From the coast of San Francisco in the United States to the suburbs of Australia, it is not difficult to spot solar panels on rooftops.
But did you know you can put a wind turbine on a roof, too?
We are not talking about the enormous turbines used in wind farms, whose giant blades are transported to the site on specialised heavy vehicles. There is a much smaller category called micro-wind turbines, designed to generate electricity close to where it is used.
It may sound futuristic, but the technology already exists. Some of these turbines are small enough to fit within the footprint of a dining table — or even smaller.
One example is the German-made SkyWind NG. The patented system has an all-metal construction, with a rotor about 1.5 metres in diameter, and weighs roughly 19 kilograms. The manufacturer offers versions with rated outputs of 600 watts and 1,000 watts.
SkyWind Energy GmbH, based near Hanover, Germany, says its systems are already being used for decentralised, on-site electricity generation in residential buildings, industrial facilities and critical infrastructure such as radio masts.
According to the company, more than 10,000 SkyWind systems are operating in more than 100 countries. This is a company-reported figure, and SkyWind does not publish a detailed breakdown of installations by country. Even so, it indicates how far the concept has travelled.
So, what can a little turbine like this actually do?
According to experts, it could run several LED lights, a Wi-Fi router, a laptop, a television and a fan at the same time, depending on their individual power consumption. However, a turbine rated at 600 or 1,000 watts does not produce that amount of electricity continuously. Its output rises and falls with wind speed.
For the 600-watt SkyWind configuration, independently tested data suggests a reference annual production of around 615 kWh at an average wind speed of 6 m/s. Averaged over a year, that works out to roughly 51 kWh per month.
Actual production can be significantly different.
A rooftop surrounded by taller buildings, trees or other obstacles may experience turbulent air and relatively little useful wind. An exposed building with a steady flow of air is a very different proposition.
In other words, you cannot simply put a turbine on any empty patch of roof and expect the same results.
And this is not really a DIY weekend project.
A rooftop turbine needs a suitable mounting system, proper electrical connections and appropriate safety measures. Structural requirements and local regulations may also need to be considered.
But this is what makes the idea fascinating.
What if a building could generate some of its own electricity from the wind passing over it?
It does not necessarily have to be a choice between wind and solar, either. The two technologies use different natural resources: solar needs sunlight, while wind needs moving air. In the right location, they could potentially complement each other.
Of course, a rooftop wind turbine will not make sense for every building. In many places, solar will be the simpler and more practical option.
But that is not really the point.
The interesting thing is that a machine weighing less than 20 kilograms can sit on a roof and turn moving air into electricity.