Energy savings: Are small wind turbines for the garden worth it?

You can see them all over the country in the fields and in the sea: wind turbines that produce energy. But only a few households use this technology to produce their own heat and electricity. Why?

As long as the wind blows, you could use it to generate electricity and even heat for your own home. Because there are wind turbines for the roof or the garden – the small wind turbines. However, this is only worth it in some places because the wind is not just wind.

How do small wind turbines work?

Small wind turbines or small wind turbines, as they are also known, are installed on rooftops, but mostly on masts in the garden or in the open field. The wind sets them in motion and the energy they produce is converted into direct current.

In the case of micro wind turbines, this results in a direct current of 24 or 28 volts, which can be used, for example, for LEDs, battery charging or electric fences for paddocks, explains Joachim Sroka. He is the second president of the Federal Small Wind Turbine Association. Slightly larger systems are more interesting for homes. They can be used to generate alternating current with 220 volts or high current with 400 volts.

“The basic principle of horizontal axis wind turbines is the same in the small and micro class as well as in the gigawatt class in the field,” says graduate engineer Sroka. But the size of the turbines differs enormously: “Small and micro wind turbines have rotor heads with a diameter of one to four meters.” And Sroka advises mast heights between twelve and 24 meters. If the system is no more than ten meters high, you usually don’t even need a building permit.

Can this also be used to generate heat?

Electricity from small wind turbines can be used to run heating rods in the buffer or hot water storage tank. Or to operate a heat pump.

What is the advantage of small wind turbines compared to other heating technologies?

Especially in the cold time of the year, the greatest amount of energy can be generated with a wind turbine, i.e. when heating is also needed. That’s the problem with a solar thermal system: It most likely needs its power from fall through spring, but it produces about 80 percent of its annual output from March through October.

What are the disadvantages of this type of power generation?

Prices. “A wind turbine is a relatively high investment that only pays for itself over a long period of time,” says Sroka. Those interested must have 5,500 euros per kW of output. “If you want to supply an entire household, and initially only with electricity for your own needs, without heating, then you need a system with five kW only in winter.”

In addition, you may need a building permit, and in many places the authorities are skeptical about this, reports Joachim Sroka, whose company sells and installs such systems. Because there were ideas of the gigantic wind turbines, which were previously known from the fields, in the middle of the town.

“Also, there is usually not enough space available in residential buildings to install such a system,” says Sroka. “Due to the construction law, you have to take into account shadows, sound insulation and free areas. There are currently very restrictive specifications here.”

However, there are options for private homes: low systems up to ten meters high, which do not require a permit. However, the main wind direction at your location must be barrier-free for these systems to be worthwhile. “But that’s rarely the case,” says Sroka. Therefore, it recommends mast heights of 12 meters or more. “Better even 24 meters”, but at this height the mast is very expensive due to static.

What locations are suitable for small wind turbines?

Here comes into play what has already been mentioned for the smallest turbines less than ten meters high: the wind, its direction and strength.

1. Free main wind direction

The wind needs a boost: According to the energy advisory service of the Baden-Württemberg consumer advisory center, even a house or a bush that is located in the main wind direction of the system reduces the energy output. And even a forest that starts 50 meters from the house can have a correspondingly unfavorable effect on electricity production.

2. Average wind force

And then, as a matter of principle, the wind in the region must be strong enough. According to Joachim Sroka, the average wind speed in Germany is between two and eight meters per second. In the north, especially on the coasts, it is seven to 7.5 meters per second, inland only two to 4.5 meters per second. “I advise not to choose a place where the wind blows an average of less than three meters per second.”

The German Wind Energy Association and the energy advisors of the consumer advice center also recommend checking the wind zone of the property during planning. “In general, proximity to the coast or altitude is an advantage, but there must also be free space in front of and behind the turbine.” That can make a significant difference: a good location offers eight times more performance compared to a bad location with an average wind speed that is only half as high.

Lars Möller, energy expert at the Lower Saxony consumer advisory center, comes to a similar conclusion: “Small systems only make sense in the open area and then as high as possible and not on the roof and with enough constant wind, usually hardly ever.” For him, this even means that the systems are “not an option worth considering” for normal households. He would only address the subject in a very special environment, like that of the farmers.

If I don’t have a good spot in the garden, are there alternatives?

There are also small wind turbines for the roof, but energy advisers at the Baden-Württemberg consumer advice center don’t think this is a good idea. The building would ensure that wind currents are turbulent and performance is poor. And the vibrations from the rotors could be perceived as disturbing directly in the house.

But Sroka knows of a solution: “There are initial ideas to share heating with larger systems.” So several homes in the neighborhood come together and get their heating current from a larger system. Advantage: The larger the system, the lower the system costs per kW of output, which are then shared among multiple households.

Should I produce electricity or thermal heat with the wind turbine?

The Sroka expert recommends covering one’s own electricity needs as a priority for single-family homes, “because currently electricity costs are higher than heating costs.” In the second step, it would cover the heating heat with the excess production of electricity.

Either way, he recommends a hybrid solution: “If you just rely on wind power, it’s not without risk. Because if you have a calm week, you don’t get power. But in general, calm is associated with good weather.” The photovoltaic system takes over here and supplements the energy mix.

The energy advisers at the consumer advice center in Stuttgart also see it this way: A combination offers more independence, “in summer mainly through photovoltaics, in winter through a lot of wind.”

How do I recognize good small wind turbines?

“You can have a big problem if the inverter does not meet the DIN VDE-AR-4105 standard,” says Sroka. “Recently this technical regulation has appeared that requires very specific tolerances for investors. All energy generators require this certification to be able to connect the system to the electrical network”. In addition, Sroka advises clarifying whether all the documents required for the building permit are supplied with the system.

The NRW consumer advice center advises critically evaluating the nominal output of the system specified by the manufacturer. The deciding factor is performance at low wind speeds, not in the rare high winds.