A solar panel requires light from the sun to generate electricity. One solar panel is made up of many photovoltaic (PV) cells – usually 60 cells on a modern rooftop solar panel. When photons from the sun land on a PV cell, a chemical reaction in the cell liberates electrons, causing them to flow and create a DC (Direct Current) electric current. This DC current is fed into a solar inverter that converts it to an AC (Alternating Current) electric current at mains voltage to power any standard mains equipment; Mains voltage is 110V in the US, but varies from country to country.
To achieve maximum output from a solar panel, it needs to be facing directly towards the sun, with no shading or cloud cover. However, solar panels will still generate electricity when they’re not in direct sunlight, albeit at a reduced level.
To some extent, solar panels can even generate power at night time powered by sunlight reflected from a full moon, however, the amount is fundamentally insignificant.
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Solar Panel Rated Output and How it's Calculated
To be able to fairly compare the output of one panel with another, each panel has to be measured with a standardized amount of sunlight falling on it. Panel manufacturers around the world have agreed to standardize on a sunlight concentration of 1000W/m2 and each panel’s rated output is measured using this level of illumination.
This full sun value is an average value for means of comparing panels only. The maximum amount of solar illumination that can fall on a panel varies significantly depending on where on the earth it is situated, and what time of year it is (thus what angle the sun is at in the sky).
Depending on location, time of year and atmospheric conditions, actual sunlight concentration can exceed 1000W/m2 – meaning it is sometimes possible for panels to generate more than their rated output. However at the North and South poles, even during the summer, sunlight concentration never reaches 1000W/m2, and so a solar panel at the poles will never achieve its rated value, even in full sun.
Note that this standardised sunlight illumination value has nothing to do with the size of the panel itself – this 1000W/m2 is just the concentration of the incoming solar rays.
Factors that Affect Panel Efficiency
If one solar panel is twice the size of another solar panel, the larger one will generate twice as much electricity, but this tells us nothing about the efficiency of each panel.
Panel efficiencies are measured as a percentage – the amount of solar energy landing on the panel that gets converted into electrical energy. The higher this percentage, the better the panel – regardless of its size.
The most efficient commercially available solar panel available at time of writing is the Maxeon Sunpower 450W panel, which has a conversion efficiency of 22.2%. This means that for every 100W of sunlight energy that lands on the panel, 22.2W of electrical energy is generated.
Effect of Temperature on Efficiency
Solar panels decrease in efficiency the hotter the PV solar cells become. The he rated output of every solar panel is measured at an ambient temperature of 25 degrees Celsius. If the ambient temperature goes below or above 25°C, the output of the panel increases or decreases accordingly. This amount by which the output power varies is called the temperature coefficient. Different panel types will have different temperature coefficients.
A modern solar panel will have a low temperature coefficient in the region of 0.3%/°C. This means that for every degree above 25C, the panel’s output will drop by 0.3%, given the same amount of sunlight. For example, a panel with a rated output of 300W and temperature coefficient of 0.3%/°C will generate only 295.5W when the ambient temperature of the panel is 30 degrees Celsius.
Different Panel Technologies
Silicon is the base material in the manufacture of all commercially available solar panels; But the way they are manufactured varies. The two primary types are called monocrystalline silicon and polycrystalline silicon, commonly abbreviated to just ‘mono’ or ‘poly.’ This refers to the way in which the silicon crystal wafers within the PV cells are grown; Mono solar cells are more efficient than poly, but are more expensive to manufacture.
Another variation is called the heterojunction intrinsic thin layer PV cell, commonly abbreviated to HIT. This refers to the way in which the silicon crystals are assembled. HIT cells are more efficient than non-HIT cells because they are able to generate a higher percentage of their rated output at lower illumination levels.
For example, on an overcast day, a 300W HIT panel may produce 150W, whereas a non-HIT panel would only be able to produce say 120W in the same situation.
Panel Mounting angle
To obtain the maximum amount of electricity from a solar panel, it ideally needs to be mounted such that it faces directly towards the sun – perpendicular to the angle of incoming sunlight.
This angle depends on whereabouts on the globe the panel is situated. The further away from the equator, the steeper the angle will need to be to keep the panel facing directly towards the sun (see diagram below). The closer to the equator the panel is, the more flat it must be mounted to directly face the sun.
If the panel is situated exactly on the equator, it should be mounted completely flat to achieve maximum output power.
Solar Tracking Systems
The sun moves through the sky from sunrise to sunset each day. For maximum output, solar panels should face east in the morning to align with the rising sun, and then track the sun’s position as it moves through the sky from east to west.
Solar tracking systems are fairly common, but the improvement in efficiency is variable and dependent on the ambient temperature of the location where the solar panel is situated. For example, in a cold city like Berlin, Germany, an efficiency improvement of 39% can be achieved with a solar tracking system, whereas an efficiency improvement of only 8% is achieved in a hot city like Aswan, Egypt.
Also, the tracking system itself requires electricity to power it, thus decreasing the efficiency gains by about 5-10%; Meaning, solar tracking is not viable in hot locations, as the tracker uses more electricity than is gained.
Panel Mounting Direction
In the Northern hemisphere, solar panels should never be mounted on a north-facing roof. This is because from sunrise to sunset, the sun remains on the southern side of the sky so a north facing roof will remain in shade all day.
The opposite is the case in the Southern hemisphere – solar panels should never be mounted on a south-facing roof, since the sun is always in the northern side of the sky.
If you have a roof with east- and west- facing elevations, it is still beneficial to install solar panels, since the east-facing panels will be facing the sun in the morning, and the west-facing panels will be facing the sun in the evening.
Although this configuration is not quite as efficient as panels mounted on a south-facing roof. Studies have shown that 6 east facing + 6 west facing solar panels generate about 80% of the power of 12 panels all mounted on a south-facing roof.
Solar Generation at Night?
Solar panels can generate at night time, powered by sunlight reflected from a full moon. However, the amount of photons reaching the panel is greatly reduced, and studies have shown that the output of the panel is only 1/345th of its normal rated output.
So, for example, a 3450W solar array would generate only 10W. This is below the switch-on level for solar inverters, and so no AC mains electricity would be generated. However, if the DC output of the solar array was connected directly to a DC LED lighting system, it could feasibly power a few small LED lights.