Sun tube performance figures

How much light will you get from a sun tube?

A slightly misleading headline but then giving figures for how much light you will get from a sun tube is totally misleading. This is the UK. We get sun and rain, clouds and fog. Light levels are constantly changing and so will the light levels from the sun tube you choose. The only guide that we can give with any guarantee of accuracy is how one system will compare to another.

Ideally ever sun tube system should display an efficiency figure. If they did, you the consumer, would be able to gauge fairly accurately how one system would compare to the next – but to then attribute this to the equivalent number of light bulbs would be a nonsense. So far, only two manufacturers have made this information available and they are Solarspot and Solatube.

Let me take these two manufactures as an example and explain how the maths works. Both of these companies have been independently tested and have been given figures of:

D-38 vs 290DS

The difference between the two efficiencies is 44% but that’s not where the story ends. You then have to take into account the diameters of the tubes. Because the Solarspot is slightly wider than the Solatube, the D-38 ‘pipe’ has a greater volume which means the actual difference in light levels (over the 2.2m length as tested) would be around 70% more for the Solarspot.

People ask us what difference does it make if there is a bend in the sun tube, or if the dome is on a north or south facing roof. As long as the two units are given the same set of criteria; location, length, aspect, configuration etc then the basic efficiency differences will remain the same.

The only other independent test information available is for the flexible sun tube from Velux and SolaSkylights. The tests were carried out for Solatube, to compare them to their 290DS rigid unit. The test figures show efficiency figures for the two manufactures of 3.8% for the Velux and 1.5% for the SolaSkylight system.

The table above compares the Solarspot D-38 against the Velux and SolaSkylights
The table above compares the Solarspot D-38 against the Velux and SolaSkylights

Compare this to the Solatube 290 DS with a figure of 40% and, because they all have the same diameter (350mm) the maths is simple, you can see that it would be 10.5 times brighter than the Velux and 26.6 times brighter than the SolaSkylight unit. That flexible unit isn’t looking such good value anymore, is it?

So to summarise, if you were to consider a Solatube 290 DS or a Velux Flexible Sun Tunnel, the difference between the two units will always be roughly 10 times more light from the Solatube over the Sun Tube – whatever that may be due to the exterior light levels.