Hopper Design, LLC


Toward Healthy and Sustainable Homes

Shade Study

A shade study allows you to determine if your site will work for passive or active solar collection, that is, whether you can get enough uninterrupted sunlight per day at a given location on your property to make solar collection a useful option.

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Figure 1. The purpose of a shade study is to find the apparent arc of the sun at your location for the shortest day of the year to determine if solar collection is possible at your site. On December 21 the sun is at angle Φ (phi) to the south and angle Θ (theta) to the southeast and southwest.

The description given here applies for passive collection such as for direct gain through south facing windows for space heating. It also applies for active collection such as for thermal collectors to make hot water for domestic use and space heating as well as for solar electricity generation with photovoltaics.

Figure 1 above is appropriate for the northern hemisphere, and shows a house facing south receiving sunlight on the shortest day of the year, December 21st (the lower curve), and June 21st, the longest day (the upper curve). (Solstice is a curve half way between these, not shown for clarity.) We will pay special attention to the lower curve since we wish to receive as much sunlight as possible in the winter for passive solar gain and for active solar thermal collection for domestic hot water and space heating.

What you need to know

In order to do a shade study you need to know the apparent arc the sun traces out on the shortest day of the year at the proposed site of solar collection. Objects that appear above this arc shade your site and should be removed if possible. You can reconstruct this arc if you know the direction of true south and the angles indicated in the figure above.

One way to get true south is from solar noon, the time at which the sun is at its zenith.1 The NOAA Sunrise/Sunset Calculator can find solar noon at your location given your latitude and longitude. If you know the time at which sunrise and sunset occur, the average is also solar noon. Now find an object that is plumb, such as a straight vertical tree. The shadow this object makes at solar noon is oriented true north-south.

The easiest way to determine true south is with a GPS. Avoid using a compass unless you know the magnetic deviation in your area - magnetic compasses deviate about 17 degrees to the West in New England and can vary significantly locally.

Once you know which direction is true south you can determine the southeast and southwest directions.

The angles Θ and Φ are dependent on your home's latitude, which you can find online. This search should get you started. Look up the angles for your given latitude in the table below. Now all you need is a way to measure the angles - you can purchase an angle locator at your local lumber or hardware store for about $10 (see figure below).

Location Site latitude Apparent arc of the sun on Dec 21
Solar angle Φ
S (12 noon)
Solar angle Θ
SE/SW (9 a.m./3 p.m.)
New York City 40° 27° 14°
Boston, MA 42° 25° 12°
Montpelier, VT 44° 23° 11°
Quebec 46° 21°

Times given in the table above are in solar time. Taking these times to be standard time is acceptable for the level of accuracy needed here. For more precise solar angles (also called solar elevations) see the NOAA Solar Position Calculator.

What you do

As indicated in Figure 2 you use the angle locator to determine the apparent arc of the sun. The sun on December 21 will be at angle Φ to the south and angle Θ to the southeast and southwest. Connect the three readings in an arc to see what obstructions exist at your site. Be sure the line you determine with the angle locator hits below the sill of your windows for passive solar gain or below the foot of your proposed collection area for active solar energy production.

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Figure 2. Determine where to cut with the angle locator.

If the target collection area of the site is shaded, trees will need to be trimmed or cut to provide the required sunlight. You may want to cut a little more to accommodate the growth of vegetation. Shading of 40% to 60% can occur with just the leafless branches of a deciduous tree in winter.

If you can collect uninterrupted daylight for 6 hours on December 21, you will receive 90% of that day's total available energy. At many locations it is hard to obtain 6 hours of unshaded sunlight on those short days when the sun is low. Do not accept any less than 4 1/2 hours, or 80%, of that day's total available energy. If you receive less than this and cannot avoid the shade, it would be better to find a more appropriate location for solar energy collection.

The Solar Pathfinder is a shade analysis tool for accurately determining the available solar energy at your site.
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Figure 3. A typical shade analysis using the Solar Pathfinder is shown above. Hand drawn lines represent treetops; the central area is unshaded. Over the course of the year about 15% of the available solar energy is lost to shade. This is an acceptable amount of shade. The shading at your site should not exceed 20% over the course of the year.