Part I: Start HereChapter 1. Reading Your Site

The One-Afternoon Solar Site Survey

A single afternoon of measuring shade, orientation, and usable area before you buy materials.

Difficulty: Beginner
Cost: $0–15

published

A site survey answers three questions before you buy a sheet of glazing or a solar module. Which surfaces face the winter sun? What shades them, and for how much of the day? How much of that surface can you actually use?

One clear afternoon is enough for a first pass. You are making a sketch and a few measurements, not a construction drawing. If the sketch says the south wall is in shade until 1 p.m. in December, that fact is worth more than a careful collector plan on the wrong wall.

This guide assumes a site in the northern hemisphere, where the winter sun lives in the southern sky. In the southern hemisphere, swap south and north and keep the same method.

Difficulty

Level: Beginner
Time: One afternoon, plus a note at solar noon if you can get it
Skills: Measuring, sketching, and reading a compass or a clock

Cost estimate

$0–15

A tape measure and a notebook are enough. A hardware-store compass is optional if your phone already has one. Do not buy a collector to test the site.

Materials

Item	Qty	Notes
Notebook and pencil	1	Write the date, the time, and which way you were facing.
Tape measure, 25 ft	1	Longer is useful for a yard, not for a roof you should not be walking.
Straight stick and a sunny patch of level ground	1	Used at solar noon to find true north and south.
Compass or phone compass	1	Only after you correct for magnetic declination. The stick is the check.
Camera	1	Photograph the south and west views so you can mark trees later.

What you are trying to learn

Sunlight on a surface depends on the direction the surface faces, its tilt, and whatever stands in front of it. A wall that faces true south gets the most winter sun in most of North America. East and west walls get strong low-angle sun in summer mornings and afternoons, which is a cooling problem more than a heating resource. A roof that faces east or west can still make electricity, but it will make less than the same roof facing south, and the difference belongs in the PV sizing guide rather than in a guess.

Shade is not a yes-or-no label. A tree that shades a wall until 11 a.m. in June may shade it until 2 p.m. in December, because the winter sun is lower and farther toward the south. The survey has to name the season you care about. For heat and for winter PV, that season is December and January. For cooling, it is June through August, especially the west side.

Plan view: true south is toward the bottom of the page. A tree south of the house throws its winter noon shadow north, toward the south wall. The inset is a stick at solar noon.

Find true south without trusting the compass alone

A magnetic compass points at magnetic north, not true north. The difference is magnetic declination, and it changes with where you live and slowly with the year. Look up the declination for your town from a current geophysical source, then correct the compass. Do not copy a correction from a book that was written for a different state.

The outdoor check is simpler. On a sunny day, set a straight stick upright on level ground. Solar noon is the moment the sun is highest, which is also the moment the shadow is shortest. At that moment, in the northern hemisphere, the shadow points true north. The opposite direction is true south.

Solar noon is not always 12:00 on a clock. Time zones, daylight saving time, and your distance from the middle of the zone can move it by the better part of an hour. If you cannot stay to watch the shadow shrink, look up the solar noon time for your town on the date you are outside, mark the shadow then, and label it.

Once you have that line, stand with true south in front of you and sketch the house, the garage, the fences, and the trees. Mark which walls and roof planes face within about 30 degrees of true south. Those are the heating and winter-PV candidates. Mark the west walls separately. They are the summer shade candidates.

Estimate winter shade with a stick and a tape

A first decision needs one answer, and you can get it without a sun-path machine: does a tree, a neighbor's roof, or your own eave block the low winter sun?

At solar noon near the winter solstice, the sun's altitude above the horizon is about 90 degrees minus your latitude minus 23.5 degrees. At 40 degrees north that is roughly 26 degrees. A 20 foot tree standing due south of a wall throws a noon shadow on the order of 20 divided by the tangent of that altitude. At 26 degrees the tangent is about 0.5, so the shadow is about 40 feet long. If the tree is 25 feet from the wall, the wall is in shade at noon in late December. If the tree is 60 feet away, noon sun still reaches the wall, and you then check the morning and afternoon by eye.

Do this for the objects that actually stand to the south of the surface you might use. Write the object, its height as well as you can measure or pace it, and its distance. A photograph with a person standing at the base gives you a scale later.

Summer shade is the reverse problem. The noon sun is high, so a modest overhang or a nearby tree crown blocks it. Low sun from the west still slips under a shallow overhang. That is why the shade guide treats west windows differently from south windows.

Measure the usable area

For each candidate surface, write down:

Width and height you could actually cover, after you subtract windows you are not willing to block, vents, meters, and doors.
The tilt. A wall is vertical. A roof pitch can be read with a phone inclinometer from the ground if a gable end shows the rafter line, or from the attic. Do not climb a roof to get a number you can see from the yard.
What is behind it. A south wall of a living room is a different project from a south wall of an unheated garage.
How you would get air, water, or wire from that surface to the place that needs the energy. A perfect roof with no path for a duct is a weak air-heater site.

Also note legal and practical limits you can see from the ground: a historic-district street face, a neighbor's easement, a power line over the only sunny patch, a roof that sags or is already at the end of its shingles. They are not solar measurements, but they stop projects just as often.

What to do with the sketch

Rank the surfaces. A first project wants sun in the season you care about, a short path to the load, and a place you can work without a scaffold. The window air heater wants one sunny window. The wall collector wants a south wall of a shop. PV wants a roof or a ground patch that stays clear, which the PV sizing guide turns into kilowatts only after this sketch exists.

If every south surface is shaded in winter, spend the next project on conservation or on summer shade. A collector there will disappoint you.

Pick a clear day and print nothing
Take the notebook, tape, stick, and camera. Write the date. If you can, choose a day near winter for a heating decision, or visit twice, once in winter light and once in summer.
Mark true south
Look up solar noon for your town. Plant the stick, mark the shadow at solar noon, and label north and south. Correct a compass with current declination only as a backup.
Sketch and photograph
Draw the buildings, trees, and the neighbor's roof. Stand on the south side and on the west side and take photos. Note the clock time on each photo.
Pace the obstacles
Measure or pace the height and distance of anything that can shade a candidate wall or roof. Use the noon altitude for your latitude as a first check, then confirm by watching the real shadow if the season is right.
Measure usable rectangles
For each sunny surface, write width, height, tilt, what room is behind it, and how energy would get from there to where you use it.
Rank and stop
Choose one surface for the next guide, or decide the site is a conservation project first. Do not order glazing from a memory of a sunny July afternoon.

Difficulty

Cost estimate

Materials

What you are trying to learn

Find true south without trusting the compass alone

Estimate winter shade with a stick and a tape

Measure the usable area

What to do with the sketch

Pick a clear day and print nothing

Mark true south

Sketch and photograph

Pace the obstacles

Measure usable rectangles

Rank and stop

Further reading