Solar Radiation Analysis

The sunlight landing on any surface is not one thing. It arrives as three components, and good analysis keeps them separate.

Direct (beam) radiation is the sharp, shadow-casting sun that travels straight from the solar disc. On a clear day it is the biggest component - the beam normal to the sun can approach 900-1000 W/m2. Its strength on your surface depends entirely on angle: a face square-on to the sun collects the full beam, while a face at a grazing angle collects almost none (the cosine effect). This is why orientation dominates everything that follows.

Diffuse radiation is the light scattered by the whole sky dome - the reason a shaded courtyard is still bright and an overcast day still lights a room. On a clear day it might be 10-20 percent of the total; under full overcast it is effectively 100 percent, because the beam has been scattered away. Diffuse light comes from all directions, so a surface's view of the sky matters more than its precise orientation.

Reflected radiation bounces up off the ground and nearby surfaces. Its size depends on the albedo - fresh concrete or sand can throw back a quarter of what hits it, water or dark asphalt very little. It mostly matters for lower storeys, light-wells and the undersides of shades.

Added together on a horizontal surface these give GHI - global horizontal irradiance, the headline climate number. The beam-only value perpendicular to the sun is DNI (direct normal), and the sky-only part is DHI (diffuse horizontal). A weather file stores all three, hour by hour, and every serious solar tool reconstructs your surface's share from them.

A single W/m2 reading is a snapshot. Design decisions need the accumulated dose, so analysis integrates the flux over time.

The workhorse quantity is incident radiation on the plane of the surface - often called plane-of-array (POA) irradiance for panels. For any tilted, oriented face the tool takes each hour's DNI, DHI and ground reflection from the weather file, works out the sun's position for that hour and location, applies the cosine angle between the sun and the surface normal, adds the sky and ground contributions the surface can actually 'see', and lands on a W/m2 for that hour. Do that for all 8,760 hours of the year and sum, and you get cumulative annual radiation in kWh/m2/yr - the number that actually drives design.

The magnitudes are worth memorising. A well-oriented surface in sunny India collects very roughly 1,400-1,900 kWh/m2/yr; a poorly oriented or self-shaded one far less. To sanity-check: a location with an average GHI of about 5 kWh/m2/day works out to 5 x 365, near 1,825 kWh/m2/yr on the horizontal - which is why India's solar resource is so strong.

Everything depends on the EPW weather file you feed in. It is a typical-meteorological-year record - representative, not a forecast - so results are typical-year estimates, and a coastal, hazy site behaves differently from a dry, clear one at the same latitude. Garbage weather in, garbage radiation out.

The output that makes solar analysis designable is the cumulative radiation map - a false-colour mesh painted over your geometry, hot where the annual dose is high and cool where it is low. In the Ladybug Tools workflow you feed a geometry mesh and an EPW-derived sky matrix into the incident-radiation component; it returns a kWh/m2 value per mesh face and a legend, and suddenly the building tells you where the sun goes.

Read a typical northern-hemisphere map and a clear hierarchy appears. The roof almost always tops the scale - it faces the sky all day - which is exactly why it is the prime PV real estate. The south face collects a large, steady annual dose but, crucially, most of it in winter when the sun is low and in the south; in summer the high sun grazes it. The east and west faces are the troublemakers: they take a lower annual total but concentrate it as low-angle morning and afternoon beam that stabs straight through glass, driving peak overheating. The north face (in the northern hemisphere) collects the least - mostly diffuse - which makes it the safest place for big glazing and the worst place for panels.

The two things the map guards against are (1) trusting the annual total alone - a modest annual figure on a west wall can still cause vicious afternoon peaks - and (2) forgetting self-shading: the mesh honestly shows a wing shading its own courtyard, a parapet shading a roof edge, or a neighbour's tower stealing your winter sun.

A radiation map is only useful if it changes the design. The reading translates almost directly into envelope decisions.

Where the map runs hot - south glass in a cooling climate, and especially the east/west peaks - you either reduce the glazing area, drop the SHGC of the glass, or add shading sized to the sun angles of that face. Because south sun is high in summer and low in winter, a horizontal overhang works beautifully there; because east/west sun is low all the time, only vertical fins or deep reveals help, and they cost daylight. Where the map runs cool - the north face - you can afford generous glazing for daylight and views with far less heat penalty, and you should not waste money over-shading it.

A quick worked read: suppose the south facade shows 1,650 kWh/m2/yr and the west 1,150. The instinct is to protect the south, but the west's lower total is delivered as concentrated late-afternoon beam coinciding with the day's peak temperature and peak cooling demand - so on a comfort-and-peak-load basis the west often deserves the priciest shading, not the south. That inversion is exactly the kind of thing the map reveals and intuition misses.

In hot-dry and composite Indian climates the general rule the map keeps confirming is: keep direct beam off the glass, let daylight and diffuse in. In a heating-dominated climate the logic flips - you court the winter south sun deliberately. The map is neutral; the climate tells you whether a hot face is a threat or a gift.

It is easy to treat the sun purely as something to keep out, but the same map that flags overheating also flags opportunity, and the rest of this module leans on both readings.

The high-radiation surfaces - the roof first, then well-oriented walls - are your energy harvest: the identical kWh/m2 map that warns you to shade a wall tells a PV designer where a panel will pay back fastest, which is the subject of the next lesson but one. In heating-dominated or high-altitude climates, a south face's winter dose is a passive-solar resource: you size glazing and thermal mass to soak it up in January and shade it in June, the whole winter-sun/summer-shade logic that Lesson 6.2 makes concrete.

So the discipline of solar radiation analysis is really the foundation for the whole 'sun as load and resource' module. Run it once, early, and it seeds every downstream decision: which faces to shade and how deeply, which glass to specify, where the panels go, and where the free winter heat is worth chasing. It is cheap, fast and visual - a first-week study, exactly the kind the course keeps arguing is worth the most. Do it before you commit a single window, and the rest of the module becomes a set of informed responses rather than guesses.