Solar access is the time the sun shines on specific “solar” surfaces throughout the year. Solar surfaces include windows, gardens, solar electric & thermal collectors (ground or roof mounted), etc. If you are going to use the sun, you need solar access and you need to know when you have the access, as it will vary thoughout the day and year.
Solar access varies during the day as the sun moves across the sky, it varies through the seasons as the suns rises and sets further south and crosses the horizon lower in the Winter than in the Summer. Overall solar access depends on where you build your house, its latitude. In general, you will have more potential solar access the farther south in the northern hemisphere you build, due to the tilt of the Earth’s axis and orbit. You will also typically have lower heating requirements and higher cooling requirements.
Regionally solar access still varies a lot and is limited on a piece of property by permanent structures such as nearby hills and mountains, neighboring buildings, and trees you have no right to cut.
Note: Sometimes a neighbor will cut a tree if you ask, but you can’t require it, especially if they and the tree were there first. It is interesting that solar access has come up in courts recently, but usually it is because someone builds an addition that shades a neighbor’s pool, as it did with one Atlanta Braves pitcher while I lived in that sunny area some years ago.
Solar access also depends on the height of a surface on your property. A point on the ground may be shaded, but if collectors are located on the future roof at that same location but 20+ feet up they may have full access to the sun. By the same token, a point in the sun this year at a particular height may become shaded in future years as nearby trees grow.
More than solar access is involved in quantifying the effective solar energy that will strike a surface at a particular time of the year, including primarily clouds on a minute-to-minute basis and tilt of the surface. I will cover tilt when I discuss PV and Solar Thermal Collectors. Clouds are factored into computer model data and charts and maps of “Average Sun Hours” for different locations. The important point in selecting a site and designing your house is, “Will you have good solar access?”
The vast majority of homes built in the last 60 or 70 years did not take the sun into consideration. What is unsettling is that homes being built today are just as ambivalent with respect to the sun. You drive through the countryside, where large amounts of good farm land is being converted into residential property, locations where the builders have a choice, and you see houses mindlessly oriented toward the street. What a pity, what a waste!
This is the first step that requires you to know where you will be building. Not only must you pick the lot, but eventually the exact location on the lot, as even a few feet can significantly affect your access to the sun and how you might want to orient your house at that site, design the spaces within the house and place your solar panels.
But first, determining your solar access should be a critical component in selecting your building lot. If you are going to make good use of the sun you can’t ignore or gloss over solar access. Here are two ways that require only a compass or a watch to find Solar South, your reference point for this analysis.
Typically, just standing in front of a lot with a compass, knowing where solar south is, and having an understanding of what you are planning to build is sufficient to see if you will have sufficient access to the sun and whether or not the foundation will be high enough to drain to daylight (another important aspect of site selection). If the lot meets those tests, then there is a lot of due diligence that must be done to decide if you want to make an offer. What all else needs to be investigated is beyond the scope of this discussion, but includes items like: water (supply & purity), soil type & drainage (perk test), location of bedrock or ledges (exploratory dig), school district, taxes and tax policy, local codes, traffic patterns, local businesses, past businesses and possible soil or water pollution, drive times to your key destinations, etc. Nevertheless, if you don't have adequate solar access and drainage, then move on.
Sometimes elevations can be deceiving and sometimes you may be able to move a little dirt on the property (significant dirt is expensive to haul from anywhere off site); this is when you need a transit and a sharp pencil.
I looked at dozens of potential home building sites – most I could rule out without leaving my car, due to trees, hills or other buildings, or for lack of drainage. Still others could be ruled out by standing at the roadside with just a compass.
The remaining sites required a more careful evaluation. I used a Solar Pathfinder, which is a remarkable device invented in 1978 (Bernard McGrew Haines) and still relevant today with its computer software and the ability to digitize a solar “window.” The Solar Pathfinder is a compass, a built in declination adjustment feature, a sun path map by month and a globe that reflects the horizon and shows the solar window. In the field you can easily see if there will be any major obstructions. Look carefully in the picture to the right and you can see the reflection of a tree that will have to go and a building that is in the way in the afternoon. If it is your tree, you can calculate the BTU’s you’ll gain by cutting the tree down!
Note: If trees are nearby, a little imagination is required. What will the horizon look like in 5, 10, 20, 40, 60+ years? Again, you could adjust your digital picture and calculate the estimated growth’s impact.
Wanting a 100% Solar Home requires good solar access. The site we selected is nearly perfect in terms of access and drainage. I will have to monitor a few trees as they mature on the East corner; they may someday affect us. Not a problem – hey, I have a chain saw and will need some fire wood!
There is a lot written about “adjusting” your house’s orientation to accommodate local micro climates (clouds burning off in the morning, for example), or shading (neighbor’s trees or buildings or a nearby hill), or a spectacular view, or heaven forbid “the street.” In my case, I had none of these issues. But if you do, take solace in the fact that you will get over 97% of the sun’s energy if you are within 10% of true south. Even if you need to be off south by as much as 22 degrees, you will still get almost 92%.
Here is an example: Imagine you had a large building standing towards the south and its right edge (from your view) shaded out the sun on winter days until noon. This would cost you 50% of your potential solar energy. However, if you could shift the house to your right, such that you would see the sun at 10am, you would now lose just 20% of the potential sun energy! The reason for such a large increase is that most of the sun energy happens between 10am and 2pm (~60%) at our latitude.
However, if your goal is winter heating, be wary of rotating more than 25° or sacrificing more than 10% with a group of infringements; solar heating is a difficult challenge and there is no sense in tying one arm behind your back – look for a better site.
Dan Gibson is Reporter and Chief Coordinator of Our Energy Independence Community and Chief Energy Auditor and Solar Designer for www.HomeEnergyAdvisors.com. He previously spent 5 years performing home energy audits in NYSERDA’s Home Performance program. He is also currently “finishing” their 100% Solar Home – Yes, three years in the making and it's not finished yet. He can be reached at DanG@OEIC.us or at (518) 899-2400.