The "Minimum" Solar Box Cooker

Solar ovens can cook food or pasteurize water using only the power of the sun.  How does a solar oven work?  The simple answer is that it is designed to absorb more heat than it releases.

The “Minimum” Solar Box Cooker is a simple cardboard box cooker that can be built in a short time for very little expense. 

The designers of this cooker, named it the “Minimum Solar Box Cooker” because, at the time, it represented the simplest design they could devise.  What they didn’t communicate with that name was that this is a full-power cooker that works very well, and is in no way minimum as far as its cooking power goes.

How to Build a Simple Cardboard Box Solar Cooker

What You Will Need

Two cardboard boxes.  We would suggest that you use an inner box that is at least 15 inch x 15 inch (38 cm x 38 cm), but bigger is better.  The outer box should be larger than the small box all around, but it doesn’t matter how much bigger, as long as there is a half inch (1.5cm) or more of an airspace between the two boxes.  The distance between the two boxes does not have to be equal all the way around.  Also, keep in mind that it is very easy to adjust the size of a cardboard box by cutting and gluing it.

One sheet of cardboard to make the lid.  This piece must be approximately 2 to 3 inch (4 to 8 cm) larger all the way around than the top of the finished cooker (the outer box).

One small roll of aluminum foil.  One can of flat-black spray paint (look for the words “non-toxic when dry”) or one small jar of black tempera paint.  Some people have reported making their own paint out of soot mixed with wheat paste.

At least 8 ounces (250 g) of white glue or wheat paste.

One Reynolds Oven Cooking Bag®. These are available in almost all supermarkets in the U.S. and they can be mail-ordered from Solar Cookers International.  They are rated for 400 °F (204 °C) so they are perfect for solar cooking.  They are not UV-resistant; so they will become more brittle and opaque over time and may need to be replaced periodically.  A sheet of glass can also be used, but this is more expensive and fragile, and doesn’t offer that much better cooking except on windy days.

Building the Base

(Click on any of the pictures to see a larger image.)

Fold the top flaps closed

Fold the top flaps closed on the outer box and set the inner box on top and trace a line around it onto the top of the outer box, Remove the inner box and cut along this line to form a hole in the top of the outer box (Figure 1).

Decide how deep you want your oven to be.  It should be about 1 inch (2.5 cm) deeper than your largest pot and about 1″ shorter than the outer box so that there will be a space between the bottoms of the boxes once the cooker is assembled.

Using a knife slit the corners of the inner box

Using a knife slit the corners of the inner box down to that height.  Fold each side down forming extended flaps (Figure 2). Folding is smoother if you first draw a firm line from the end of one cut to the other where the folds are to go.

Glue aluminum foil to the inside of both boxes and also to the inside of the remaining top flaps of the outer box.  Don’t bother being neat on the outer box, since it will never be seen, nor will it experience any wear.  The inner box will be visible even after assembly, so if it matters to you, you might want to take more time here. Glue the top flaps closed on the outer box.

Set the inner box inside the outer box

Place some wads of crumpled newspaper into the outer box so that when you set the inner box down inside the hole in the outer box, the flaps on the inner box just touch the top of the outer box (Figure 3).  Glue these flaps onto the top of the outer box.  Trim the excess flap length to be even with the perimeter of the outer box.

Finally, to make the drip pan, cut a piece of cardboard, the same size as the bottom of the interior of the oven and apply foil to one side. Paint this foiled side black and allow it to dry.  Put this in the oven so that it rests on the bottom of the inner box (black side up), and place your pots on it when cooking.  The base is now finished.

Building the Removable Lid

Building the removable lid

Take the large sheet of cardboard and lay it on top of the base.  Trace its outline and then cut and fold down the edges to form a lip of about 3″ (7.5cm). Fold the corner flaps around and glue to the side lid flaps.  (Figure 4).

Orient the corrugations so that they go from left to right as you face the oven so that later the prop may be inserted into the corrugations (Figure 6).

How to make the lid fit well

One trick you can use to make the lid fit well is to lay the pencil or pen against the side of the box when marking (Figure 5).

Don’t glue this lid to the box; you’ll need to remove it to move pots in and out of the oven.  To make the reflector flap, draw a line on the lid, forming a rectangle the same size as the oven opening. 

Lid, reflector and prop

Cut around three sides and fold the resulting flap up forming the reflector (Figure 6).  Foil this flap on the inside.

To make a prop bend a 12″ (30cm) piece of hanger wire as indicated in Figure 6.  This can then be inserted into the corrugations as shown.

Next, turn the lid upside-down and glue the oven bag (or other glazing material) in place.  We have had great success using the turkey size oven bag (19″ x 23 1/2″, 47.5cm x 58.5cm) applied as is, without opening it up.  This makes a double layer of plastic.  The two layers tend to separate from each other to form an airspace as the oven cooks.  When using this method, it is important to also glue the bag closed on its open end.  This stops water vapor from entering the bag and condensing.  Alternately you can cut any size oven bag open to form a flat sheet large enough to cover the oven opening.

Improving Efficiency of your Solar Oven

The solar oven you have built should cook fine during most of the solar season.  If you would like to improve the efficiency to be able to cook on more marginal days, you can modify your oven in any or all of the following ways:

Make pieces of foiled cardboard the same size as the oven sides and place these in the wall spaces.

Make a new reflector the size of the entire lid (see photo above).

Make the drip pan using sheet metal, such as aluminum flashing.  Paint this black and elevate this off the bottom of the oven slightly with small cardboard strips.

Before using you new Solar Cooker for the first time

Put on the lid, with the lid reflector propped open, and aim the cooker toward the sun and “cook” the cooker for several hours to drive out the last bit of moisture and any paint or glue fumes.

We would recommend “cooking” a new solar oven whether it was homemade or commercially manufactured to burn off any fumes there may be do to adhesives, coatings etc.

Food Cooking in Solar Box Oven

We enjoy building our solar cookers as much as we enjoy cooking in and experimenting with them and we hope that you will too!

If you would prefer to purchase a pre-built solar cooker, here are two models that have good reviews and are highly recommended.

Global Sun Oven – Solar Cooker	Hybrid Solar Oven

Many solar cooker designs are portable, allowing for solar cooking at work sites or while pursuing outdoor activities like picnics or camping.

So whether you build a solar cooker yourself or purchase one that is ready to use, you will enjoy the benefits of clean non-polluting solar cooking with free solar energy direct from the sun.

Sources:

The original plans for the solar cooker in this project are from:  Solar Cookers International – Solar Cooking Archive – “Minimum” Solar Box Cooker - These solar box cooker plans are also available in other languages.

Resources:

Principles of Solar Box Cooker Design

There is a wide variety of solar cooker designs, many of them very simple to build from inexpensive, easy-to-obtain materials.

Visit SolarCooking.org to learn more about the plans that they have made available.

Do you use a Solar Cooker?

Share your experience with others and help spread the word about natural sun powered solar cooking by leaving a comment below.

Thanks,
-Red

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Simple as Flipping a Switch to Abundant Energy Supplies

In our modern day environment, flipping a switch to abundant energy supplies is usually an unconscious act with little awareness of the critical issues that lie behind it.  But many of the world’s inhabitants do not have a switch to flip.

About half the inhabitants of the Earth cook over wood fires — 45% of the world’s wood is used as fuel.  But there’s not enough to go round:  some 2.4 billion people are now facing shortages of fuel wood.

Meanwhile deforestation helps cause climate change, floods and soil erosion, increasing environmental degradation, and increasing poverty and hunger.

Smoky Indoor Cooking Fire

Wood smoke from cooking fires is a major source of air-pollution and an outright killer:  it causes respiratory infections which kill at least 5 million young children each year.

At the same time millions of people do not have access to clean drinking water, causing widespread disease, especially among children:  the WHO says diseases spread through contaminated water cause 80% of the world’s illnesses.

There’s a simple answer to help ease some of these problems, that can save millions of trees, provide clean, safe drinking water, doesn’t pollute, and that costs very little – a cardboard box.

Most cardboard boxes end up in the garbage dump.  Recycling them saves wood, saves water, prevents pollution and saves a lot of energy.

Kyoto Box Solar Cooker

But, while recycling 60 or so cardboard boxes might save one tree, one cardboard box solar cooker can save at least one whole tree all by itself, each year.

Solar box cookers are easy to make — one cooker designed to be simple enough for 10-year-olds to build without special tools in less than an hour is now helping to feed refugees in Africa (and to save trees).

Solar cookers can cook, bake, braise, stew and fry food and can also be used to pasteurize water using only the clean free energy from the sun.  The 3 most common solar cooker designs are parabolic, box and panel cookers.

Parabolic Solar Cooker

 Parabolic Cooker:  The sun’s rays are captured in a reflector which focuses them at a point under a pot.  The effect is like a stove top burner or a campfire.  Temperatures can reach above 400 degrees Fahrenheit, hot enough to fry food.  Also called a Curved Concentrator solar cooker.

Wooden Solar Box Cooker

 Box Cooker:  The sun’s rays are received in an insulated black box with a transparent lid which lets in the sun’s rays.  Inside the box, this sunshine turns to heat which is trapped in the box.  The effect is similar to the oven in your kitchen.  Temperatures can reach around 300 degrees Fahrenheit.

CooKit Panel Solar Cooker

 Panel Cooker:  A combination of the two systems which is portable and less expensive.  Temperatures can reach around 250 degrees Fahrenheit.  (This is ample because cooking begins at around 180 degrees Fahrenheit.)

Many solar cooker designs are portable which allows for solar cooking anywhere that the sun shines.  So whether you build a solar cooker yourself or purchase one that is ready to use, you will enjoy the benefits of clean non-polluting solar cooking with solar energy direct from the sun.

In our next post we will show you how to build a simple and inexpensive cardboard box Solar Cooker that can be built in a few hours and would make a great family project.

Do you use a Solar Cooker?

Share your experience with others and help spread the word about natural sun powered solar cooking by leaving a comment below.

Thanks,
-Red

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A Simple Box Solar Cooker

The Benefits of Solar Oven Cooking

Solar cooking is the simplest, safest, most convenient way to cook food without consuming fuels or heating up the kitchen.

Many people choose to solar cook for these reasons.  But for hundreds of millions of people around the world who cook over fires fueled by wood or dung, and who walk for miles to collect wood or spend much of their meager incomes on fuel, solar cooking is more than a choice – it is a blessing.

How Do Solar Ovens Work?

There are a few different styles of solar ovens to choose from, but all function on the same basic principle by converting sunlight (solar energy) into heat. 

There are panel cookers, made up of reflective panels that surround the cooking pot.  Generally, the pot is placed in a plastic cooking bag that helps to trap the heat.

Parabolic concentrators or curved cookers that cook at high temperatures by concentrating and focusing reflected light onto the pot.  Because curved concentrators cook at high temperatures, they need careful adjusting and constant supervision.

Box cookers consist of an insulated box, a glass door and a reflective panel.  Food is cooked inside the box where the heat is trapped.

There are many different variations of these three main types of solar cookers.  You can find solar oven plans and build one yourself, purchase a solar oven ready to use, or even invent your own.  All solar ovens benefit from the use of a dark cooking pot with a tight-fitting lid.

Benefits to Households using Solar Oven Cookers

At moderate solar cooking temperatures food doesn’t need to be stirred and won’t burn – food can simply be placed in a solar cooker and left to cook, unattended, for several hours while other activities are pursued.

In the right circumstances it is possible to put a solar cooker out in the morning and return home in the late afternoon to a hot meal ready to eat.  Pots used for solar cooking are typically easy to clean.

Many solar cooker designs are portable, allowing for solar cooking at work sites or while pursuing outdoor activities like picnics or camping.

So whether you build a solar cooker yourself or purchase a solar oven that is already to use you will enjoy the benefits of clean non-polluting cooking with direct solar energy from the sun.

Global Sun Oven – Solar Cooker	Hybrid Solar Oven

Do you use a Solar Cooker?

Share your experience with others and help spread the word about natural sun powered solar cooking by leaving a comment below.

Thanks,
-Red

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Jon Bohmer with the Kyoto Box Solar Oven

Inventor turns cardboard boxes into eco-friendly solar powered oven

When Jon Bohmer sat down with his two daughters to create a simple project they could work on together, he didn’t realize they’d hit upon a solution to one of the world’s biggest problems for just $5: A solar-powered oven.

The ingeniously simple design uses two cardboard boxes, one inside the other, and an acrylic cover that lets in the sun’s rays and traps them. 

Black paint on the inner box, and silver foil on the outer one, help concentrate the heat.  The trapped rays make the inside hot enough to cook casseroles, bake bread and boil water.

What the box also does is eliminate the need in developing countries for rural residents to cut down trees for firewood. About 3 billion people around the world do so, adding to deforestation and, in turn, global warming.

By allowing users to boil water, the simple device could also potentially save the millions of children who die from drinking unclean water.

Bohmer’s invention won the FT Climate Change Challenge, which sought to find and publicize the most innovative and practical solution to climate change.

“A lot of scientists are working on ways to send people to Mars.  I was looking for something a little more grassroots, a little simpler,” Bohmer said.

Bohmer’s contest win notwithstanding, solar cooking with a cardboard oven isn’t new.  Two American women, Barbara Kerr and Sherry Cole, were the solar box cooker’s first serious promoters in the 1970s.  They and others joined forces to create the non-profit Solar Cookers International — originally called Solar Box Cookers International — in 1987.

Further, the organization’s executive director, Patrick Widner, said that the plans for a solar box cooker were found in a book published by the Peace Corps in the 1960s.

“We are pleased that Mr. Bohmer has taken up the cause and interest of the 95 member organizations and 160 individuals of the Solar Cookers Worldwide Network,” Widner said.  ”It would be a pleasure to work with Mr. Bohmer in Kenya where we have been promoting the use of solar cookers for ten years.”

Bohmer, a Norwegian-born entrepreneur based in Kenya, said he also had been looking at solutions “way too complex, for way too long.”

“This took me about a weekend, and it worked on the first try,” Bohmer said. “It’s mind-boggling how simple it is.”

The contest was organized by the Forum for the Future — a sustainable development charity — and the Financial Times newspaper.

Bohmer’s invention beat about 300 other entries, including a machine that turns wood and other organic material into charcoal, wheel covers that make trucks more fuel efficient by reducing drag, and a feed supplement for livestock that reduces the methane they emit by 15 percent.

Bohmer named his invention the Kyoto Box, after the international environmental treaty to reduce global warming.

The box can be produced in existing cardboard factories. It has gone into production in a factory in Nairobi, Kenya, that can churn out about 2.5 million boxes a month.

Bohmer has also designed a more durable version, made from recycled plastic, which can be produced just as cheaply.

He envisions such cardboard ovens being distributed throughout rural Africa.

“In the West, we cook with electricity, so it’s easy to ignore this problem,” he said.  ”But half the world’s population is still living in a stone age.  The only way for them to cook is to make a fire.

“I don’t want to see another 80-year-old woman carrying 20 kilos of firewood on her back.  Maybe we don’t have to.”

Source: CNN/ EcoSolutions

The design of the Kyoto Box Solar Oven, while simple, does have a number of shortcomings:

Kyoto Box Solar Oven

The reflectors will not stay in position with even the slightest wind.

When the sun is not straight overhead, in order to make use of the reflectors, the entire box has to be tipped up to face the sun. This causes the cooking pot to slide around and spill food.

Rather than have four reflectors, it is much more desirable to have a single rear reflector like the The “Minimum” Solar Box Cooker pictured below.

The "Minimum" Solar Box Cooker uses a single back reflector that is very easily tilted up and down to track the sun instead of tilting the entire cooker as is the case with the Kyoto Box.

The “Minimum” Solar Box Cooker is a simple box cooker that can be built in a few hours for very little money. This is a full-power cooker that works very well, and is in no way minimum as far as its cooking power goes.

Many solar cooker designs are portable which allows for solar cooking anywhere that the sun shines.  So whether you build a solar cooker yourself or purchase a light weight solar oven that is ready to use, you will enjoy the benefits of clean non-polluting solar cooking with solar energy direct from the sun.

Do you use a Solar Cooker?

Share your experience with others and help spread the word about natural sun powered solar cooking by leaving a comment below.

Thanks,
-Red

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Using Solar Energy to Produce Electricity

Solar Energy Associations and Societies

The International Solar Energy Society
http://www.ises.org/

The Society has been active in advancing the cause of renewable energy since 1954 and is present in more than 50 countries; the Society supports its members in the advancement of renewable energy technology, implementation and education all over the world.

American Solar Energy Society
http://www.ases.org/

Established in 1954, the American Solar Energy Society (ASES) is the nonprofit organization dedicated to increasing the use of solar energy, energy efficiency, and other sustainable technologies in the U.S.

Solar Energy Industries Association
http://www.seia.org/

Established in 1974, SEIA works to expand the use of solar technologies, strengthen research and development, remove market barriers, and improve education and outreach for solar energy.

Solar Electric Power Association
http://www.solarelectricpower.org/

The Solar Electric Power Association is a nonprofit organization, formed in 1992 as the Utility Photovoltaic Group. From national events to one-on-one assistance, SEPA is the go-to resource for unbiased and actionable solar intelligence. SEPA is comprised of 375 utility and solar industry members. Breaking down information overload into business reality, SEPA takes the time and risk out of implementing solar business plans and helps turn new technologies into new opportunities.

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This is just a short list of Solar Industry Associations and Societies.  There are many other state and local level Solar Energy Associations and Societies but I tried to just list some of the major (non-state and local) associations and societies to keep this post a reasonable length.  I will try to put together a full list in the future.

If you have any organizations that you would like to add to this list please post them in a comment below.

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A solar cell or photovoltaic cell is a device that converts light directly into electricity by the photovoltaic effect.

Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays.  Photovoltaics is the field of technology and research related to the application of solar cells in producing electricity for practical use.  The energy generated this way is an example of solar energy (also called solar power).

Solar cells are usually made from silicon, the same material used for transistors and integrated circuits.  The silicon is treated or “doped” so that when light strikes it electrons are released, so generating an electric current.  (See – How do Photovoltaic Cells Generate Electricity?)

There are three basic types of solar cell

Crystalline solar cells are wired in series to produce solar panels.  As each cell produces a voltage of between 0.5 and 0.6 Volts, 36 cells are needed to produce an open-circuit voltage of about 20 Volts.  This is sufficient to charge a 12 Volt battery under most conditions.

Monocrystalline Solar Cells

Monocrystalline – made from a single large crystal, cut from ingots.  Most efficient, but also the most expensive.  Somewhat better in low light conditions (but not as good as some advertising hype would have you believe). 

Polycrystalline Solar Cell

Polycrystalline – basically cast blocks of silicon which may contain many small crystals.  This is probably the most common type right now.  Slightly less efficient than single crystal, but once set into a frame with 36 or so other cells, the actual difference in watts per square foot is not much.

Although the theoretical efficiency of monocrystalline cells is slightly higher than that of polycrystalline cells, there is little practical difference in performance.  Crystalline cells generally have a longer lifetime than the amorphous variety.

Amorphous Solar Cell

Amorphous - technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications.  They are made by depositing a thin film of silicon onto a sheet of another material such as steel.  The panel is formed as one piece and the individual cells are not as visible as in other types.

The efficiency of amorphous solar panels is not as high as those made from individual solar cells, although this has improved over recent years to the point where they can be seen as a practical alternative to panels made with crystalline cells.  Their great advantage lies in their relatively low cost per Watt of power generated.  This can be offset, however, by their lower power density; more panels are needed for the same power output and therefore more space is taken up.

Vaporware – this is a 4th type – one that pops up in the news once in a while proclaiming to be the next major breakthrough that will make plastic spray on solar cells that will cost around 5 cents a watt, or some similar claim.  None have reached production yet as of this writing.

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A Single Solar Cell with Soldering Leads Attached

A solar cell is a device that converts photons from the sun into electricity.

In general a solar cell that includes both solar and non-solar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell.

Fundamentally, the device needs to fulfill only two functions: photo generation of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity.  This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics. 

Solar cells have many applications. 

Historically solar cells have been used in situations where electrical power from the grid is unavailable such as pumping water in remote areas without power systems, Earth orbiting satellites, consumer products, such as handheld calculators or wrist watches and remote radiotelephones.  More recently solar cells are being used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.

Solar cells are widely regarded as one of the key technologies moving us towards a sustainable energy future.

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A Solar Photovoltaic Array

A solar cell is any device that directly converts the energy in light into electrical energy through the process of photovoltaics.

The term “photovoltaic” comes from the Greek Word meaning “light”, and the name of the Italian physicist Volta, after whom the volt (and consequently voltage) are named.  It means literally of light and electricity.

The photovoltaic effect was first recognized in 1839 by French physicist Alexandre-Edmond Becquerel. However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions.  The device was only around 1% efficient.

Russell Ohl patented the modern solar cell in 1946.  Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells.

The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent.  This milestone created interest in producing and launching a geostationary communications satellite by providing a viable power supply.

Sputnik 1 in orbit around the Earth

Russia launched the first artificial satellite in 1957, and the United States’ first artificial satellite was launched in 1958.  Russian Sputnik 3, launched on 15 May 1958, was the first satellite to use solar arrays.  This was a crucial development which diverted funding from several governments into research for improved solar cells.

Three generations of solar cells

Solar Cell in Researchers Hand

Solar Cells are classified into three generations which indicates the order of which each became important.

At present there is concurrent research into all three generations while the first generation technologies are most highly represented in commercial production.

First Generation

First Generation technologies involve high energy and labor inputs which prevent any significant progress in reducing production costs.

Single junction silicon devices are approaching the theoretical limiting efficiency of 33% and achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years.

Second Generation (Thin Film)

Second generation materials have been developed to address energy requirements and production costs of solar cells.  Second generation technologies are expected to gain in market share.

The most successful second generation materials have been cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon.

Third Generation

Third generation technologies aim to enhance poor electrical performance of second generation (thin-film technologies) while maintaining very low production costs.

Current research is targeting conversion efficiencies of 30-60% while retaining low cost materials and manufacturing techniques.

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Photovoltaics or PV for short can be thought of as a direct current (DC) generator powered by the sun. 

When light photons of sufficient energy strike a solar cell, they knock electrons free in the silicon crystal structure forcing them through an external circuit (battery or direct DC load), and then returning them to the other side of the solar cell to start the process all over again.

The voltage output from a single-crystalline solar cell is about 0.5V with an amperage output that is directly proportional to cell’s surface area (approximately 7A for a 6 inch square multicrystalline solar cell).  Typically 30-36 cells are wired in series (positive to negative) in each solar module.  This produces a solar module with a 12V nominal output (~17V at peak power) that can then be wired in series and/or parallel with other solar modules to form a complete solar array to charge a 12, 24 or 48 volt battery bank.

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Solar energy is universal and will work virtually anywhere, however some locations are better than others.

Irradiance is a measure of the sun’s power available at the surface of the earth and it averages about 1000 watts per square meter.  With typical crystalline solar cell efficiencies around of around 14-16%, we can expect to generate about 140-160W per square meter if solar cells are exposed to full sun.

Insolation is a measure of the available energy from the sun and is expressed in terms of “full sun hours” (i.e. 4 full sun hours = 4 hours of sunlight at an irradiance level of 1000 watts per square meter).

Obviously different parts of the world receive more sunlight and will have more “full sun hours” per day than others.

The solar insolation zone map (below) will give you a general idea of the “full sun hours per day” for your location.

World Solar Insolation Map

This solar insolation map shows the amount of solar energy in hours (peak sun hours), received each day on an optimally tilted surface during the worst month of the year. (Based on accumulated worldwide solar insolation data.)

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