But before being taken in by the claims of equipment manufacturers and suppliers, it is always prudent to check the reality of your renewable energy resource to avoid dissappointment a couple of years down the line due to poor energy yields, and thus a much longer payback than first expected.
With PV this is fairly straightforward. Our 1.08kW system at East Cambusmoon (central/west of Scotland) has produced around 800kWh per annum over each of the last 6 years; the same setup would probably priduce 10-20% more on the south coast of England. In other words, expect around 800kWh per kW of solar PV capacity installed in the UK; a bit more in southern England, a bit less in northern Scotland! This assumes the panels are facing south and tilted at an angle of 20 degrees less than your position of latitude (eg. if you're at 56 degrees north, the panels work best tilted at 36 degrees from the horizontal - conveniently the pitch of many a roof!). It goes without saying they shouldn't be in shadow by any nearby obstructions!
But the wind is a bit more tricky. At any one time it's fickle, inconsistent, and above all very site specific. On the plus side a few things are predictable; it's windier near to the coast and high on the hills, it tends to blow harder during the day than at night, and more often in winter than in the summer. These patterns also loosely correlate to energy use, such that a wind turbine is more likely to generate electricity when its needed.
So how do we work out whether a site is any good? The 'near-to-coast' and 'high-as-possible' rules are a good start, followed by a quick check on the well-worn NOABL database which provides the estimated annual mean windspeed on each 1km square of the UK. The database however carries a long list of health warnings, as it takes no account for local topography and obstructions, and simply boils down to a computer model.
The best way is to measure and record the windspeed and direction where the turbine is intended to be installed. For 'big wind' we go to great lengths to do this, using superbly engineered wind-tunnel calibrated instruments mounted at several levels on a mast which needs to be at least 2/3 the intended hub height of the turbines. The recorded data - typically wind speed, direction, temperature, and sometimes humidity and rainfall - are transmitted daily via the cellphone network to the office for collection and analysis.
Whilst the high cost and complexity of this kit is prohibitive for the prospective home-generator, working out whether to invest the equivalent cost of a high-end family car in a wind turbine warrants somewhat more than poking a wet finger in the air.
Data is recorded onto an SD card, which can be removed from the logger and the data uploaded every few weeks or so to a website which provides an analysis of the results, thus requiring no specialist analytical knowledge. Wind geeks can also pour through the raw data by downloading to Excel.