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Cost Benefit Analysis for Windsave system

Mark Brinkley has bought a domestic wind turbine. If you don’t know who Mark is, he is author of cult self-build manual ‘The Housebuilder’s Bible’. I couldn’t resist running the numbers Windsave quote on their website to see how long Mark will have to wait to see a return on his investment of £1498.

I’ve kept things simple and ignored inflation.

For a saving of 1MWh per annum, assuming a cost of 8p/kWh, Mark would save £80 per annum, around 1/3 of his electricity costs. The simple payback for the wind turbine comes out at nearly 19 years. Of course if electricity costs are higher, the payback looks more attractive. If electricity cost 12p/kWh, payback would come down to 12.5 years.

The lifespan of the product is quoted by Windspan as being 10 years for normal operating life, with 15-20 years quoted as highly achievable.

There are quite a few assumptions in the calculation above, variables such as wind speed, location and electricity price will all play their part, as well as future inflation, but the numbers to me don’t look very enticing just yet - they’re just about breaking even (which is much improved on the situation a few years ago). I also haven’t looked at any tax benefits which may exist - comments (as always) are welcome.
More information on payback, cost benefit analysis and whole life costing can be found at BSRIA. They have published a useful article about Whole Life Cost Analysis. It’s not the most rivetting read but if you need to know about CapEx, OpEx, ECA’s, NPV and WLC, it’s a good starting point and overview and does a good job at explaining why ECA’s can result in a lower NPV for equipment.

**UPDATE** The man from Windsave, he say no. Mark makes the point on his post that a critical factor is the average windspeed. Average windspeed’s for the country can be found in CIBSE Guide A and also the BWEA.

A few formulae shed some more light on the feasibility of domestic windpower. Fan power is directly proportional to the air speed³. The table below shows the numbers taking the value of 100W at 6m/s and extrapolating out.

The graph illustrates the power law, and why low speeds are such bad news for power output.

The other factors which need to be taken into account are the roughness of the land and the surrounding topography. CIBSE use terrain coefficients, the effects of which I have shown below for the 12m/s example.

Terrain	k	a	speed (m/s)	Power (W)
Open, flat country	0.68	0.17	12.07	814
Country with scattered wind breaks	0.52	0.2	9.89	448
Urban	0.35	0.25	7.47	193
City	0.21	0.33	5.39	72

As you can see from the figures above (based on a building 10 metres high), what works really well in open country, is not much use in an urban setting. If you are contemplating a wind turbine, it is worth asking the questions:

• what is the average wind speed
• what is the effect of surrounding buildings and countryside
• can the building take the load

This will help you answer how much energy you are likely to save (if this is your primary reason for buying one).  Of course, if you just like the look of it and want to be ’seen to be green’, then go for it.  Most home purchases would never need to come under the detailed scrutiny I have inflicted on this poor wind turbine (although it is perfectly feasible to run calculations for dishwashers for example).

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