Solar irradiation is the solar energy arriving on one square metre over a given period, measured in kWh/m². Annual global horizontal irradiation ranges from roughly 950–1,200 kWh/m² in Germany and the UK to about 1,500 kWh/m² in Türkiye and 1,700–1,900 kWh/m² in southern Spain. It is the resource figure every solar yield and payback calculation starts from.
What is the difference between irradiance and irradiation?
Irradiance is power; irradiation is energy. Irradiance (W/m²) describes how strongly the sun shines at one instant — up to about 1,000 W/m² under a clear midday sky, which is also the reference value used in collector testing. Irradiation (kWh/m²) integrates that power over time: an hour, a day or a year. The two are related exactly like the speedometer and the odometer of a car. Solar professionals also compress daily irradiation into "peak sun hours": 4.2 kWh/m² per day equals 4.2 hours of the reference 1,000 W/m².
How much solar energy does your location receive?
Annual global horizontal irradiation (GHI) varies about twofold across Europe and its neighbourhood.
| Region | Annual GHI (kWh/m²) |
|---|---|
| UK, northern Germany | 950 – 1,100 |
| Germany, national mean (DWD, 1991–2020 era) | ≈ 1,100 |
| Türkiye, national mean (SEPA atlas) | ≈ 1,530 |
| Central Spain | 1,700 – 1,800 |
| Southern Spain, Andalusia | 1,800 – 1,900 |
Even the lower end of that table supports viable solar water heating: Germany, at roughly 1,100 kWh/m², maintains Europe's largest solar thermal installed base. Türkiye's plateau regions around Konya — where Solimpeks has manufactured collectors since 2001 — sit comfortably above the national average, which is one reason thermosiphon water heaters are ubiquitous there.
Global, direct and diffuse radiation: which does a collector use?
Global radiation is the sum of direct beam radiation and diffuse radiation scattered by clouds and atmosphere — and glazed collectors harvest both. That matters in northern climates: in Germany roughly half the annual solar energy arrives as diffuse radiation, so a flat-plate collector keeps producing under overcast skies at reduced power. Only concentrating collectors, which must focus beam radiation, depend on direct sun. This is why solar water heating remains productive in climates the intuition writes off — the physics runs on kWh/m², not on postcard weather.
How do tilt and orientation change the numbers?
Tilting a surface toward the sun raises its annual harvest above the horizontal figure — typically by 10–15% at optimal tilt in central Europe. The annual optimum lies around 30–35° facing south there; steeper tilts favour winter yield, shallower ones summer. Orientation is forgiving: up to 45° away from due south costs only a few percent annually. System designers therefore quote irradiation in the collector plane, not the horizontal value, and choose tilt to match the load profile — steep for heating-season support, moderate for year-round hot water.
How do you turn irradiation into expected collector yield?
Multiply the in-plane irradiation by the system's average conversion efficiency. A central-European roof receiving 1,100 kWh/m² feeding a well-designed hot water system that converts 35–45% of it delivers roughly 350–500 kWh per square metre of collector annually; the same system under 1,800 kWh/m² in southern Spain delivers proportionally more. The instantaneous physics behind that average sits in the collector efficiency curve, and the share of demand the result covers is the solar fraction. Free, authoritative data sources remove the guesswork: the EU's PVGIS gives monthly and annual irradiation for any location, the World Bank's Global Solar Atlas covers the world, and Solar Keymark datasheets state each collector's calculated annual output at defined reference climates.
Frequently asked questions
What are peak sun hours?
Daily irradiation re-expressed as hours of full reference sunshine at 1,000 W/m². A site receiving 4.5 kWh/m² per day has 4.5 peak sun hours. The figure makes quick yield estimates easy: multiply by collector area and efficiency.
Where do I find reliable irradiation data for my address?
PVGIS, the European Commission's free tool, returns monthly and annual irradiation for any coordinates including optimal tilt values. The Global Solar Atlas offers worldwide maps, and national weather services such as Germany's DWD publish long-term means.
Does solar water heating make sense below 1,200 kWh/m² per year?
Yes — Germany and Austria built Europe's largest solar thermal markets at those levels. Payback stretches compared with Mediterranean sites, but 50–65% hot water solar fractions are routine, and subsidy schemes in these countries improve the economics further.
Is irradiation on the collector higher than on the ground?
A tilted south-facing collector intercepts typically 10–15% more annual energy than a horizontal surface in central Europe, because it faces the sun's average path more squarely. Yield software always converts horizontal data to the collector plane.