Table of Contents
Understanding How Solar Potential Varies
Solar energy is available everywhere on Earth, but not in the same amount or at the same time. Solar potential describes how much useful solar energy a location can provide, typically over a year. It depends mainly on the quantity of sunlight, the local climate, and how often the sky is clear enough for solar technologies to work effectively.
It is important to distinguish between theoretical potential, which is how much solar energy reaches a given area of land, and practical or technical potential, which is how much can realistically be converted into electricity or heat after considering land, technology, and practical constraints. In this chapter, the focus is on how geography and climate shape the resource itself, rather than on specific technologies or system design.
Solar potential is primarily governed by:
- Latitude and angle of the sun.
- Local climate and cloudiness.
- Atmospheric conditions such as humidity and aerosols.
- Seasons and daily patterns of sunlight.
Latitude And The Path Of The Sun
The position of a place on Earth, described by its latitude, has a strong effect on solar potential. Near the equator, the sun is high in the sky for much of the year, and day length changes only slightly between seasons. This produces relatively stable and high annual solar resources. In the tropics and subtropics, many locations receive between about $1{,}600$ and $2{,}500$ kilowatt-hours per square meter per year, often written as $1{,}600$ to $2{,}500 \ \text{kWh/m}^2\text{/year}$.
At higher latitudes, closer to the poles, the sun’s path is lower in the sky on average, and the total annual energy is smaller. Seasonal differences also become more pronounced. Long summer days can give very high solar output in some months, while winter days can be very short with the sun low on the horizon, which reduces both the intensity and duration of sunlight. As a result, annual solar potential in high latitudes can be significantly lower, even though summer months may still offer very good conditions.
This effect appears even between countries at similar levels of economic development. For example, southern European regions, closer to the equator, receive more solar energy per square meter than northern European regions. The same pattern holds in North America, with the southern United States generally having higher solar potential than Canada on an annual basis.
Climate, Clouds, And Solar Resource Quality
While latitude is important, it is not the only influence. The local climate also shapes how much sunlight actually reaches the ground. Two places at similar latitudes can have very different solar potentials if one is often cloudy and the other is usually clear.
Regions with dry, clear air often have some of the best solar resources. Deserts in subtropical belts, such as parts of North Africa, the Middle East, Australia, and the southwestern United States, combine relatively high sun angles with very low cloud cover. These areas can exceed $2{,}000$ or even $2{,}500 \ \text{kWh/m}^2\text{/year}$ of global horizontal irradiation, which is the technical term for all sunlight reaching a horizontal surface.
In contrast, regions with frequent cloud cover, fog, or heavy rainfall can have significantly lower solar potential, even at similar latitudes. Coastal zones exposed to persistent marine clouds, or mountainous regions where moist air rises and cools, may experience regular overcast conditions, which reduce the amount of direct sunlight. Here, the sunshine is more diffuse, meaning scattered by clouds and particles, and the total annual energy resource can be lower and more variable.
Humidity also plays a role. High humidity increases scattering and absorption of solar radiation in the atmosphere. Urban air pollution, smoke, and dust can further reduce the clarity of the sky. This combination affects not only the total energy but also the fraction that arrives as direct beam sunlight compared to diffuse light, which in turn is important for some specific solar technologies covered elsewhere.
Seasonal And Daily Variations Across Regions
Solar potential is not only about how much energy arrives over a whole year. The pattern throughout the days and seasons is equally important. In equatorial regions, day length is nearly constant, close to 12 hours of daylight year round. Monthly solar energy values tend to be relatively uniform, except where strong wet and dry seasons exist. In many equatorial locations, a rainy season can bring more clouds and reduce solar output for part of the year, while the dry season can offer very high, consistent sunshine.
In mid-latitude regions, such as much of Europe, North America, and parts of East Asia, there is a clear difference between summer and winter. Summer days are long, and the sun reaches a higher angle, which increases solar intensity on the ground. Winter days are shorter, and the sun is lower in the sky, leading to lower energy per day. This seasonal variation means that solar potential in these regions is high in some months and much lower in others, even if the annual average is attractive.
Closer to the poles, seasonal contrast is extreme. In some high latitude areas, the sun may not rise at all for parts of winter, while summer can bring almost continuous daylight. Over a year, the total solar resource can still be useful, but the timing is highly uneven. This has important implications for how solar is used alongside other energy sources and storage, which is addressed in other chapters.
Daily cycles are similar everywhere in the sense that solar energy is only available during daylight, rising from zero at sunrise, peaking around solar noon, then falling to zero at sunset. However, the maximum height the sun reaches, and how quickly it climbs and descends, differs by latitude and season, influencing the exact shape and size of the daily solar energy curve in each region.
Continental Patterns Of Solar Potential
Different continents show recognisable patterns of solar resource. Across Africa, many interior and northern regions have some of the highest solar potentials in the world, because of clear skies, low cloud cover, and relatively low latitudes. Some coastal or tropical forest areas with higher humidity and cloudiness have lower solar resources, but much of the continent remains very favorable for solar use.
In Asia, solar potential varies widely. Arid and semi-arid regions in West Asia, parts of Central Asia, and interior areas of South Asia can have very high solar irradiation. In contrast, tropical monsoon regions and humid subtropical zones experience strong seasonal cloud and rainfall patterns that lower solar output during rainy seasons. High mountain areas add further complexity due to altitude and local cloud formation.
Europe generally has moderate solar potential, with higher values in the Mediterranean region and lower values towards the north and northwest, where cloud cover is more frequent and sun angles are smaller. Despite this, many European countries still have enough solar resource to support large solar industries, due in part to long summer days, technological advances, and supportive policies, topics which appear in later parts of the course.
North America combines high solar resources in the southwest with more moderate values in the northeast and in parts of Canada. Interior dry regions often perform well, while coastal and high latitude areas experience more cloud cover and stronger seasonal variation. Central and South America also show strong solar potential in many places, especially in dry highland regions and some subtropical areas, although tropical rainforests and cloud forests reduce solar access locally.
In Australia and parts of Oceania, many inland and desert regions have excellent solar resources. Cloudier coastal belts and tropical islands affected by frequent storms and clouds have lower solar energy per year, though still often sufficient for practical uses.
The Role Of Altitude And Terrain
Altitude and terrain can modify solar potential within the same latitude and climate zone. At higher elevations, the atmosphere is thinner, so there is less air mass for sunlight to pass through. This can increase solar intensity at the surface compared to nearby lowlands. Mountain plateaus at sufficient altitude often receive high levels of solar radiation, as long as clouds are not persistent.
However, complex terrain also creates local shading and microclimates. Valleys may be shaded by surrounding peaks for part of the day, reducing effective solar hours. Slopes facing the equator receive more direct sunlight than those facing away, which influences the best orientations for solar collectors, a topic discussed in the chapter on orientation, tilt, and shading. Local wind patterns and cloud formation around mountains also affect the actual hours of bright sunshine.
Snow cover can further influence solar potential. Fresh snow reflects sunlight strongly, which can sometimes increase the light reaching tilted panels through reflection. At the same time, snow accumulation on solar surfaces can temporarily block light until it melts or is removed. These combined effects mean that mountainous areas can have both advantages and practical challenges for harnessing solar energy.
Measuring And Comparing Regional Solar Resources
To use solar potential in planning, it must be described in standard terms. One common measure is annual global horizontal irradiation, summarised as the total solar energy that reaches one square meter of perfectly horizontal surface in one year. This is expressed in $\text{kWh/m}^2\text{/year}$. Another related concept is average daily irradiation, often quoted in $\text{kWh/m}^2\text{/day}$, which gives a sense of the average energy available on a typical day over a year.
A typical measure of regional solar potential is:
Annual Global Horizontal Irradiation (GHI), usually given in $\text{kWh/m}^2\text{/year}$.
When comparing regions, it is common to classify solar potential into broad bands such as low, medium, high, and very high. For instance, some temperate regions with frequent cloud cover might see around $1{,}000$ to $1{,}200 \ \text{kWh/m}^2\text{/year}$, while sunny subtropical deserts can exceed $2{,}200 \ \text{kWh/m}^2\text{/year}$. Even the lower end of this range can be sufficient for extensive solar deployment, especially when technologies are efficient and costs are low. The higher end simply indicates that more energy is available from a given surface area.
Because many factors affect the local resource, solar potential is often mapped using satellite data, ground measurements, and long term climate records. These maps reveal detailed regional differences that are not obvious from latitude alone. However, the general picture is that most inhabited regions of the world receive enough solar energy to support significant solar use, even if some areas are especially favorable.
Regional Solar Potential And Energy Planning
The way solar potential varies across regions influences how solar energy fits into broader energy systems. Areas with high and steady solar resources can expect more predictable production and smaller seasonal swings, which simplifies planning for grid integration and storage. Regions with strong seasonality or frequent cloudy periods need to consider how solar will combine with other resources, such as wind, hydropower, or storage, to provide reliable energy.
Nevertheless, regional differences in solar potential are only one part of the overall picture. Land availability, environmental considerations, infrastructure, social acceptance, and policy frameworks also shape how much solar energy is actually used. A region with only moderate solar potential but strong policy support and good infrastructure can deploy more solar capacity than a sunnier region with weak institutions or limited grid access.
Understanding that solar potential is a geographically variable but widely available resource helps to explain why solar can contribute to energy systems in almost every part of the world. The precise role it plays in each region depends on how the natural resource patterns described in this chapter interact with technology, economics, and policy, which are explored in later sections of the course.