A modern wind turbines will be designed to work for 120 000 hours throughout their estimated life-span of 20 years. This would be the turbine operating for approximately 66% of the time for two decades. This is far more than modern car engine which is built to last for 4 000 to 6 000 hours of use. This equates to an average of 49 minutes driving a day over the same two decades.
From experience, the maintenance costs of a new turbine will be very low but as the turbine ages these costs will increase.
Studies done in Denmark on the 5000 wind turbines installed in the country since 1975 has demonstrated that each new generations of turbines has had lower repair and maintenance costs than the previous generation. (The studies compared wind turbines which were built and erected at approximately the same time, but which belong to different technological generations).
Older wind turbines have an annual maintenance cost are on average 3% of the original cost of the turbine. Because newer turbines are usually quite substantially larger you get an economy of scale, lower maintenance costs per kW of rated power. This is simply because you do not need to service a large turbine any more often than a small one. Couple this with the constant development of new materials and techniques and you will make savings on the maintenance costs. For modern machines the estimated maintenance costs are in the range of 1.5% to 2% of the original investment per annum.
Most of maintenance cost of a wind turbine will be a fixed amount each year for regular servicing, but is can be preferable to base the maintenance cost on a per kWh rate. This is purely because of tear and wear increasing on the turbine with increasing production, so there is a balance between savings compared to cost.
As well as the economies of scale which you would benefit from based on the size of the turbine, there may be savings that can be made when operating wind parks over individual turbines. These economies would be based on the six-monthly maintenance visits, surveillance and administration, of the site. This would be saving on a per kW basis.
Every machine has a set design lifetime based on how long the parts are expected to last. Some of the components within a wind turbine are subject to more wear and tear than others. Generally a moving part wears out faster than a static part and an exposed component faster than its shielded alternative. So the parts which wear out fastest are the rotor blades and gearboxes.
In many cases, when a wind turbine comes to end of its technical design life, it may be more cost effective to refit the existing turbine to increase its lifetime rather than replace it. A major overhaul would include a replacing some of the internal workings and the rotor blades. In many cases the tower itself would be in good condition and safe for a considerable while.
Although the typical price of replacement components (set of rotor blades, a gearbox and generator) is 15% - 20% of the price of a new turbine, a thorough check has to be made of the existing components to be sure that they are safe and suitable.
The components of a wind turbine are typically designed to remain operational for twenty years. It would be quite easy, and hardly any more expensive to design and build some of the components to remain operational for far longer. However, because most of the major components would be very expensive to build for a longer life span, it would be a waste to have a whole turbine standing idle because one part failed years earlier than the rest.
By agreeing on a twenty year design lifetime, an economic compromise is met which guides the engineers who develop new components for wind turbines. When planning new components they know that it will be expected to work reliably for two decades. They have to show that their planned components will have little chance of failing within twenty years of installation.
The design lifetime of a component compared to its actual lifetime means that a wind turbine can last far longer than originally planned. How long it will continue working depends on the build quality of all of the turbine components, how well put together and the local environmental conditions. Environment isn't just the wind factors, like how much turbulence is experienced at the site, but also the air density, average humidity, even seismic factors.
An offshore turbine may last longer, simply due to the fact that with no obstacles to the wind there is lower turbulence at sea. This would in turn result in lower maintenance costs, but this would be balanced by the increased cost in accessing the turbine to effect any maintenance.