Solar Energy Efficiency 101



Solar energy efficiency maxes out at about 15%. I can hear you screeching, 15%? That's all the efficiency I get from an investment of thousands of dollars of state of the art solar equipment?!! At 15% is solar efficiency high enough to make a solar power plan in my home sensible?” But before your head explodes, consider this. Mother Nature is way less efficient. Green plants take the sun's rays and transform them into chlorophyll. They use this sweet chlorophyll as a food source. And they only convert the solar rays that hit them at a miserable 3% solar energy efficiency rate. (I know, they add H2O and CO2, but you get the idea.) And they support themselves rather well, as well as you, me and all the animals that eat them.

But is that really enough for our needs? While solar energy efficiency rates of over 25% have been reached in perfect lab situations, most commercial solar panels are right at the 15% mentioned above. Out of every 100 electrons they receive, they convert 85 to energy, and lose 85 to “who knows where” in the form of heat or lost energy reflected back into our atmosphere. And as it turns out, that is more solar energy efficiency than we will ever need. Most residences can harvest enough to supply all their practical needs from a 100 or 200 square foot solar PV array. If you covered the top of a small tool shed with collectors, you'd have plenty.

But here is a handy output formula for those math lovers out there.

(PV Array Wattage) x (Ave. Hours of sun) x 75% = Daily Watt-Hours

Average hours for your area can be found at http://rredc.nrel.gov/solar/pubs/redbook/ . This figure runs from 4 to 5.5 hours in most major U.S. cities, based on National Renewable Energy Lab data from 1960 to 1999.

Now, let's use some national averages, and a common residential solar system setup to crunch the numbers.

We will take a SMA Sunny Boy 2500 inverter and couple it with eighteen Sharp 165-watt PV modules. Let's settle down in lovely Sacramento, Ca. for our example, where 5.5 average hours of sun per day falls to earth. The PV Array Wattage is obviously the total watts in our PV system, so we multiply 165 watts per module times our 18 modules, and we get 2,970 watts total.

Now let's employ an “adjustment multiplier” that accounts for high and low estimates, weather, humidity, some shading and a zillion other “what ifs.” We'll use 75%. Plug in the formula above and you get 2,970 watts x 5.5 hours x 0.75 which equals 12,251 watt-hours. Transfer this to a measurement we know, kilowatt hours, and we get 12.25 kWh per day. This is an annual average, and will be higher in summer and lower in winter.

After testing in real world situations, we know this to be a reliable estimator for your U.S. situation, barring extremes like shading, excessive temperatures and other meteorological extremes. When employing this formula you will have a conservative estimate. Most solar power customers report a pleasant increase in production above these figures. Long term Redbook charts use an “uncertainty plus 9%” factor, so your true numbers will vary, but using this system gets you within about 5% true production.

Now let's look at how solar energy efficiency is dictated by PV cell structure. A PV module is simply an assembly of individual PV cells that are run in parallel and connected in series to form a larger panel. This provides optimum performance and solar energy efficiency. The total amount of energy that a module can produce is the product of the size of the panel and the amount of sunlight that hits it. A common residential module will put out about 200 watts of usable power.

The most important part of the cells that make up the module is their silicon composition structure. Single crystalline cells can be used as is, or cast into polycrystalline structures by combining several. And these may used as is as well, or then covered with a thin film known as amorphous silicon.

Single crystal cells are more efficient and more expensive than polycrystalline models, who in turn have greater solar energy efficiency, and a higher price, than amorphous cells. Monocrystalline cells reach 15% to 20% efficiency, and will cost you around $3.48 per watt on average. Polycrystalline cell modules operate at 12% to 15% efficiency and cost approximately $3.29 per watt, and amorphous technology can only reach 4% to 14% solar energy efficiency, and costs about $2.50 per watt. The amorphous technology is seeing technological advances in leaps and bounds, and as a result is cheaper all the time.

Now that we understand cell structure as it dictates efficiency, let's look at the four main factors that predict or influence solar energy efficiency, regardless of cell structure. A module's 1-V curve, amount of incidental sunlight, cell temperature, and shading are the most influential factors in figuring a PV module's output.

1-V curve simply expresses a module's current versus voltage ratio. Modules produce better when paired with a load which resembles them. Hooking up a small solar cell from a garden light would hardly power a water heater's energy needs. This is an example of a very low 1-V curve.

Incidental sunlight is known as the amount of the sun's light that reaches a module. This base indicator dictates the high end possibility of what a module can produce in best case scenarios. A module can not harness more sun, or incidental sunlight, than it receives, so this number is important to max out. Proper placement of your panels is vital to getting solar energy efficiency right in this part of the equation.

PV cell temperature is also extremely important. PV cells don't operate well in high heat, and function more efficiently at cooler temperatures. Heat causes this low performance level in all electronics, and is why we are told not to leave our cell phones in our cars. It may seem exactly opposite to common solar power sense, but just remember that sun LIGHT and not sun HEAT are what we are collecting. The same power we are trying to harness is very powerful in its extreme.

Finally, shading has an obviously negative effect on a PV module, and the solar energy efficiency of the unit. If a module is shaded only 5%, it can lose up to 50% of its output.

So, we have learned that at 15% solar energy efficiency, and a 100 to 200 square foot solar cell array, hooked to the appropriate inverter, we can replace about 40% to 50% of the common household's electrical needs in the U.S. with average sunlight of 4.0 to 5.5 hours per day. So to answer the question, “Is solar energy efficiency high enough to make a solar power plan in my home sensible” the answer is a resounding YES!


Back to Solar Energy Facts
From Solar Energy Efficiency Back to Home Solar Power Guide