Solar · shading

What is your skyline costing you?

Shading is usually reduced to one annual percentage. This models the skyline direction by direction instead, because the same obstruction can cost nothing in June and most of a tall tree to the south in November.

Location

Where is the array?

Searches use OpenStreetMap. Nothing is stored.

Or pick an example location

Or enter coordinates directly

Latitude

Longitude

Searching or using your location fills these in, so you can always see exactly which point the figures are for.

Climate comes from PVGIS, the European Commission's reference dataset — the same source this model is validated against, for examples and searches alike.

Your panels

Which way does the roof face?

The direction you would face standing on the roof looking down the slope.

How steep is the roof?

Measured up from flat. Most pitched roofs are between 30 and 45 degrees.

How many panels

Type any number — there is no upper limit.

Watts per panel

The number on the label.

Your skyline

How high things rise above flat, looking in each direction. Pick a situation, then adjust.

What is around you?

Fills the eight angles below. Change any of them afterwards.

N → NE

Degrees above flat. Rarely costly — the sun is only here at midsummer dawn in the far north.

NE → E

Costs early summer mornings. A two-storey house across a street is roughly 15 degrees.

E → SE

Costs winter mornings, when the sun rises here and stays low all day.

SE → S

Costs late winter mornings — the sun is low and already working hard by this point.

S → SW

The expensive one. This is where the sun sits at midday, lowest in the months you generate least.

SW → W

Costs winter afternoons, and it costs them at the same rate the south-east arc costs mornings.

W → NW

Costs summer evenings. Mature trees are 25 to 30 degrees; a distant ridge can be more.

NW → N

Rarely costly, for the same reason as the first arc: the sun is barely ever there.

5.1%of the year lost to the skyline

277 kWha year, from 5.40 kWp

38%lost in Nov, the worst month

0%lost in Apr, the best

This skyline costs 5.1% of the year — and 38% of Nov, against 0% of Apr.

Those two figures describe the same obstruction, and only one of them is the number people are usually given. An annual percentage is an average over a year in which the loss was near zero for months at a time and severe for others — which is exactly the shape a single number cannot carry.

The same loss, two ways of describing it

Solid bars are what this array makes with the skyline you described; the pale backdrop is what it would make with none. The dashed line is the same annual loss spread evenly across the year — what you would get by entering one percentage into a calculator. Where the line sits above the bars it is overstating winter; where it sits below them it is inventing a summer loss that does not happen.

Monthly generation in kWh. The last column is what a single annual percentage would have predicted for that month.
Month	No skyline	With skyline	Lost	Flat derate would say
Jan	143	101	29.4%	136
Feb	221	163	26.4%	210
Mar	442	427	3.4%	420
Apr	647	647	0.0%	614
May	701	701	0.0%	666
Jun	719	719	0.0%	683
Jul	748	748	0.0%	710
Aug	648	648	0.0%	615
Sep	541	539	0.5%	514
Oct	340	295	13.3%	323
Nov	202	126	37.9%	192
Dec	122	84	30.9%	116

Which direction is costing you

What each direction costs, measured by clearing that one arc and leaving the rest in place.
Looking	Skyline height	Clearing it would return
N → NE	5°	nothing measurable
NE → E	5°	nothing measurable
E → SE	8°	nothing measurable
SE → S	25°	118 kWh a year
S → SW	28°	151 kWh a year
SW → W	20°	8 kWh a year
W → NW	8°	nothing measurable
NW → N	5°	nothing measurable

Looking S → SW, the skyline rises to 28° and costs about 151 kWh a year on its own. That is the obstruction worth measuring properly before it is worth arguing about.

Why a tree to the south is the expensive one

The sun is lowest in winter and highest in summer, and it spends the middle of every day in the direction of the equator — south from London, United Kingdom. An obstruction there blocks the sun when it is both lowest and most productive, and it does so in the months when a system generates least to begin with. An obstruction to the north, at the same height, may cost nothing at all.

That is also why clearing the worst arc is worth so much more than trimming everything a little. Shading loss is not spread evenly across the sky, so neither is the value of removing it.

How this is modelled

Each of the eight arcs carries a skyline height. For every interval of the year the model asks whether the sun sits above or below that height in the direction it happens to be, and where it is below, the beam is blocked and the diffuse and ground-reflected light is not. That distinction is the whole physics of the page: a tree blocks the sun, it does not block the sky, so a shaded array on an overcast day loses very little and on a clear winter morning loses nearly everything.

What this does not model

Eight arcs is a coarse skyline. A narrow chimney falling between two arcs is averaged across 45 degrees, which understates its bite and overstates its width. A surveyed profile would have far more bins.
Near shading on the panel itself is a different problem. This treats the array as a point. A shadow crossing part of a real array interacts with bypass diodes and string wiring, and can cost far more than the shaded fraction of the area — that is module-level electrical behaviour, and it is not modelled here at all.
The skyline does not change. Trees grow and lose their leaves; this profile is one shape held for the whole year.
Reflections are ignored. A pale wall opposite returns light this model does not count.
The heights are yours. The named situations place familiar surroundings on a plausible scale; none of them is a measurement of your garden. An elevation angle is easy to measure with a phone level, and doing so is worth more than any refinement here.