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.