When you decide to hire a contractor to install solar panels in your property, one of the things he will discuss with your is their efficiency (also known as conversion rate).

Solar panel efficiency measures what amount of sunlight hitting the panel is converted into electricity for use in the household. All photovoltaic (PV) modules have such measurements in their labels.

Due to the ongoing advances in solar cell technology, the highest efficiency solar panels can reach almost 23 percent efficiency. But the average efficiency of solar panels falls between the 15 to 18 percent efficiency range. 

The solar panel efficiency is determined by the total panel size, cell layout and panel design. This is based on the number and type of cells used, the busbar design and interconnection of the cells. Even the color of the panel protective backsheet can effect efficiency as a black backsheet absorbs more heat which reduces cell efficiency.
Picking panels with the highest efficiency is pivotal, since it would mean less modules and more power for your home. But that’s not all you need to pay attention to. 

How is efficiency calculated?

Solar panel efficiency is measured under what is known as Standard Test Conditions (STC) in a lab. The test involves throwing 1,000 watts of irradiance (light) per square feet on the panel at 77 degrees Fahrenheit. All manufacturers must produce panels that meet or surpass their advertised wattage under STC.

But real-world conditions rarely fall into these perfect scenario, so the efficiency of the solar panels you get varies. External factors such as orientation, location, time of year, shading, dirt, and cell temperature influence how much power is derived from the solar panels. 

The National Renewable Energy Laboratory (NREL) found in a study that solar panel output falls by an average of about 0.8% annually. In other words, after 1 year, your solar panel will produce 99.2% of the energy it did when it was new; after 10 years it will produce 92% of the energy it did when it was new. Degradation is inevitable and occurs over time, but some panels are able to continue to function at higher efficiency over time than others.

For this reason, it’s also important to consider the manufacturer’s power warranty.  This guarantees a minimum amount of power for the first 5, 10, and 25 years of the panel’s life. If a panel output is lower than promised, the manufacturer will replace, repair or reimburse you for the panel.  

More efficient panels

There are two types of solar panels: monocrystalline and polycrystalline. Because of the higher silicon content used in the design, monocrystalline (black color) panels are more expensive, but they tend to have a higher level of efficiency, so fewer panels are needed in most roof installations to meet the same power requirements. 

With lower amounts of silicon utilized in polycrystalline, or multi-layered, silicon panels (blue hue color), they are often more affordable. They also come with a design to help reduce this efficiency loss.

You can help improve the efficiency of your solar panels. Cleaning in addition to what is done naturally by rain will boost panel output, since on average they will have even less dirt on them.

A study conducted recently found out that cleaning solar panels only improved their output by one percent. This may not seem like much, but if you live in a particularly dusty area or have a lot of pigeons that like to stick around your PV modules, you could be losing a lot more by the accumulation of bird feces and other dirt. 

The inverter

This device, which converts the DC (direct current) electricity generated by your panels and transforms it into AC (alternating current) to be used by your appliances, also has an effect on efficiency.

Microinverters and power optimizers that connect directly to each panel rather than the entire array (like the string inverters) boost efficiency. This is because panels in a string will perform at the rate of the lowest-performing module. 

Other factors that influence solar panel efficiency is the design of the array itself. If your solar panels are not set up correctly (in a true south direction in the Northern Hemisphere), it is possible that they will not produce enough electricity to meet your usage needs. Also, you want to make sure you get the maximum sun exposure. You can achieve this by eliminating any shade or obstacles preventing your panels from getting direct sunlight. 

Even temperature plays a role. Higher temperatures can reduce output and lower efficiency. This is more evident in Polycrystalline solar panels that tend to have slightly lower heat tolerance than monocrystalline solar panels. This technically means that they perform slightly worse than monocrystalline solar panels in high temperatures. 

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