A short while back a colleague asked me for advice how to best install solar panels on his roof. He was afraid that the shadow cast by a dormer would decrease the output of the panels. At first I couldn’t convince him that it wouldn’t be a problem, but suddenly I had an idea.

A while ago I was experimenting with Google Sketchup (), a fun free application to create 3D drawings in a simple way. It’s possible to draw a simplified house literally in seconds. The program also offers the option of projecting shadows for every day of the year and every time of day. This makes it very easy to see if solar panels (or a solar hot water system) suffers from shade from parts of the house, trees or other buildings. Although a simulation like this can’t calculate any performance loss due to shading in kWh or MJ, but it can give a very good overview of locations on a building where it would definitely not be a good idea to install solar panels.

I’ll try to give some tips in this article how to construct simplified buildings and quickly determine if solar panels in certain locations on those building suffer from shade effects. This will not be a complete Sketchup course, if only for the fact that I’m just a basic-level user myself.

1 Download, installation and configuration

You can download the application here (), after which it can be installed. Installation is easy and won’t be problem for anyone who has ever installed a computer program before. As far as I know Sketchup is only available for Windows.

There is only thing that needs to be configured and that’s the users’ location, or else the position of the sun won’t be realistic. Click Window -> Model Info, select the country and city you’re in. It’s also possible to enter longitude/latitude location, but for this application that kind of accuracy isn’t necessary, unless the closest city in the list is really far away from where you are.

2 Building a houe

It might be a good idea to watch a few of the instruction movies () which Google has put online to learn the basics. Building a simple house is very easy – the secret is to keep the building as simple as possible. It’s possible to build a very detailed house when you spend a bit more time with the software, but as long as the measurements are accurate a simplified house will suffice for this purpose. I will refer to some of the instruction movies. To help you getting started, here’s a Sketchup-file of two fictitious houses as pictured on the right. The rear house is a copy of the front house. All images in this article can be clicked to view them in full size.

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Starting to draw a house is easy; draw a square or rectangle of the desired size on the ground. The house in this example has a square base of 8×8 m. Raise this square to the desired height, in this case 6 m.

The result should be the cube as pictured on the right.

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Use the Line tool to draw the roof ridge over the centre of the cube, and then use the Move/Copy-tool to raise it the desired distance – in this case 3 m.

With only a few mouse clicks, we’ve created a shape that’s recognisable as a house.

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The next step is adding the solar panels and a few details that could cast shadows, in this example a dormer and a small flue.

The solar panels, windows and door are simple rectangles, which I have coloured using the Paint Bucket tool. By copying the solar panel to the facade it keeps its angle, but if necessary that can be changed with the Rotation tool.

I started the dormer a rectangle on the roof, and used separate lines the draw the protruding part. This takes some practice, but is not difficult. I drew the flue on the ground next to the house, and moved it to the roof after it was finished. It consists of two cylinders of differing diameter stacked on top of each other. To make the flue stand out a bit more I colored it silver grey.

Finally I copied the completed house and moved it 20 m backwards, so that we can observe how the shadow of the front house may affect the back house. I found the tree in front of the house in the Google Sketchup database by using the Get Models tool. This database is filled with hundreds of thousands of objects of people, bicycles, solar panels to houses, ships, etc. etc. etc. It’s possible to upload objects you’ve created into this database.

These houses face south, but if necessary they can be rotated to any direction with the Rotation tool.

Only one thing to do, and that is to activate the shadows. Click on Window -> Shadows -> Display Shadows.

3 Simulate

Now that the building phase is over we can see how the different panels are affected by shadow. With the Shadow Settings sliders every day of the year and every time of the day can be selected.

Around the 21st of June the sun is above the horizon the longest and farthest, so potentially the output of a solar panel is the highest on that date.

It’s clear that the panel on the right side, next to the flue, isn’t placed well; until about 12:00 the flue casts a shadow on the panel and that’s very bad for the output.

The panel next to the dormer is also not placed very well; it receives shade from 14:00 onwards and that’ll greatly reduce output.

Only the panels near the top of the roof and the one above the door have no shadow throughout the day.

No one will install a panel right next to a dormer or a flue of there’s room elsewhere on the roof, but sometimes one has no choice. Shadow effects become an even bigger factor in larger solar installations in which many solar panels are put in series. Partial shading of one panel will immediately reduce the output of the entire string. A simulation in Sketchup can demonstrate the need to divide the installation in smaller strings or even to move the flue to another location.

As a final fun exercise I’ve drawn a tiny village of six houses, all with solar panels and each with its own shadow-problem. You can download the Sketchup- file here to experiment further.

You can play with it and see if you can solve the shadow-problems by for instance moving the panels around.

I hope this article has been of some help. One of the things I should do now is draw our own house in Sketchup. That will however, for now, have to remain on my ‘someday’ list because I have a lot to do and I know our shadows situation quite well. The Google Sketchup simulation did convince my colleague and he installed the panels on his roof. I’m sure they’ll produce a large amount of power.

Update: Here is a very interesting document () with more information about how to work accurately with shadows in Sketchup.