Solar Energy on Earth: Solar Constant, Spectrum and Potential
How much solar energy reaches Earth? Solar constant, atmospheric losses, global distribution and the enormous potential of solar energy explained.
Solar Energy on Earth
The sun is a gigantic fusion reactor radiating approximately 3.8 x 10^26 watts every second. Only a tiny fraction of this reaches our planet — yet even this fraction exceeds humanity’s entire energy demand by a factor of thousands. This chapter explains the physical fundamentals of solar irradiance on Earth.
The Solar Constant
At the edge of Earth’s atmosphere, an average of 1,361 W/m2 of solar energy strikes a surface perpendicular to the solar radiation. This value is called the solar constant and is continuously measured by NASA’s SORCE satellite.
Total Power on Earth
Earth intercepts with its cross-section (circular area with radius 6,371 km) a total of approximately 174 petawatts (174 x 10^15 W) of solar energy. This equates to:
- 1.5 x 10^18 kWh per year (1.5 quintillion kWh)
- In one hour, more energy than humanity uses in an entire year
- Approximately 10,000 times the global primary energy consumption
Sonnenenergie im Vergleich zum Energiebedarf
Die Sonne liefert in einer Stunde mehr Energie, als die gesamte Menschheit in einem Jahr verbraucht. Klicken Sie auf einen Wert für Details.
Gesamter Primärenergieverbrauch der Menschheit (ca. 580 EJ/Jahr, Stand 2024).
Die Sonne liefert 4.944× so viel Energie wie globaler energiebedarf
Theoretisch reicht die Sonneneinstrahlung auf der Erdoberfläche, um den Globaler Energiebedarf 4.944‑fach zu decken.
Quellen: NASA Earth Fact Sheet, IEA World Energy Outlook 2024, Bundesnetzagentur
Losses in the Atmosphere
Of the 1,361 W/m2 in space, only about 1,000 W/m2 reaches Earth’s surface under clear skies and perpendicular incidence. The atmosphere absorbs and reflects a significant portion of the radiation.
Atmosphärische Absorption der Sonnenstrahlung
Von 1.361 W/m² im Weltraum erreichen etwa 925 W/m² die Erdoberfläche bei klarem Himmel und senkrechtem Einfall (AM1.5 Standard).
The AM1.5 Spectrum
The solar spectrum at Earth’s surface differs significantly from the space spectrum. The standard AM1.5 (Air Mass 1.5) describes the reference spectrum at a sun elevation angle of about 48 degrees — typical for mid-latitudes like Germany, Austria and Switzerland.
Under Standard Test Conditions (STC) for solar modules:
- Irradiance: 1,000 W/m2 (AM1.5 Global)
- Cell temperature: 25 degrees C
- Wind speed: 1 m/s
Global Distribution of Solar Irradiance
Solar irradiance is not evenly distributed across the Earth. The main influencing factors are:
- Latitude: At the equator, radiation strikes almost perpendicularly; at the poles, at a very shallow angle
- Cloud cover: Dry desert regions receive more direct radiation
- Altitude: Thinner atmosphere at high altitudes = less absorption
- Albedo: Snow and ice reflect up to 90% of radiation
Globale Sonneneinstrahlung im Vergleich
Jährliche Globalstrahlung in kWh/m² auf einer horizontalen Fläche. Wählen Sie Regionen zum Vergleich.
Süddeutschland (Bayern)
Beste Standorte in Deutschland, Föhn-Effekte
Quelle: PVGIS (Photovoltaic Geographical Information System), Europäische Kommission; NASA Surface Meteorology and Solar Energy
The DACH Region in Global Context
Germany, Austria and Switzerland lie between the 47th and 55th parallels and receive 1,000-1,250 kWh/m2 of global irradiance annually. This is significantly less than the Sahara (~2,500 kWh/m2), but still sufficient for economically viable use of photovoltaics.
Seasonal Variations
Due to Earth’s axial tilt of 23.4 degrees, solar irradiance in the DACH region varies strongly throughout the year. In June, about 7 times as much energy hits a horizontal surface as in December.
Saisonale Schwankungen der Sonneneinstrahlung
Monatliche Werte für Süddeutschland (~48°N). Jahressumme: 1.132 kWh/m². Das Verhältnis Sommer zu Winter beträgt etwa 7:1.
Die Erdachse ist um 23,4° geneigt. Im Sommer steht die Sonne in der DACH-Region bis zu 65° über dem Horizont, im Winter nur 17–19°. Der flachere Einfallswinkel im Winter verteilt die gleiche Energie auf eine größere Fläche – und der längere Weg durch die Atmosphäre verstärkt die Absorption.
Quelle: PVGIS (EU Joint Research Centre), DWD (Deutscher Wetterdienst)
Why Does Irradiance Vary So Much?
Three factors reinforce each other:
- Sun elevation: In winter, the sun is low (17-19 degrees in Munich), in summer high (up to 65 degrees). A shallower angle spreads the same amount of energy over a larger area.
- Day length: From ~8 hours in December to ~16 hours in June — twice as much time for electricity generation.
- Atmospheric path: At low sun elevation, light traverses a thicker atmospheric layer, leading to stronger absorption and scattering.
How Much Solar Energy Hits My Roof?
The actual amount of energy on your roof depends on location, roof orientation and tilt angle. Our calculator gives you an initial estimate:
Wie viel Sonnenenergie trifft mein Dach?
Berechnen Sie, wie viel Solarenergie auf Ihre Dachfläche trifft und wie viel Strom Sie daraus gewinnen könnten.
Hinweis: Vereinfachte Berechnung basierend auf Durchschnittswerten. Für eine genaue Planung nutzen Sie unseren PV-Rechner oder PVGIS.
Summary
| Parameter | Value |
|---|---|
| Solar constant (AM0) | 1,361 W/m2 |
| Earth’s surface (AM1.5, STC) | ~1,000 W/m2 |
| Total power on Earth | ~174 PW |
| Global irradiance DACH | 1,000-1,250 kWh/m2/year |
| Global energy demand | ~18 TW |
| Solar/demand ratio | ~10,000:1 |
| Summer/winter ratio (DACH) | ~7:1 |
The solar energy reaching our planet exceeds human demand by an enormous factor. The challenge is not the available quantity, but efficient conversion, storage and distribution — precisely where photovoltaics comes in.
Frequently Asked Questions
How much energy does the sun radiate in total?
The sun’s total luminosity is approximately 3.828 x 10^26 watts. Earth intercepts only about 0.000000045% of this (1 in 2.2 billion). Nevertheless, this amount is about 10,000 times the total energy demand of humanity.
Why is the solar constant on Earth ~1,361 W/m2 and not more?
The value results from the sun’s luminosity spread across a spherical surface with the radius of the Earth-sun distance (approx. 150 million km). The further away, the less energy per area — the so-called inverse square law.
Is solar irradiance in Germany sufficient for photovoltaics?
Yes. With 1,000-1,250 kWh/m2 per year, a typical 10 kWp system in Germany generates about 9,000-11,000 kWh of electricity annually — enough for 2-3 households.
What do AM0, AM1 and AM1.5 mean?
AM stands for “Air Mass”. AM0 = space (no atmosphere), AM1 = sun at zenith (shortest path), AM1.5 = standard condition at 48 degrees sun elevation. The higher the number, the more atmosphere the light traverses.
How much area would be needed to power the whole world with solar?
At 20% module efficiency and German irradiance levels (~1,100 kWh/m2), an area of approx. 500 x 500 km (roughly the size of Spain) would suffice to cover the entire global electricity demand. In the Sahara, the area would be even smaller.
Sources: NASA Earth Fact Sheet, IEC 60904-3, PVGIS (European Commission), DLR, Fraunhofer ISE, IEA World Energy Outlook 2024
Table of Contents
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