupant activity inside the building. Both contribute to the heating load, but they are calculated separately because they follow different patterns: solar gains peak at midday; occupant gains peak in evening and night.
- What is the utilisation factor and why does it change solar gains sign?
- The utilisation factor is the fraction of a solar gain that is actually used to meet heating demand. If 1 kWh of solar energy arrives during heating season when the building needs heat, utilisation is high (perhaps 0.8). If the same 1 kWh arrives in summer when heating is off, utilisation is zero and the gain becomes a cooling load, a liability not an asset.
- How do solar heat gains appear on a Slovak energy certificate?
- The energy certificate calculated under STN 73 0540/STN 73 0605 includes solar gains in the annual energy balance. The calculation accounts for window orientation, shading, thermal mass, and utilisation factor. A well-oriented home with south-facing glazing and adequate thermal mass will show lower specific heat demand than an equivalent building without solar capture.