In what sense of sustainable? I assume you mean profitable, since you use "economical" as a synonym in the text.
A short, brutal answer is that a small unit in an urban area is unlikely to generate enough electricity to even cover maintenance costs, so it's not ever going to be profitable. The problem is that smaller turbines are more expensive ($/watt), wind in urban areas is more erratic and people tend not to live in exposed areas (a city will be built in a valley rather than on a hilltop). Urban planning rules make it difficult to build a decent tower so generally an urban turbine will have to be low which makes all those problems worse.
The ATA did a desktop study in 2007 (pdf here) The study concluded that:
- the urban environment is too complex to model with any accuracy; therefore
- the use of anemometers would be advised to check the wind resource at any
- urban turbines have performed poorly overseas and payback periods are likely
to be long
- bird strike and electromagnetic interference are unlikely to be significant
- turbines are still expensive, with installation and grid connection costs adding
around $10,000 to the cost of the turbine
Note this key point:
In order to gather case study information for this report, a number of small wind
turbine owners were contacted to take part in an online survey. The survey was
designed to learn about several aspects relating to installation of small turbines,
including costs, output, planning, noise and siting issues. Nine respondents described
their experience with 10 turbines.
Most of the respondents to this survey live in windy rural areas and have purchased
turbines that are readily available in Australia. Typically the turbines have been sited
on hills or raised areas and away from houses.
So the study did include actual experience from people who have small wind turbines on their properties.
Anecdotally, there's this slightly cynical thread on Whirlpool where a number of people observe that the turbines on an office building rarely rotate. I've done a "sustainability tour" of that building and another where in both buildings the power output of the turbines was under 1% of capacity. For comparison in a wind farm the capacity factor is typically over 30%.
The answer seems to be that you would need to do this for the exact site you're considering:
- establish how big and how high your turbine can be
- install a tower that high with anemometers at intervals up the tower
- log the wind speed, direction and variability over at least a year
- work out from that what power output you can expect from the turbine(s) you are considering, at the heights you're allowed to put them.
- negotiate with the power company to establish a connection regime
- install the wind turbine
- negotiate with the neighbours if any problems arise (in Australia "wind turbine syndrome" is a real problem for turbine owners, even if the medical issues are contested)
At step 6 you should have some idea of profitability, but you will have spent quite a lot of money to get there. Assuming it looks profitable, you can then install the turbine and test your assumptions.
Once you've got the turbine up and running you can calculate the payback period for your installation based on actual output, real running costs, and how much (if any) payment you get from the power company for what you feed in to the grid.