So much sunlight to the square inch
Walt Whitman captured the concept nicely in this phrase, describing the cheer radiated by a person in whose company he had found himself.
Essentially, insolation is a measure of the power of the sun's light over time and surface area. The higher the number, the more intense the sunlight exposure.
Units of measure
Insolation measures sunlight intensity over time. A simple analogy is to think about sun-bathing - if it doesn't take long to get tanned, then insolation is higher. If you're somewhere relatively dark or shady, then you'd never get a tan - the insolation is low.
Another way to think of it is as "light bulb density": if you filled out an area of your lawn with 100W light bulbs, it would be brighter than the same area filled with 25W bulbs. Similarly, when an area receives more sunlight, it has a higher insolation.
Insolation is typically measured in units of kW/m2 (power per unit area) or kWh/m2(hours of power per unit area). But the units don't really matter too much for our use case - we're more interested in relative insolation: when and where do we get more sunlight or less.
Orientation matters
You don't see sun-worshippers on the beaches standing up to get tanned in summer. Instead, they lie down. That's because flat ground receives much higher insolation than a vertical surface: the tilt (or slope) of a surface relative to the sunlight makes a big difference.
Similarly, a south-facing vertical wall during a northern hemisphere summer receives much less insolation than a flat lawn; but the same wall in mid-winter receives more insolation than the lawn, because the sun remains low in the sky for most of the day.
But of course, the same does not apply to a north-facing wall: the northerly aspect receives no direct sunlight expect during the height of summer.
Isn't this all obvious?
You might say "yes", but many people underestimate the extent to which the sun moves through the seasons. Additionally, when viewing a property with a view to buying or renting, it's often hard to know exactly what direction you are facing, if the area is unfamiliar or the street layout does not align with the standard compass points.
Also, the way the sun interacts with different aspects of a property can be counterintuitive at first. Read on for some examples!
Time of Day and Time of Year
On Flat Ground
The amount of sunlight reaching a surface varies across the day. On flat ground, it typically increases from sunrise until the sun reaches its highest angle above the horizon at solar noon, when it lies due south (for folks in the northern hemisphere), and then decreases again until sunset:
The chart above shows the horizontal (i.e. flat ground), clear sky (i.e. not accounting for clouds/weather/obstructions) insolation. You can see also that on this particular day (20 September) in this location (the north of England), the insolation is less than it is at mid-summer (Jun 21, dotted line) and much more than it is in mid-winter (Dec 21). The numbers on the chart show the estimated total insolation for each hour of the day, e.g. at 1-2pm (13:00-14:00) is it ~0.47 kWh/m2.
The app will also show you how the daily average insolation varies month-by-month across the year - again, this is pretty intuitive: summers are 'hotter' and winters are 'cooler':
The chart shows (same location in the north of England) that the daily average insolation in September is 3.6 kWh/m2, around half of the Jun value and nearly 10x the December value. What does 'daily average insolation' mean? It's what you get if you calculate the daily insolation for each day of the month and then average the results.
Finally, we can select a particular hour of the day and see how the insolation varies across the year for that particular time period:
This shows the average insolation from 5-6pm (17:00-18:00) by month. Looking at the September value (0.2 kWh/m2), it is higher than the hourly insolation from 5-6pm on Sep 20 (daily chart, above, 0.13 kWh/m2), as you might expect for the monthly average vs a day in the latter half of September where the days are becoming shorter and sun lower in the sky.
On a wall or window
Things are very different when we think about a vertical surface, such as a wall or window of a house. Here, direction or 'aspect' is everything. Thinking again about the north-facing wall - the only time of day during summer that it is likely to receive any direct sunlight is at the ends of the day, when the sun is either rising in the north east or setting in the north west. An east-facing wall (or window) receives morning light, but nothing in the afternoon. A north-west facing wall receives late afternoon and evening light in summer, and so forth.
For a southwest-facing wall (or window), here's the insolation by hour-of-day for Sep 20, again in the north of England:
As you might expect, for a southwesterly aspect, the sun starts to intensify in the mid-afternoon. The eagle-eyed reader might notice that the peak insolation is higher on September 20 than on June 21. Why is that? It's back to the sun-bathing analogy above, but this time, reversed: because the sun is lower in the sky in September than in June, the light falls more directly on the SW wall.
Across the entire year, things look very different than for horizontal insolation:
The peak months are now April and August rather than June.
Also, you can look at average daily insolation for the southwest wall for only 5-6pm across the entire year:
Strange, right? April - September are all pretty much similar, while Nov - Feb get nothing. Why is that? Here in the north, the sun sets early - at 3:39pm on Dec 21. The sun is more or less set by the time it starts shining on the southwest wall, and certainly long before 5pm in the afternoon.
Tilted surfaces
You might immediately think of solar panels: indeed, these are tilted surfaces. For fixed panels, these are oriented in a particular compass direction and with a particular angle of tilt. However, Insolation (the app) is not primarily concerned with solar power installations, where many other factors must be considered (including roof area, material and installation cost, efficiency, maintenance, lifespan, cost of electricity, interconnection regulations and more).
Instead, for the home buyer, tilted surfaces come into play when considering items such as skylights placed in sloping roofs, and sloping gardens or driveways. Here's an example of a 25° pitched roof, facing north:
A skylight in this roof would receive a small amount of light in the middle of the day in September, much more in June (the sun is higher in the sky for longer) and none at all in December.
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