Though the pros and cons of solar power continue to be debated, solar is starting to make a serious contribution to our energy resource mix. Solar energy systems work with a reliability unmatched by other energy technologies, at least when the sun shines.

A number of means have been created to harness energy from the sun. The development of these technologies dates back at least twenty-five centuries to Ancient Greece as shown in this Department of Energy Solar History Timeline .

Technically, most of our energy resources are forms of solar energy. The wind wouldn’t blow without the sun. The sun causes evaporation of water that ends up in rivers that are used for hydro electrical power; without the sun, water would be eternally locked in a solid state, ice. No biofuels can exist without the sun. And by most accounts, all fossil fuels exist because the sun allowed things to grow.

Solar Power, as the term is commonly used, applies to energy systems that produce energy directly from sunlight.

Most solar systems collect heat created from light striking an object. Some concentrate light with mirrors or lenses and can produce enough heat to start a fire. And light can be converted directly into electricity through the Photovoltaic Effect , first discovered in 1839 by French physicist Alexandre Becquerel.

Part of this discussion of the pros and cons of solar power will apply to all aspects of solar energy. In some sections, we will focus on photovoltaic electrical generation since that is the most cost efficient use of solar energy and gets more efficient every year.

[Note: While a directed solar collector coupled with a Sterling Cycle generator has the record for the most energy efficient system - at 30% efficiency - photovoltaic systems mounted on existing rooftops are simpler and far more cost effective; hence the claim that photovoltaic is proving to be the most cost efficient use of solar energy.]

Though solar power has not generated as much controversy as the Pros And Cons Of Wind Power debate, concerns about solar power focus on the same areas listed here:

Pros and Cons of Solar Power Environmental Friendliness

Pros and Cons of Solar Power Reliability

Pros and Cons of Solar Power Cost

Pros and Cons of Solar Power Availability

Pros and Cons of Solar Power Aesthetics

Pros and Cons of Solar Power Sustainability



All forms of solar power are extraordinarily clean. They produce zero pollution when operating.

Though there is some concern about the toxicity of cadmium used in photovoltaic collectors, solar power generators generally entail no more manufacturing concerns than any other energy related product.

Solar power does suffer environmental shortcomings similar to wind power. Insolation (not to be confused with insulation) refers to the concentration of solar energy available per unit of area. It’s usually measured in terms of watts per square meter.

Rutgers University Solar Data shows that the upper atmosphere of Earth is bathed in sunlight concentrations of about 1.37KW/m2 (1,370 Watts, or 1.37 Kilowatts per square meter). But the energy output of a solar collector will be far less.

Before reaching a solar collector on the ground, sunlight is filtered through our atmosphere and loses part of its energy. Solar collector output is also affected by angle to the sun and latitude; the greater the angle to the sun, the less efficient the collector. For stationary collectors, the angle of the sun is constantly changing as the Earth turns. The only time a stationary collector operates at peak efficiency is at midday. And no solar collector converts 100% of sunlight into useful energy (either thermal or electrical).

The end result of these energy losses is that energy supplies of 1,370 Watts per square meter will result in average solar collector energy output in the United States of less than 100 Watts per square meter.

Because of that low power density, solar collectors covering millions of acres would be required to replace a large portion of the energy generated by existing power plants. Some environmentalists are concerned about permanently covering huge land masses with solar collectors.

But, though solar energy does require a lot of space, a lot of that space can be found on rooftops of existing structures, and can produce solar energy without occupying any extra land area.

How much energy?

We'll put some numbers together, and I'll warn you in advance that the conclusions will be debated. As is the case with all energy resource projections, some will argue that the numbers are high, others that they are a bit low. The pros and cons of solar power debate even applys to hard data.

My hope here is that we finally have some kind of reliable, ball park estimate of our solar capabilities. You can adjust the data up or down, however you see fit.

Let's get started.

Using some rough estimates, let’s see what we can do with rooftops in the U.S. Census Bureau Housing Data puts the number of households in the U.S. at about 126 million. About 100 million of these households are single houses, duplex or fourplex units, or mobile homes.

A number of factors influence usable solar collecting area of a rooftop. Size of the roof, orientation, portion of the roof that faces south, shading from trees, shading from other properties, and a host of other factors will increase or decrease the usable area.

Understanding that there are a lot of variables, let’s make the fairly loose assumption that the average economically usable roof area for installing photovoltaic collectors is about 1,077 square feet, or 100 square meters. Let’s also just use the 100 million properties mentioned above to stay on the conservative side of things. That gives us an area of 10 billion square meters (about 2.5 million acres) of surface on which to install photovoltaic collectors, without using any new land.

Moving along, we see that this NREL Solar Map shows an estimated average U.S. insolation of 5KWh/m2/day (5 Kilowatt hours per square meter per day), or an average of about 417 Watts per hour on a stationary, one square meter, south oriented collector.

Current photovoltaic conversion efficiency is about 15% and getting better, much better. Some experimental models are yielding Over 40% Efficiencies .

If we look ahead just a bit and assume an average 20% conversion efficiency for solar cells being installed in the near future, we see that insolation of 5KWh/m2/day input would yield 1KWh/m2/day (an average of about 83 Watts per square meter per hour of daylight) of electrical output.

With an average electrical output of 1KWh/m2/day, a 100 square meter rooftop would generate an average of 100KWh/day. And 100 million of these units could produce 10 billion KWh/day or 3.65 trillion KWh per year.

U.S. Department of Energy Electrical Consumption Statistics show that we consumed a little over 4 trillion KWh in 2005.

If you add available rooftop area from the rest of the housing units mentioned above, and then add the rooftop area on our warehouses, factories, and countless other business buildings, it looks like we could provide over 100% of our electrical needs from rooftop installed photovoltaic collectors with space left over for extra collectors to recharge car batteries or produce hydrogen for auto fuel.

Then we can add the area covered by driveways and parking lots. Adding it all up, it looks like solar power has no land use problems.



Generally speaking, solar power is more reliable and predictable than wind power. The sun rises daily, without fail; we can’t say the same for wind in most areas.

Solar power does suffer intermittency problems similar to wind since the sun doesn’t shine at night. And it doesn’t work well on cloudy days. But even these problems are eased because the intermittency is 100% predictable, and while clouds diminish solar cell productivity, it doesn’t eliminate it.

Solar power has a distinct reliability advantage over wind power. But it is intermittent, and large-scale use of solar power will have to address that problem.

We will discuss that next.



Once installed, solar collectors require little maintenance and provide free energy. Solar cells do wear out, but their lifespan is measured in decades. And the durability of photovoltaic cells improves almost every year.

Though they operate cheaply after installation, solar cells historically have been an expensive way to generate electricity. But that is changing rapidly.

Solar cell costs get cheaper per kilowatt every year, and we may be on the verge of a breakthrough that will make them financially competitive with conventional forms of electrical generation.

Because of its intermittency, however, as power supplied to a grid by solar power exceeds about 20%, additional cells and some form of Grid Energy Storage will be needed to provide enough power to meet daytime demand and store energy for dark or cloudy periods. (See Pros and Cons of Wind Power for a discussion of grid penetration problems posed by intermittent energies like wind and solar.)

The main determining factors for economic viability of solar energy of a given site are the number of cloudless days per year, latitude, the cost of the solar collectors, and the percentage of grid demand supplied by solar collectors.

Though there are some cost hurdles yet to overcome, the price of solar electric energy technology is getting cheap enough that solar power, even for our automobiles, is looking more and more like an exciting possibility that is happening now.



Power from the sun is universally available, but on a periodic basis. The sun doesn’t shine at night, sorry. And the further north or south of the equator you go, the greater the intermittency problems because of long winter periods of darkness that can last essentially for weeks on end.

That being said, solar cells can generally be located where they are needed. Solar cells work best in sunny areas. People love sunshine, and most population centers have enough sunshine to take advantage of solar generated electricity in a big way.



Solar cells aren’t unpleasant to look at. In fact, their metallic blue luster has a decent amount of eye appeal. For most of us, covering every rooftop in the world with blue solar cells wouldn’t be a bad thing.

But a new generation of photovoltaic cells is coming online that are available in colors like brown and black. So we will have a choice of rooftop colors similar to what we have now. Besides that, how often do we really look at rooftops anyway?

Aesthetics problems may only happen if we start covering roadways or pastureland with solar collectors. For reasons discussed above, that’s unlikely to happen.

Unlike wind power, solar power seems to have few aesthetics problems.



So, just how much sunshine is available for our use?

Using data referenced by the Stanford University Wind Power Study , we see that total annual global Btu consumption is about 7,000-10,000 Mtoe, or an average of 8,500 Mtoe.

[Btu: British thermal unit; the amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit.

Mtoe: Million metric tons of oil equivalent; or about 7 million barrels of oil (42 U.S. gallons per barrel); or 11.63 TWh; or 40 Trillion Btus.

TWh: Terrawatt hours – One million Megawatt hours, or one billion kilowatt hours, or about 3.44 Trillion Btus]

Converting 8,500 Mtoe to Terrawatt hours gives us an annual global energy consumption of 98,855 Terrawatt hours.

Our Earth is about 12,746 kilometers in diameter (8,300 miles). That gives an area exposed to sunlight of 127.5 trillion square meters. As discussed above, the sun delivers 1,370 Watts of energy per square meter at the outer edge of our atmosphere.

Multiplying meters by Watts we see that our planet is bathed with almost 175 trillion Kilowatts that provides extraordinarily reliable annual energy supplies of 1,530,000 Terrawatt hours. That’s over 15 times our total annual global energy consumption from all resources including oil, coal, gas, nuclear, wood, and so on.

Solar power will last as long as the sun shines. But when it stops shining, we’re done. Then again, lack of electricity will be the least of our problems when that happens. Don’t worry, though, most scientists agree that the sun will continue shining for at least four billion more years .



For detailed solar info including some great kids pages, visit Green Planet Solar Power Facts . _______________________________________________________________

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