Wind Turbines versus Solar Panels

This prepping and survival blog post compares wind turbines to solar panels — the two most common off-grid power sources.

A 100-watt solar panel sells (as of this writing) for around $200 to $250, making the price about $2 to $2.50 per watt. The cost per watt goes higher with fewer watts at roughly $3/watt for 50 watts and $5/watt for 10 watts. We should expect the price per watt to go up for smaller units because there is a certain minimum cost to make any product. But right now 100W panels are the sweet spot. The price per watt drops significantly at that price-point, but it is still at an affordable level.

However, a 100W solar panel is not going to give you 1200 watt-hours of power for 12 hours of daylight. First, the 100W rating is always full sun. Second, the full-sun rating may be somewhat overstated.

Let’s look at a real world example: the Maine Solar House. This property is the solar house of a private family, but they have placed detailed information online. They have 16 solar panels of 4′ x 6′ each, for a total rated power of 4200 watts (262.5 watts for each panel). How much power are they producing per month? In April of 2013, they obtained 466 kWhrs. If we multiply 4200 watts times 12 hours of daylight times 30 days, we get 1512 kilowatt-hours per month.

So the Maine solar house is getting 30.8% of the 12-hours per day figure. This makes sense because the angle of the sun and occasional cloudiness will reduce panel output considerably. Any solar house will produce more power in summer and less in winter. But this April figure is probably a good approximation of the average. So a 100W solar panel might produce 360 watt-hours per day (30% of 1200 watts). That’s my rough estimate; actual numbers will vary greatly depending on the particular panel and the climate conditions.

Now let’s take a look at wind turbines. There is an ancient expression in commerce: “Let the buyer beware”.

I’ve been looking into wind turbines as a power source from some time now, and I’m dismayed to find that many companies overstate their wind turbine specs. The American Wind Energy Association has published standards for wind turbine specs here [PDF]. The most important number is the “rated wind speed”. That is the wind speed used to calculate the power output of the turbine in watts.

A solar panel has a rated power of 100 watts if it produces 100 watts in mid-day full sun (ideal conditions). But you do not have mid-day full sun most of the time. Similarly, a wind turbine is rated at say 1,000 watts if it produces that amount of power at its rated wind speed. But you won’t have that wind speed most of the time.

The power output of a wind turbine is equal to the power in the wind times the efficiency of the turbine in turning that wind power into electricity. Huge multi-megawatt off-shore wind turbines placed high above sea level have an efficiency of 25 to almost 40 percent efficiency. But a small wind turbine, even one with an excellent design, in your backyard is probably about 20% efficient at best. And an inefficient design might be more like 10% efficient.

The power in the wind is calculated as the wind speed cubed times the density of air times one half times the swept area of the turbine (all in metric units) to obtain the power in Watts. Notice that the speed of the wind in that formula is cubed. So any increase in rated wind speed makes the rated turbine power seem much greater. (The swept area is the area from which the turbine draws its power by having its blades rotate through a particular cross-section of the air.)

The AWEA standard for rated wind speed is 11 m/s (24.6 mph). Some companies use a higher number, so that their wind turbine will have a higher power rating. Let’s say a company uses 12 m/s instead of 11 m/s. At 20% efficiency and an air density of 1.225, the power ratings are as follows:

* At 11 m/s, there is 815 watts of power in the wind, times 20% efficiency for 163 watts per square meter of swept area.
* At 12 m/s, there is 1058 watts of power in the wind, times 20% efficiency for 212 watts per sq m of swept area.
* At 15 m/s, there is 2067 watts of power in the wind, times 20% efficiency for 413 watts per sq m of swept area.

So by choosing a higher wind speed for the rated power, a company can claim a higher power rating. The standard is 11 m/s, but some companies rate as high as 14 or 15 m/s. This vastly over-estimates the wind power that you will actually get from a turbine.

Another problem with rated power is the value used for air density. The usual figure used is 1.225 kg/m3. But that assumes zero percent humidity, an altitude of sea level, and a temperature of 14 degrees centigrade (57.2 F). At 20 degrees (68 F), 500 meters of altitude, and 50% humidity, the air density is about 8% less, and so is the power output.

Then there is the efficiency of the turbine. Most companies don’t state an efficiency. But based on the power in the wind and the rated power, some companies are essentially claiming an efficiency higher than an off-shore multi-megawatt multi-million dollar turbine. So when you combine all of the above factors, the rated power of a wind turbine may have little to do with its actual power output.

I suggest that you figure on getting at most 150 watts per square meter of swept area from a small wind turbine. That takes into account a lower air density (1.126) and a fairly good efficiency (20%). A few wind power companies use AWEA standards for rating their turbines. One such company is called Windspire Energy. Their 1200 watt wind turbine has 7.43 sq m of swept area for 161.5 watts per sq m. Take away about 8% for a lower air density than the ideal, and you have about 150 watts per sq m. (The company previously put day-to-day real world test numbers online for their turbine, and the numbers held up.)

How much power will you get per day, month, or year from that 150 watts/sq m ?? Well, that’s another disappointing tale. You won’t have a steady wind at the rated wind speed of 11 m/s. And when the wind is faster, most turbines will not produce more power than at the rated speed. When the wind is slower, the power output falls dramatically — again because the power is dependent on the wind speed cubed. The AWEA standard for calculating AEP (annual energy production) is based on a 5 m/s average wind speed, not the 11 m/s for the power rating. The Windspire turbine is 1200 watts rated power (1.2 kW), but only 2000 kWhr per year by AWEA specs.

If you could obtain the rated wind speed 24 hours a day, 365 days a year, you would get 10,512 kWhr per year from a 1.2 kW rated turbine. The figure of 2000 kWhr is about 19% of that value. So for each sq m of swept area, you are getting about 270 kWhr/year, or about 737 watt-hours per day. That’s about equal to the power output of two of the 100W solar panels.

Right now, solar panels are significantly less expensive than wind turbines, in terms of the power that you get per day. What I’d like to find is an off-the shelf (or easy to assemble kit) wind turbine, of about 1 to 2 square meters swept area, for about $100 to $200 dollars. But the price-point of small wind turbines is just not there yet. So I would say that solar panels are the better deal.

– Thoreau

6 Responses to Wind Turbines versus Solar Panels

  1. Solar panels (PV) are a good way to have some source of electric power in situations where connection to the grid is impractical. Such as a mountain cabins or RV’s. The cost of the power is excessive, so excessive that it simply isn’t practical for power where grid power is available. The ONLY reason PV is used on homes like the one in Maine shown in the article is because the government and the utility heavily subsidizes it for political reasons. So if you want a small backup that would allow you to have a few lights at night and power a radio or laptop for a few hours everyday then PV is worth the price. If you have a notion that you can somehow build a PV system that will “pay for itself” and then provide you free power then you are a victim of the PV political propaganda.

  2. Agreed. While on the surface the solar panels seem economical, in practical terms the cost of the panels doesn’t even come close to the total. Unless you live in an RV loaded with 12 Volt appliances, you need to convert the power output from the panels into something that standard appliances need as a usable form with an inverter. A quality whole-house inverter can run into thousands of dollars. Similarly, unless your day rises and sets with the sun, any use of the solar power at night (or cloudy days?) requires a storage system of some kind AKA batteries. Not the Walmart automotive batteries, but special deep cycle batteries like for a golf cart or indoor forklift. Again, quite expensive and necessitates maintenance plans and skills.

    Rarely am I aware of a totally off-grid system based solely on wind. They are adjuncts to the cloudy-day failings of the panels or are supplemental to the grid-tie.

    • True, but I’m thinking more along the lines of a smaller power system for charging phones, 12-volts, laptops, etc. The “whole house off-grid” concept is an expense beyond my reach.

  3. I’ve crunched the same numbers and found them both to be horribly expensive and inefficient. I have an idea that if you have a running stream or river on your property that you can get entry-level nearly domestic grade hydro-electric generators, the so-called “micro hydro” turbines. I can pick up a 6kw micro hydro turbine for about $16 900 (I’m not in the US, so I’m converting). Even though practically you can only expect it to run at about 40% capacity (2400w), it will do that pretty much 24 hours a day (we don’t really get frozen rivers or even snow here). It’s still not enough to recover the investment in the hydroelectric turbine anytime soon, but it seems to be the most efficient way.

  4. Adding a couple of motors and light sensors you can obtain much more power for a little work. Just have the panel array migrate with the sun. I could probably build this for you if I had the parts. My only arduino projects right now are dealing with water.

  5. We’ve been living off-grid going on 7 years now. Agreed with GWTW and Hank, it’s not an economical solution – but a necessity here. At 45 degrees north latitude, a good rule of thumb I found is “[Rated Wattage of the panels] X [5 hours of sun] X [.75] = watt hours in an average day”. About a $20k solution with the Xantrex inverter, Rolls-Surrette batts, and misc equipment back then (without installation). Haven’t had to replace the batts yet, but when we do it’ll be about another $3-4k. Have a Air-X 400W wind turbine as well, and while I admit it’s placement/height doesn’t follow the recommendations, it provides essentially nothing unless the wind speed is over about 25MPH.