What type of renewable energy system should I install?

Before you buy a wind turbine and/or solar panels, start implementing energy conservation and energy efficiency measures first.  Many of them are cheap (or FREE!) and you can immediately began to reduce your utility bills and save money.

You effectively have two choices for producing renewable energy at your home or business: wind turbines or solar panels.  Small, grid-connected residential wind turbines are generally considered to be anything with less than rated output of 25 KW (please see the below explanation about the fallacy of using the rated output of a wind turbine to select a model to purchase).  Almost all solar panels have a rated output of less than 300 watts.  The following information can help you determine if it makes more sense to try and install a wind turbine, solar panels, either of them, or neither of them.

You can see some general pros and cons about wind turbines and solar panels.

The beauty of either solar panels or a wind turbine at your home or business is the energy is being produced where it will be used.

The United States is blessed with a tremendous amount of renewable energy resources.  It has more available wind energy than any other country in the world, and enough solar energy falls on the U.S. every day to power the entire Country for a year.  But solar and wind energy are inconsistent, are not evenly distributed, are diffused and occasionally weak.

The energy in solar and wind is much less concentrated, and much harder to extract, than energy in fossil fuels.  The difficulty of using solar and wind energy is in trying to convert it to electricity cheaply and efficiently enough for it to be considered a reasonable alternative to fossil fuels.

If your goal is to maximize the amount of energy you obtain from your renewable energy system, while minimizing investment costs and maintenance issues, then it really gets down to four considerations:

  1. How much wind energy and solar energy is potentially available at your property?
  2. How much of that available wind and solar energy at your property can you reasonably expect to convert to energy, and at what cost?
  3. Are there any laws, ordinances, zoning regulations or physical obstructions that could limit your ability to install and efficiently operate a solar panel or wind turbine?
  4. What is the total cost to purchase, install and maintain a solar panel or wind turbine compared to the potential energy production?

These four considerations are individually addressed below.

Potentially Available Energy
The first consideration may seem like the most difficult to answer, but it can be objectively determined.  Common sense will tell you that the available wind energy or solar energy varies depending on your geographic location.  A property in Arizona has more available solar energy than a property in Vermont; and a property on the open prairie in South Dakota has more available wind energy than a property in a heavily wooded area of Georgia.

The federal government spent decades measuring and compiling the amount of wind energy and solar energy across the entire U.S.  When measured consistently over a long period of time, solar energy and wind energy can be averaged very accurately.  That information provides a good overview of the solar and wind potential in your area.

The results of that massive effort by the federal government are shown on the following Solar Radiation Map, and the below Wind Resources Map. The higher the solar radiation number, the better a solar energy system will work.  The higher the average wind speed, the better a wind turbine will work.

SOLAR ENERGY

The Solar Radiation Map lists the average annual solar radiation amounts in kilowatt hours per square meter per day.  (The two maps that follow the average annual solar radiation map are the solar radiation map for the months of December and July, so you can see how dramatic the seasonal fluctuation is.)  Because of the long period of time over which the solar radiation was measured, the map reflects the seasonal fluctuations and other factors (clouds, dust, etc.) that can influence solar radiation during the year.

The following table shows the approximate total potentially available solar energy per square foot per year.  The table is set up that way to allow you a better comparison with the potentially available wind energy in your area, which is best measured over a year because of the constant fluctuations (and seasonal variations) in wind speeds.  The table shows the average annual kilowatt hours of solar energy per square foot for each of the solar radiation classes in the U.S.:

SOLAR ENERGY TABLE
kWh / square meter / day   kWh / square foot / year
6 to 7   204.4 to 237.3
5 to 6   168 to 204.4
4 to 5   135 to 168
3 to 4   102.2 to 135

The information in the table above will allow you to make some rough estimates of how many square feet of solar panels you’d need to buy to generate the amount of electricty you want.

For example, say you live in an area of the Country that receives between 167.9 and 204.4 kWh of solar energy per square foot per year, and you’re looking at buying some solar panels that are 15% efficient.  You could expect to generate between 25.2 and 30.7 kWh per square foot per year from the solar panels.

If you wanted to generate a third of your electricity usage from the solar panels, and you used 15,000 kWh per year of electricity, you’d need to generate 5,000 kWh per year from the panels.  So you’d need to buy between 163 and 198 square feet of solar panels (5,000 divided by 30.7 and 25.2).

The disclaimer near the bottom of the solar radiation map should be repeated here: “Though useful for identifying general trends, this map should be used with caution for site-specific resource evaluations because variations in solar radiation not reflected in the maps can exist, introducing uncertainty into resource estimates”.

In other words, you may live in an area with more pollution and/or dust than the sites where the solar radiation monitors are located, and your property might have more shade or shadows than the monitor sites.  The information in the above table was derived from the map, so the same caution would apply to the table and the estimates that followed it.

WIND ENERGY

The United States Wind Resources Map lists the wind power density per square meter (Remember the difference between power and energy?) at a 50 meter height (164 feet) for the listed wind classes.

The following table shows the approximate average yearly kilowatt hours of energy per square foot for the Wind Power Classes in America:

WIND ENERGY TABLE
Wind Power Class   Wind Speed at 50 m in MPH   kWh / square foot / year at 50 m height
3   14.3 to 15.7   145 to 156.7
4   15.7 to 16.8   156.7 to 168
5   16.8 to 17.9   168 to 201.7
6   17.9 to 19.7   201.7 to 218
7   19.7 to 24.8   218 to 236

Please note that the energy amounts shown in the wind energy table above are based on available wind at 164 feet above the ground.  Remember that the wind is generally slower and more turbulent, with less available energy, closer to the ground.  If your wind turbine will be mounted significantly closer to the ground than 164 feet in height, the potentially available energy per square foot of rotor swept area may be considerably less than what is shown in the table.

With even a quick review of the total annual potential available energy from solar and wind listed in the above tables you can see very similar amounts of energy from the two sources – in certain classes.

If you happen to live in an area with similar amounts of wind and solar energy, then use other facts to decide what system to buy (cost, zoning and HOA restrictions against wind turbines or solar panels, obstructions to solar or wind, etc).

However, if you live in an area with greater potential solar energy or wind energy then you need to decide if the potential energy is greater enough to purchase either solar panels or a wind turbine.  This is where it becomes important to determine the total cost to install solar panels or a wind turbine plus the overall efficiency of those respective systems in generating electricity, and compare those costs and system efficiencies with the total available potential solar and wind energy.

The average per square foot solar energy and wind energy shown above represents the total amount of solar energy and wind energy potentially available per year for all areas of the U.S., and account for the seasonal fluctuations.  Those numbers are are all the energy we have to work with.  The trick is to increase the efficiency of solar panels and wind turbines, and install them widely enough to meet our energy needs.

The preceding estimates should help you understand that a wind turbine will work in areas with lower average wind speeds, and a solar panel will work in areas with lower solar radiation, they just won’t work as well.

Designing, engineering and manufacturing a solar panel or wind turbine that can convert sunlight or wind efficiently enough to economically compete with energy dense fossil fuels that can just be pumped or dug out of the ground is extraordinarily difficult.  The reality is that energy produced from small renewable energy systems can’t generally compete with energy produced from fossil fuels on a cost basis.

Unless a carbon tax or a carbon cap and trade system significantly increases the cost of energy produced from fossil fuels, energy produced from small renewable energy systems may not be able to compete with energy produced from fossil fuels until fossil fuels begin to run out and their costs increase.  Of course if we wait until then to seriously begin the transition to renewable energy, an energy disaster can not be avoided.

The efficiency of a solar panel or wind turbine (that is, the percent of the potentially available wind energy or solar energy typically converted to electricity) should also be considered.

How Much Can Be Converted?
In a perfect world, solar energy systems and wind turbines would each be able to respectively convert 100% of the solar energy and wind energy that reach them.  But they can’t. And, unfortunately, they aren’t even close. The best solar panels have an efficiency of about 20%, which means they convert about 20% of the available solar energy that strikes them.  An extremely efficient small wind turbine system can convert maybe 35% of the wind energy that hits it.

Why are those efficiency percentages so low?

For solar, it is problems with trying to engineer the materials of the solar cells to convert sunlight at ever higher percentages and ever lower costs.  Scientists and engineers have spent decades trying different natural and man-made elements, and endless combinations of those elements, in the search for cheaper and more efficient solar panels.  But most solar panels are still produced from naturally occurring silicon that has been treated, which becomes electrically charged in sunlight.  Nevertheless, the cost of producing energy from solar panels has decreased some 90% over the past 25 years.

For wind, it is a matter of physics.  Wind turbines work by slowing the wind that passes through the rotors, and converting the kinetic energy of the wind into electricity.   The only way a wind turbine can convert 100% of the wind energy is if it completely stopped the wind passing through it.  But if it stopped the wind passing through it, then there would not be any more wind passing through it to turn the rotor.  According to Betz’s Law, named for the German physicist who calculated it, the maximum theoretical limit of the amount of energy in the wind that a wind turbine can convert is 59.6%.  All turbines also have aerodynamic losses, turbulence losses, generator losses, etc.

Generally speaking, the wind turbine industry is older and more mature than the solar panel industry.  That is one reason why the most efficient wind turbines are currently about twice as efficient as the most efficient solar panels.

Natural and Man-made Obstructions
In addition to the overall low efficiency of the solar energy and wind energy systems, natural and/or man-made obstructions at your property can reduce the efficiency of those systems even further.  You’ll need to spend some time and effort to determine the impact of the natural and/or man-made obstructions on the available solar and wind energy at your property, to better understand which type of renewable energy system is right for you; and to determine the best location on your property for a solar panel or wind turbine.

Examples of natural obstructions are trees, tall bushes, cliffs and hills.  Man-made obstructions might be houses, buildings or walls.

For a solar panel, the issue of obstructions is pretty straight forward: does any shadow or shade cross the proposed location of the solar energy system at any time during the year? If so, how much of the year does a shadow  or shade cross that location; and what time of the day does it happen?  Obviously, if a shadow or shade only crosses the location a few days per year in the early morning or late afternoon that would not be considered a serious obstruction.

For a wind turbine, it is much more difficult to measure or gauge the effect of obstructions.  Wind flows over and/or around any obstruction.  But the obstructions can decrease wind speed and make the wind more turbulent, which decreases energy yield and increases maintenance costs.

Rating systems have been developed to allow people to try to calculate the effect of obstructions on a proposed wind turbine installation location.  The rating systems assign a value to the type of topography and vegetation in the vicinity of a proposed wind turbine site, from a low value assigned to open water to a higher value assigned to open, treeless fields, up to a high value assigned to an area that is heavily wooded or has numerous buildings.  But the rating system values are somewhat subjective in the middle to upper values of the rating systems, and are difficult to use for any type of meaningful calculations.

A much simpler, and perhaps no less effective, solution is to use the general rule that the bottom of a wind turbine’s blades should be at least 30 feet above the top of any obstruction within 500 feet from the proposed wind turbine location.  Think about that for a minute, and you’ll begin to understand the difficulty of finding a good location on a suburban or urban property to place a wind turbine.

The issue of obstructions is a crucial consideration when trying to find the best site on your property for a solar panel and/or wind turbine, or even when trying to decide what to buy.  It’s important to spend the time to get a good understanding of the obstructions, because even a 15% reduction in wind speeds at a site will result in a 50% reduction in the amount of energy a turbine will produce.  Similarly, shade or shadows can dramatically reduce the output of solar panels.

Please see the attached checklists to help you decide what type of renewable energy system may work best for you. Checklists coming soon.

For Texas, New York and New Jersey property owners: If you live at a suburban location with houses and large trees creating wind and solar obstructions for any wind turbine and solar panel site so that neither a wind turbine or solar panel will work effectively, and you’d like to help America become more energy independent, sign up to get your electricity from Green Mountain at www.greenmountain.com/.  Green Mountain is an Austin, Texas-based electricity company that obtains electricity from large wind farms.

Laws, Ordinances and/or Zoning Regulations
If you are in an urban or suburban area, there may be ordinances and/or zoning regulations limiting the installation of wind turbines or solar panels.  You should check with your town or city government, and your homeowners association (HOA), to obtain a clear understanding of any possible limitations or restrictions to installing a wind turbine or solar panel.  Make sure you know the particulars of your desired system (wind turbine height, rotor diameter, etc.) before you contact the city and HOA. As a general rule, towns and cities typically have more limitations or restrictions on wind turbines than on solar panels.  Trying to use a tower with guy wire supports to mount your wind turbine in a suburban or urban area may be impractical, because of spacing limitations for anchoring the guy wires.

Just because your city or HOA may have ordinances and/or zoning regulations prohibiting the installation of a wind turbine or solar panel, it does not mean you won’t be able to obtain a variance to the regulation and install the system you want.  Obtaining a variance may involve getting the written consent of your immediate neighbors, and filling out and submitting the necessary forms to the city. Check with your city to understand how you may obtain a variance, if city ordinances and/or zoning regulations limit your ability to install a renewable energy system.

Experienced solar panel or wind turbine installation contractors should know the process to obtain a permit for installation (or a variance and permit if necessary), and can do the work for you.  But is doesn’t hurt for you to have a clear understanding of the requirements of your city and/or HOA to ensure the process is satisfactorily completed.

Even if you have a permit for installation of your renewable energy system, it’s probably a good idea to talk with your immediate neighbors about the installation to let them know what you’re planning to do.

System Costs vs. Energy Production

Obviously, the larger the wind turbine or solar panel system you buy the more energy you can produce.  The other consideration is how large of a wind turbine or solar panel system you can afford to purchase and/or finance to harvest the energy.  Perhaps, for that reason, this section should have come first.

It may be possible to buy a larger renewable energy system than you think, by taking advantage of the federal, state and local purchase incentives (see the exhaustive list of incentives at www.dsireusa.org) and by effective financing (link to the HOW tab section on financing).

The overall cost of the wind turbine or solar panels is a much more important consideration than the efficiency of the system. You’ll want to purchase a wind turbine or solar panel that produces the greatest amount of energy per dollar of investment.  Buying a 5% more efficient system at a 25% higher cost doesn’t make sense.  If you can find a more efficient system at the same, or close to the same price, buy it.  Otherwise, buy the cheaper system.

Thin film solar panels are not nearly as efficient as traditional solar panels. But thin film solar panels cost a fraction as much as traditional solar panels. That’s why the per watt cost of electricity from thin film solar panels could very well cost you less than traditional solar panels.

Keep in mind there has been little independent testing or rating of wind turbine or solar panel performance by certified testing laboratories or agencies.  So you should be careful about overly enthusiastic claims by manufacturers.

As an example, wind turbine manufacturers can use any speed they want to rate their turbines on maximum rated output (so called name plate rating). Consequently, different models and different manufacturers achieve the maximum rated output at different wind speeds – and that is not a fair comparison.  One turbine might have a rated output at 14 MPH, while another might have a rated output at 28 MPH.

Since the power of wind increases by the cube of the wind speed (if wind speed is doubled, wind power goes up eightfold), the second turbine would need eight times as much power to achieve its maximum output.  For that reason, you should ignore the name plate rating when comparing wind turbines – they won’t provide meaningful information.  Focus instead on the annual energy output.

The huge increases in wind power associated with increases in wind speed is the reason why using an average wind speed to estimate the potential energy production of a wind turbine may be understating the actual energy production.

For example, the average of six MPH and twelve MPH is nine MPH. But since the energy in twelve MPH wind is eight times the energy in six MPH wind, and assuming the wind blew at 12 MPH half the time and 6 MPH half the time, the average energy would be more than the energy in a constant nine MPH wind.

Wind speeds vary constantly, so even obtaining an average wind speed is difficult.  It’s much more useful to look at the actual energy production of a wind turbine over the period of a year, where the wind speeds at that location have been continuously recorded and than graphed.

If you’d like to be able to calculate what the approximate payback period might be for your investment in a renewable energy system and/or what your return-on-investment may be, please see the information and example calculations on this page: System Cost Payback Period & Return-On-Investment (ROI).

Findsolar.com and www.nabcep.org/ have extensive lists of solar energy system installation contractors.  Find Solar is a service of the American Solar Energy Society, and the North American Board of Certified Energy Practioners (NABCEP) is a national certification organization for professionals in the renewable energy field. You should be able to find a contractor in your area.  Even if you want to get your system this way, you might want to do your own research on the products offered by the various solar panel manufacturers (link) so you can confirm the installation contractor can install the system you want.

About the Author

Mark H. Witte is a strong proponent for energy efficiency and renewable energy, and believes individuals should have more control over how the energy for their homes is produced.