Wind · Towers and maintenance
Everything decided so far has been about the machine. The machine is the part people shop for, argue about and photograph. It is not the part that determines how much electricity arrives.
Height determines output. Structure determines land. Access determines what every service visit costs for the next twenty years. Those three decisions are all made when the tower is chosen, and none of them can be revisited cheaply afterwards.
Start here
The site assessment guide worked a single machine through four mounting heights and left the ladder standing. It is worth restating here, because from this point on every structural decision is a negotiation with it.
| Hub height | Screened annual output | Gain over previous step |
|---|---|---|
| 30 ft | 1,912 kWh | baseline |
| 60 ft | 2,545 kWh | +33% |
| 100 ft | 3,142 kWh | +23% |
| 140 ft | 3,646 kWh | +16% |
Same rotor, same generator, same wind. The only variable is how far the machine sits above the ground. Between the bottom and the top of that ladder the identical turbine becomes a substantially different investment, and the whole of that difference is bought with steel and concrete rather than with a better machine.
Two things in the right column deserve attention. The first is that the returns are real at every step. The second is that they shrink. Going from 30 to 60 feet buys a third more output; the last 40 feet buys a sixth. Height keeps paying, but it stops paying at the rate it did lower down, and at some point the extra structure costs more than the extra kilowatt-hours are worth.
That crossover is the tower decision. It is not answered by picking the tallest thing available, and it is certainly not answered by picking the cheapest.
Before economics enter, there is a minimum. Department of Energy guidance in the Small Wind Guidebook is that the bottom of the rotor blades should sit at least 30 feet above any obstacle within 300 feet of the tower. The Small Community Wind Handbook gives the same 30 foot clearance against anything within a 500 foot radius.
Both figures are official and they do not agree. The site assessment guide treats 300 feet as the minimum worth considering and 500 feet as the better target, and that is the reading carried forward here.
Work it through on a real property and the number climbs fast. A stand of 50 foot trees inside that radius puts the rotor bottom at 80 feet, which on a 12 foot rotor means a hub at 86 feet and a tower ordered at 90 or 100. The trees, not the catalogue, chose the tower.
The site assessment guide shows where those output figures come from and how to produce the same ladder for a specific property.
The structures
The Department of Energy divides small wind towers into two basic families, self-supporting and guyed, and notes that guyed towers also come in tilt-down versions. In practice a household is choosing between three things, because the tilt-down variant behaves so differently in service that treating it as a footnote to guyed towers hides the decision.
Each one trades money against land against access. No structure wins all three, and which of the three is scarce on a given property usually settles the question before any catalogue is opened.
Cheapest per foot of height. Hungriest for land.
A slender mast held upright by tensioned steel cables running to anchors in the ground. The mast itself carries very little bending load, which is why it can be light and inexpensive: the cables do the work of resisting the wind, and the mast mostly resists being squashed. Sections are lattice, pipe or tubing depending on the design.
The Department of Energy describes guyed towers as the least expensive option and easier to install than self-supporting towers, and they are the most common choice for home systems. The cost of that is space, dealt with in its own section below, and a set of components that must be inspected rather than forgotten.
Smallest footprint. Most expensive structure.
A tower that stands on its own, either a tapered steel tube or a free-standing lattice frame on a wide base. With no cables to take the load, everything the wind does to the rotor has to be resisted by the tower's own stiffness and carried down into the foundation as a bending force.
That is why these towers are heavier, thicker-walled and considerably more expensive than a guyed structure of the same height, and why their foundations are the largest of the three. What the money buys is ground: the tower occupies its own base and nothing more, which on a small or awkwardly shaped property can be the deciding factor.
Costs more. Changes what maintenance means.
A hinge at or near the base, and a winch or hydraulic ram to lower the whole tower to horizontal. The machine comes to the ground rather than a person going up to the machine. The Department of Energy notes two benefits: routine maintenance becomes straightforward, and the turbine can be lowered ahead of a hurricane or other hazardous weather rather than left to ride it out.
The limits are honest ones. Tilt-down towers cost more, they add a hinge and a lifting system that themselves need inspection, and they are generally offered for lighter machines. Two official figures exist for that ceiling: the Small Wind Guidebook says usually 5 kilowatts or smaller, while the Department's consumer guidance says usually 10 kilowatts or less. The 2.5 kilowatt machine used as the running example through this section sits comfortably under either reading.
| Consideration | Guyed | Self-supporting | Tilt-down |
|---|---|---|---|
| Structure cost per foot | Lowest | Highest | Moderate to high |
| Land required | Circle of half to three-quarters the tower height in radius | Foundation pad only | Guy circle plus a clear arc for the tower to lie down in |
| Reaching the machine | Climbing crew or lift | Climbing crew, lift or crane | Lower it and stand on the ground |
| Storm response | None available | None available | Can be lowered before forecast weather |
| Parts that need watching | Guy tension, cable and anchor condition | Base flange bolts and the foundation connection | All of the guyed items plus the hinge and lifting gear |
| Machine size suited | No practical ceiling at property scale | No practical ceiling at property scale | Lighter machines, per the two DOE figures above |
Cost comparisons here are ordinal rather than in dollars. Tower pricing varies with height, soil, region and how much of the labour is contracted, and a figure quoted on a page ages badly. Ask for tower, foundation and erection as separate line items on any quote and the comparison becomes straightforward to make locally.
Land
This is the point at which a plan most often meets the property and loses. The tower itself is slender and looks like it will fit almost anywhere. What has to fit is the anchor circle.
Department of Energy guidance states that the guy radius must be one-half to three-quarters of the tower height. That is not a recommendation about tidiness. Guy cables hold the mast up by pulling down and outward, and the angle at which they meet the ground determines how much of their tension does useful work rather than simply trying to compress the mast into the earth. Bring the anchors in closer and the geometry stops working.
The 100 foot tower carried through this section needs its anchors 50 to 75 feet from the base, in every direction. That is a circle 100 to 150 feet across with the tower at the centre.
At a 50 ft guy radius
0.18 acre
about 7,850 square feet
At a 75 ft guy radius
0.41 acre
about 17,700 square feet
A quarter of an acre, give or take, committed to holding up one machine. It does not have to be empty ground, but it does have to be ground that can live alongside several tensioned steel cables running from high on a mast down to fixed points at chest height and below.
Machinery paths. Anything that mows, ploughs, harvests or hauls now has fixed obstacles to work around. Guy anchors and the cables above them are struck more often than any other part of a wind installation, and almost always by equipment belonging to the owner.
Livestock and people. A cable under tension at head height in tall grass is difficult to see. Guy wires are conventionally marked with high-visibility guards on the lower sections for exactly this reason, and the marking is maintenance rather than a one-time task.
Anything that grows. The clearance rule already governs what may stand near the tower, and trees planted after installation are the quiet way a good site becomes a mediocre one over fifteen years. Plan the circle around what the property will look like at maturity, not what it looks like now.
Property lines. The anchor circle has to sit inside land you control. A tower placed to catch the best wind is worth nothing if a third of its guy radius lands on a neighbour's field.
A tilt-down tower keeps the anchor circle and adds a requirement the others do not have. When it comes down, the tower and the machine on top of it sweep through an arc and end up lying on the ground along a line as long as the tower is tall.
That strip has to be clear, and it has to still be clear in twenty years, on the day it is needed. A tilt-down tower whose landing path has quietly acquired a shed, a fence line or a young orchard is a tilt-down tower in name only.
Below grade
A foundation for a wind tower is not doing the job a shed foundation does. It is not mainly holding weight down. It is resisting a machine at the top of a lever arm trying to push the whole structure over, and in the case of a guyed tower it is resisting cables trying to pull anchors straight up out of the ground.
Those are different problems, and they are solved with different amounts of concrete depending on what the ground is made of.
Wind pushing on the rotor acts at the top of a long lever. A self-supporting tower passes that entire moment into its base, which is why its foundation is the largest of the three types.
Guy anchors are pulled upward and inward. Holding them requires either enough mass or enough engagement with undisturbed soil, and soil type changes that answer completely.
Guy tension squeezes the mast down onto its base pier. The load is straightforward but it is continuous, and it does not tolerate a pier that settles unevenly.
There is no honest general answer to how deep or how wide. The design depends on the tower, the machine on top of it, the wind loading the local code assigns to the site, the soil, the water table and the frost depth. Two identical towers on the same road can need different foundations if one sits on clay and the other on sand.
Manufacturers publish foundation drawings for their towers, and those drawings are normally conditional on a soil assumption. Where the actual soil does not match, the design has to be adjusted by someone qualified to do it. This is the stage at which a permit application and an engineer's involvement usually become unavoidable, and that is a feature of the process rather than an obstacle in it.
The Department of Energy is direct about the whole erection process, describing it as work involving the expertise of people trained in the technology, from foundation preparation through tower erection. It is worth taking that at face value.
Photograph the excavation, the reinforcement and the anchor installation before anything is backfilled, and keep the photographs with the system documents.
Every other part of the installation can be inspected for the rest of its life. This part cannot. If a question about the foundation arises in year twelve, either those photographs exist or the answer involves a digger.
Access
The horizontal-axis guide made the point that everything mechanically interesting sits at the top of the tower, and that this is invisible on installation day and central by year eight. The tower decision is where that becomes a number.
Work at height on a tower is trained professional work. It involves fall-arrest equipment, rescue planning, and procedures for making a rotor safe before anyone goes near it, and it is governed by workplace regulation written for technicians rather than for households. This guide does not describe how to do it, and a household should not treat it as a job to learn from a page.
The useful household question is different and entirely practical. Over twenty years, what does one visit to the top of this tower cost, and how many will there be?
Possible on lattice and on monopoles fitted with climbing provision, and the only option on many taller towers. It requires people with the training and equipment to work aloft safely, which in rural areas may mean a long call-out before anyone touches the machine. Availability, not price per hour, is usually the constraint.
Straightforward and comparatively quick where the tower is low enough and the ground around it will carry the machine. Both conditions bite. Lift reach falls away well below typical tower heights, and soft or sloping ground within the guy circle can rule it out in the seasons when it is most likely to be needed.
The answer for a major component change on a self-supporting tower, and the most expensive of the recurring options by a wide margin. A crane needs road access, set-up room and a clear lift path. It is worth establishing before purchase whether one can physically reach the tower site, because the answer does not improve later.
The tilt-down argument in a sentence. The recurring cost of reaching the machine falls to the effort of lowering it, and the work happens at waist height in daylight instead of a hundred feet up. On a property with no climbing crew within a comfortable distance, this is usually where the extra structural cost earns itself back.
Published operations and maintenance figures for distributed wind sit in the region of $35 to $39 per kilowatt of capacity per year. On the 2.5 kilowatt machine used throughout this section that is roughly $88 to $98 a year, which is the deduction already made when the disappointment guide took the gross value of the output down to a net figure.
That is an average across a fleet, and it is worth understanding what an average conceals. Maintenance on a small wind system is not a smooth annual charge. It is several uneventful years followed by a year with a component failure, and the size of that year depends almost entirely on how the machine is reached.
A tower that makes access cheap converts a potentially severe year into a manageable one. That is the return on a tilt-down mechanism, and it does not appear anywhere in a comparison of rated capacities.
The horizontal-axis guide lists the four questions to put to a manufacturer before buying, including how the machine is reached and what wears out first. Those questions are worth asking with the tower quote in hand rather than after it.
Keeping it standing
Blade edges, bearings and the furling mechanism belong to the machine and are covered in the horizontal-axis guide. What follows belongs to the tower, and it is the part most likely to be left off a maintenance conversation entirely.
Cables stretch, anchors settle, and seasonal ground movement changes the geometry. A guyed tower held up by cables at uneven tension is being loaded in a way its designer did not intend. Tension is checked against a specified figure, not by feel.
The most consequential item on this list and the hardest to see. Corrosion happens at and just below the soil line, where moisture and oxygen meet steel. An anchor can look sound above grade while losing section immediately below it.
On a self-supporting tower these carry the overturning load into the foundation, and they are subject to the same repeated cycling as everything else. They have a specified torque, and confirming it is a scheduled task rather than a reaction to a problem.
The tilt-down mechanism is a moving assembly that is used rarely and then asked to perform perfectly while holding a tower. Winch cable, pins and pivots need inspecting on a schedule precisely because they spend most of their life idle.
Maintenance intervals belong to the specific tower and machine, and a generic schedule invented on a page would be worth less than the manufacturer's. What a household can do is insist the schedule exists and is handed over. Ask for four things.
Small wind installations are rarely abandoned because a machine broke. They are abandoned because a machine broke at a moment when reaching it was inconvenient, and the inconvenience was permanent.
A turbine parked at the top of a tower that nobody can afford to climb is not generating anything, and it is still a structure with cables and bolts that need attention. That is the outcome the tower decision is really guarding against.
Where money gets saved badly
The tower is the largest single line on a small wind quote after the machine, which makes it the obvious place to economise. Some of those economies are sound. These four are not.
Department of Energy guidance is unusually blunt on this point: aluminium towers are prone to cracking and should be avoided. It appears as a flat instruction rather than a consideration to weigh, which in a document of that kind is worth noticing.
The reason is fatigue. A tower carrying a turbine is loaded and unloaded continuously, by gusts and by the rotor itself, and repeated cycling is what drives cracking. The failure gives very little warning and does not show up in the first seasons, which is precisely what makes it a poor trade.
The most common economy and the most expensive one, because it is paid for annually in output rather than once in steel. The ladder at the top of this page prices it. Dropping from 100 feet to 60 feet on the worked example gives back about a fifth of the annual production, permanently, in exchange for a one-time saving on structure. There are sites where a shorter tower is the right answer, but they are sites where a shorter tower still clears everything around it.
Second-hand lattice sections and repurposed communications towers circulate at attractive prices. The difficulty is that fatigue damage is cumulative and invisible, so a section that has already spent twenty years being flexed carries a history nobody can read. A tower designed for a static antenna load has also not necessarily been designed for a rotating machine applying a cyclic load at the top of it. Neither point makes used steel automatically unusable, but both put the burden of proof on the buyer.
When the anchor circle does not fit the property, the tempting fix is to shorten the guy radius and accept a steeper cable angle. This changes the structure rather than merely inconveniencing it. Steeper cables put more of their tension into compressing the mast and less into holding it upright, so the tower is asked to do a job it was not sized for. Where the circle genuinely does not fit, the answer is a self-supporting tower, not a compromised guyed one.
Roof mounting belongs on this list too, and it is dealt with at length in the guide to why most residential wind disappoints. The short version is that it fails on wind resource before it fails on structure, and it fails on structure as well.
The order of decisions
Most people arrive at wind having already chosen a turbine. It is the natural order, because the turbine is the thing with a name, a rating and a photograph. It is also backwards.
The property fixes the tower long before anyone reaches a catalogue. Obstacles set the minimum height. Available land decides whether guy cables are possible at all. Distance from a climbing crew decides whether a fixed tower is realistic to maintain. Only once those three are answered does the question of which machine to put on top become meaningful, and by then the field has usually narrowed on its own.
Measure the obstacles. What stands within 300 feet, and within 500 feet, and how tall will it be in fifteen years. This produces the minimum hub height, and it is a survey rather than an estimate.
Walk the anchor circle. Half to three-quarters of that height as a radius, marked out on the actual ground, inside your own boundaries, around whatever has to keep working there.
Find out who services towers locally, before buying anything. Their answer about distance and availability decides between a fixed tower and a tilt-down more reliably than any cost comparison.
Then choose the machine, from those that suit the tower the property has allowed, and check its certification rather than its marketing.
A great deal of disappointment in small wind traces back to that order being reversed. A machine is bought, a tower is then bought to fit the budget that remains, and the tower that fits the remaining budget is shorter than the site needed.
The result is a correctly functioning turbine producing a fraction of what it was expected to, on a structure that will still need inspecting for twenty years. Nothing failed. The order of the decisions did.
Next
The tower answers how much wind reaches the rotor and what it costs to keep the system alive. The remaining decisions are about the machine itself and about the site that justified the tower in the first place.
Where the height ladder comes from, the clearance rule in full, and how to produce the same figures for a specific property.
Read the guideThe machine that goes on the tower, how it protects itself in a gale, and the four questions to put to a manufacturer.
Read the guideThe architecture that puts the generator at ground level, and an honest account of what that convenience costs in output.
Read the guideSources
No tower price, foundation dimension or maintenance interval is published here, because each depends on the specific tower, machine, soil and local code, and a figure quoted in general would be wrong in most particular cases. Those numbers come from the tower manufacturer and from the engineer stamping the foundation design. Where the two official figures for tilt-down capacity and for obstacle clearance radius disagree, both are given above rather than one chosen quietly.