Solar panels are characterised by their ability to produce electricity without moving, unlike the rotating machinery found in hydroelectric facilities and power plants fired by fossil fuels. As a result, solar panels are not subject to mechanical wear, and their maintenance requirements are much simpler than those of other generation systems.

You have surely noticed that photovoltaic arrays are modular: regardless of total capacity, the basic building blocks are solar panels, which typically have a rated output ranging from 250W to 330W. However, this value represents the instantaneous power produced under ideal testing conditions, not the actual electricity output in field applications. The electricity produced by one solar panel in a full year depends on several factors:

• The physical properties of the solar panel itself, which change by product model and are determined by the manufacturer.
• Site conditions such as yearly solar radiation, the presence of obstacles that block sunlight, and the orientation of the surface where solar panels will be installed.

Since each project site is unique, you will find that identical solar systems in different locations show variations in their yearly output. Solar electricity production must be estimated for each project – there is no way to calculate solar generation without site-specific data. At best, you can estimate how much electricity the solar panel would produce in idealised laboratory conditions.

How the Properties of a Solar Panel Influence Power Production

Although solar panels can be manufactured of any size, most photovoltaic installations use either 60-cell modules or 72-cell modules.

• Although there are exceptions, 60-cell modules are normally rated below 300W, and 72-cell modules are rated above 300W.
• As you might expect, 72-cell modules are also around 20% larger to accommodate 12 additional solar cells.
• There is a common misconception that 72-cell modules are only for industrial and commercial use, while 60-cell modules are residential. However, either type of solar panel works in all three applications.

Assuming all other conditions are equal, a 300W solar module will deliver around 20% more electricity than a 250W module. However, solar panels do not perform at their rated power in actual projects, since site conditions differ from the ideal testing conditions.

Understanding the Concept of Peak Sun Hours

Before explaining how much electricity is delivered by each solar panel, first we must describe the concept of peak sun hours. Solar panels do not have the same productivity throughout the day – they are more productive around noon and less productive close to sunrise and sunset. If you multiply the rated power of a solar panel and the full duration of a day, you would be overestimating the electricity production drastically.

• The concept of peak sun hours (PSH) if a simplification of reality. If we assume a constant solar radiation of 1000 watts per square metre, how many hours would be required to have the same effect as the variable sunshine found in project sites?
• To illustrate the concept of PSH, assume a location gets 6,000 watt-hours of solar radiation per square metre per day. This equivalent to 6 PSH, even though the day lasts longer than 6 hours.
• If you find a spot that gets 2,000,000 watt-hours per square metre per year, this is equivalent to 2,000 PSH per year, or 5.5 PSH per day.

The theoretical energy output of a solar panel under ideal conditions is obtained by multiplying its rated wattage and peak sun hours. However, in actual projects, solar panels normally deliver between 80% and 90% of their ideal output.

Assuming 2,000 PSH per year and a real electricity output of 85% with respect to the ideal output, we get the following result for the 250W and 300W modules.

• 300W solar panel output = 0.300 kW x 2,000 h x 85% = 510 kWh per year
• 250W solar panel output = 0.250 kW x 2,000 h x 85% = 425 kWh per year

How Site Conditions Influence Production

The productivity of photovoltaic modules is strongly dependent on solar radiation, but you must also consider the physical conditions of each project site. Even if the site gets decent sunlight, some factors may prevent solar systems from operating at peak output.

Finding a spot where the sky is completely clear is very difficult – you would have to be in the middle of the ocean. In land-based locations, there are natural features such as trees and hills, and artificial obstacles like buildings and unipole signs. These obstacles block a portion of the available sunshine, so you cannot assume that PV panels will get all the solar radiation available.

The orientation of solar panels is also very important, since the incident solar radiation changes based on the direction they are facing:

• For example, if you have a solar array facing east, productivity is lowered in the afternoon when the sun is in the west
• Also consider that Australia is in the southern hemisphere, and the sun is normally in the northern half of the sky. As a result, north-facing solar arrays are more productive than south-facing installations. The opposite applies for countries in the northern hemisphere, like the USA and Canada, where the ideal orientation for solar panels is tilted to the south.

Also keep in mind that the sun’s position in the sky changes throughout the day, and solar panels are more productive when they face the sun directly. This is only possible with a tracking mechanism, but in most cases a better option is simply making the solar array larger – trackers can be cost-effective when your space for solar panels is limited.

Solar power companies normally use software to simulate factors such as solar module orientation, moving shadows and the sun’s position in the sky. The calculations described described in this article only provide a broad estimate, but they can give you a general idea of what to expect. In general, Australia has excellent conditions for solar power – the landmass gets enough sunshine to meet the country’s energy demand 10,000 times.

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