Current and future efficiency ratings of solar panels

Overall, the typical efficiency rating for a solar panel is around 16 percent, which means that of all the sunlight energy that is collected by a PV panel, only 16 percent of that energy is available as electrical power. Because there is a maximum amount of sunlight power of around 1 kW per square meter, a square meter of PV panel can only put out around 0.18 kW. There have been increases in this value, but the costs are prohibitive.

Calculating solar radiation

At sea level, on a clear day, around 1 kW of solar energy is incident on a one square meter surface (to put this in perspective, a pool pump uses around 1.5 kW's of power, and this is a lot of power). In the mountains, where the air is thinner, even more radiation is measured because much more of it gets through the thin atmosphere. However, as we all know, the sun's intensity varies over the course of a day, so it's of interest to calculate just how much solar radiation falls on a one square meter surface over the course of a day.

✓ In the summer, one can expect around 10 kWh's of energy to fall onto the one square meter surface, on a clear, cool day.

✓ In the winter, expect around 4 or 5 kWh's, under the best circumstances.

✓ When the weather is partly cloudy, in the summertime, the typical yield is around 6 kWh's.

✓ In the worst conditions, a cloudy, winter day, only around 1 kWh or less will fall onto the surface. This is why it's so cold in the winter (I probably didn't have to tell you that, did I?).

Monocrystalline solar cells have produced efficiencies of over 25 percent, while polycrystallines have achieved 20 percent. Researchers expect efficiencies will reach 30 percent, but not much higher due to the inherent physics of current technologies. This puts an upper limit on the amount of power a solar PV system will ever be able to produce on any given roof.

Efficiency may or may not be a problem. For instance, if there is unlimited surface area for installing panels (ground mounts in the middle of the desert) efficiency is of secondary concern to overall cost. In fact, the technologies that are being developed the fastest at this point put a premium on low cost, at the expense of efficiency. A new type of PV cell is being developed which is a wide, thin film that can simply be placed over an entire roof. The material, in fact, is the roof covering, and every square inch of the roof is producing power (at an efficiency of only 12 percent, maximum). Contrast this with fixed, rectangular panels that can only cover a portion of a roof.

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