I don’t know heaps about them directly, but in terms of rooftop VAWTs I guess it could depend on the type of roof, and ultimately the amount of wind that the rooftop will be exposed to.

It could be more challenging to create residential scales VAWT than it would be for commercial buildings such as the ABC building you mentioned (don’t know it off the top of my head but I’m assuming it is a least a few stories tall). I’d say a reason for this could be that as the amount of wind the turbine is exposed to reduces, so would the size of the generator, to ensure the force of the wind on the blades can generate enough counter-torque to get the blades moving and therefore generate power. Using smaller motors would definitely be possible, but you might reach a point where the amount of materials needed for each small-scale VAWT outweighs the amount of return through energy generation of each turbine, because the motor is so small, and counter-torque so small that the motor turning would only generate negligible amounts of power. EDIT: Forgot to add the context of I think there is generally more wind at higher altitudes, whether this is an in general rule or relative to the surrounds (like being in a valley vs being at the highest altitude in the region; or if you are in a low density township vs if you are in a density populated city with more structures blocking wind) I’m not certain, but it is the context for why I said residential rooftop VAWT may end up having far lower generating potential that commercial rooftop VAWT, because I’d say there is more wind on the roof of a commercial building than a residential one.

I would guess the large horizontal axis wind turbines would use large AC induction motors to generate the electricity as the blades turn. I would guess that AC motors would have some size limitations (easier to make really big ones than really small ones, not to say really big ones would present other challenges, but it would be incredibly challenging to make them under a certain size due to all the copper windings that need to fit in the motor) so once your VAWT reaches below a certain size a DC motor would need to be used. This introduces further complications, as our grid runs on AC, any DC power generation first needs to be converted to an AC waveform for the power to be injected in the grid (or used to power a load connected to the grid). This process is already performed for solar using inverters. It would also be performed for HAWTs (probably both AC to DC conversion followed by DC to AC conversion) to ensure the output (voltage, frequency, power factor) matches the grid.

It gets more complicated though, as inverters have an allowable operating DC input voltage range (these can be quite high voltages as you can place solar modules in series to increase the voltage of the generation. For example, if you put two solar modules, each with an operating voltage of 50V (arbitrary number) in series, the total voltage of that series connection will be 100V). This allows larger inverters to be used. It may not be as easy to utilise larger inverters in such a way with VAWT unless you scale up the number of them as using inverters for each individual small-scale VAWT could mean the use of a lot more materials. EDIT 2: There are cases where small inverters (known as micro inverters) are connected to every solar module in an array, so it could be argued you could do the same with small VAWT. There are also things called optimisers, which i think essentially perform the operation of a chopper (described further below) changing the DC voltage to match all the other modules before connecting to an inverter. Both of these option involve extra costs when compared to direct connection of entire strings (described further below) of modules to an inverter.

Considering using VAWT with batteries will also have added complications. Batteries store DC energy, so an AC to DC conversion would not be necessary to charge the batteries, but you would most likely still need DC to DC conversion (from memory they are called boost/buck choppers) to increase or decrease the input voltage to match the battery terminal voltage (a lot of solar inverters that can connect to batteries most likely already have these installed internally). These boost/buck choppers also have voltage input limitations, meaning they won’t operate if the input voltage is too low or too high. Therefore, to be able to use both solar and wind on say a residential rooftop, it may mean the installation of more, or retrofit of existing electronics so the power waveforms of both the solar modules and the VAWT can be transformed to match the grid or battery power waveforms. I think it’s probably unlikely that a smaller VAWT could match the voltage of multiple solar modules connected in series (known as a string), so either a second chopper would need to be added which can transform DC waveforms from a much lower voltage to match the battery terminal DC voltage (vs comparing the voltage difference between the solar string choppers input/output voltages), the choppers in the inverters would need to accept a far greater input voltage range, or as I said above, you would need to connect multiple small-scale VAWTs together to develop the necessary power waveform.

This may be one of those things where if we started designing/building/installing small-scale VAWTs about a decade ago there may have been more incentives for inverter and battery manufacturers to enable VAWT connections through the same hardware, or could be something we could consider if Aus goes down the inverter/battery manufacturing path in the future.

It could also be possible that people in the relevant technical positions have already considered all of the trade off’s and they just don’t add up to make small-scale VAWTs viable. This could be why we rarely hear about them. These things can always be subject to change though as technologies and manufacturing processes improve and change, and materials costs reduce.