Little bitty wind turbines appear virtuous, creating green power so everyone can feel good about them. In reality, they generally way underperform their specs due to turbulence on the ground and bad design. For reliable wind power, ginormous turbines, especially offshore where the wind is more reliable, are by far more efficient.
Wind turbulence and inconsistency near the ground makes it difficult to site baby turbines efficiently, make worse by ludicrously optimistic specs and lack of testing by the manufacturer.
Truth is that unless you live in a very windy place, you will be better off putting your money into solar PV. Period.
Wind turbines need wind. Not just any wind, but the nicely flowing, smooth, laminar kind. That cannot be found at 30 feet height. It can usually not be found at 60 feet. Sometimes you find it at 80 feet. More often than not it takes 100 feet of tower to get there. hose towers cost as much or more, installed, as the turbine itself. How much tower you need for a wind turbine to live up to its potential depends on your particular site; on the trees and structures around it etc. Close to the ground the wind is turbulent, and makes a poor fuel for a small wind turbine.
The world of small wind turbines is much like the wild-west of a century ago: Anything goes, and no claim is too bold. Wind turbine manufacturers will even routinely make claims that are not supported by the Laws of Physics. Energy production claims are often exaggerated, as are power curves. In fact, this is the rule, not the exception. Those manufacturers that tell the truth are the exception. Many manufacturers have never tested their wind turbines under real-world conditions. Some have never tested their turbine before selling it to unsuspecting customers. We are not joking! Because we sell grid-tie inverters for small wind turbines we have a front-row seat when it comes to actual operation of turbines of many makes and models. It turns out that some do not work; they self-destruct within days, and sometimes run away and blow their inverter within seconds (clearly nobody at the factory bothered to ever test it).
Also, vertical-axis small turbines are seldom at optimal angles to the wind. They are also installed in close proximity at each other, creating turbulence, and when installed in tandem with solar PV, the south-facing panels creating even more turbulence.
Many of these small turbines are what is called a Savonius design, which looks like two halves or a barrel stuck together. They are cheap but not very efficient, since half the turbine is blocking the wind while the other half scoops it. It barely manages to get 40% efficiency compared to horizontal axis turbines and creates a huge amount of turbulence in its wake.
As for that London skyscraper with the turbines at the top, they hardly move at all.
Then there are the turbines that are put on buildings for no other reason than to advertise “I am green!” The developer of the ugliest building in London that looks like a giant shaver actually wanted to put motors on the turbines so that would turn, because they sure don’t in the wind. Fortunately the architect refused so they just sit there.