When you review the technical specifications of solar panels, their efficiency values may give a negative first impression. As of 2021, monocrystalline solar panels are in the range of 20-23%, while polycrystalline panels are below 20%. However, conversion efficiency is not a critical factor when using sunlight - a free and virtually limitless resource.
On the other hand, when the energy inputs for power generation are fossil fuels, efficiency becomes much more important. Coal power plants are typically 30-40% efficient, while combined cycle gas plants are around 50-60% efficient.
Solar panel efficiency is only an issue when space is limited, since you may not reach the planned capacity in kilowatts. The solar panels used in commercial and industrial roofs have typical dimensions of around 80” x 40”, and the most efficient models exceed 400W. If you want to install 400 kW, you need 1000 - this represents more than 22,000 sq.ft. for the solar panels alone, plus the spacing required for maintenance purposes.
The top solar manufacturers are constantly investing millions in research and development, with the goal of improving their photovoltaic cells. However, this has paid off over time:
The efficiency of solar cells is determined by the photovoltaic material, and also how the module is wired and designed overall. Monocrystalline cells are more efficient, since they’re made of single silicon crystals of high purity. On the other hand, polycrystalline cells are made of multiple crystals, and this impacts their efficiency.
This does not mean that polycrystalline solar panels have a lower quality. They have a lower conversion efficiency due to their material properties, but there are high-quality solar modules of both types.
Many of the top-performing panels that are currently available use PERC technology, which increases productivity by up to 12%. PERC stands for Passivated Emitter and Rear Contact, and these cells have many design features that boost their efficiency.
PERC cells are often cut in half to reduce their electrical resistance losses, further increasing their power output. You will notice that many solar panels are now using 120 or 144 half-cells, instead of the traditional 60-cell and 72-cell designs. Since solar cells are cut in half, their total space requirements are almost the same and panel dimensions are not affected significantly.
As the efficiency of photovoltaic cells continues to increase, solar panels will be capable of generating more watts per square foot.
Generally, having space for solar panels is not an issue in buildings like shopping malls and distribution centers. However, multi-story buildings have a large floor area relative to their roof area - and plenty of the available space is already used by HVAC equipment. In these cases, high-efficiency solar panels will maximize the kWh output per square foot covered.
R&D has also reduced the cost of solar panels. You could expect to pay more than $10,000 per kW of capacity 20 years ago, but now home solar systems cost less than $3,000 per kW in many parts of the US. Even lower prices are possible in the utility sector, where solar arrays are being installed for less than $1,000 per kW.