Solar Panels - an overview - Brimstone Energy UK

Solar Panels – an overview

The rated power output or wattage of a panel represents the theoretical power output under ideal conditions. This rated power output is the single biggest factor that will determine the amount of electricity that a panel will generate.
10 years ago, the average panel had an output of about 250 watts. In 2023 the average panel has an output of 400 watts.

Choosing a panel

Aside from rated output, there are two key factors to consider when choosing which solar panel to buy. This is the size of the panel and its efficiency.

Size

Panels are generally about the same size. The average size is about 1650 mm in length by 991 mm wide.
It won’t surprise you to hear that some panels deliver a higher rated output simply by being bigger. This is the cheapest way of delivering an increased output. The manufacturer doesn’t need to increase efficiency or implement improved technology. It simply makes the existing panel bigger. And, “voila” you have a higher rated panel.

Take these panels from Spanish manufacturer Eurner. They have a 340 watts version called the “Eurener 340W All Black Half-Cut Mono” and a 415 watts version imaginatively named the “Eurener 415W All Black Half-Cut Mono”. . The output of the 415w panel is 25% more than the 340w version. Some of this extra output is due to increased efficiency, but most of the extra output is due to it being a physically bigger module.

Which panel to choose?

This will depend on factors personal to you. It might be that the size of your roof is the limiting factor. Therefore simply opting for bigger panels won’t be an option.

Efficiency

There are two main characteristics that help determine efficiency: panel type and temperature sensitivity.

Panel types.

This can be broken down into polycrystalline panels, which range from 14% to 19% efficient, and monocrystalline panels which are capable of reaching efficiencies of up to 25%.

If two panels are of the same dimensions but one is 15% efficient and the other is 20% efficient, the 20% efficient panel is expected to have an output increase of 33%. Therefore, to achieve a high output monocrystalline panels are the best choice.

Most panels on the market are monocrystalline panels.

Temperature sensitivity.

The rated power output of a panel is measured at Standard Test Conditions (STC), which uses a temperature of 25°C.

Output of the panel will vary as the temperature increases or decreases away from these optimal design conditions.

Depending on the temperature sensitivity of a panel the output can drop by around 0.4% per 1°C temperature increase above 25°C and for temperatures below 25°C the efficiency will increase by 0.4% per 1°C decrease.

This means that panels are most efficient when cold and least efficient when hot. However, in most climates over the course of the year the efficiency should balance out so that the effects of temperature sensitivity are negligible.

How much energy does a single solar panel generate?

Assuming the Standard Test Conditions (STC) were met, then you could make a simple calculation to accurately estimate the amount of electricity that a single panel would generate.

For instance:

6 hours x 400W = 2400Wh/day or 2.4kWh/day

However, as discussed above the Standard Test Conditions (STC) rarely apply.

The table below shows the expected annual output for a 4kWp system in London. It assumes due south and the optimal roof angle of 35 degrees. It then varies one factor in each case.

Tilt angle (°)	Orientation (°)	Shading (%)	Annual output (kWh)
35	180	0	4,068
35	180	10	3,621
35	135	0	3,726
20	180	0	3,944

Shading

Shading can have a pretty adverse effect on the system output, as it decreases the amount of sun falling on the panel (technically referred to as solar irradiance incident).

Even if only part of the array is shaded it can have a large impact on the whole array, as all the panels in the same string will perform at the level of the worst performing panel. This is simply a factor of solar panels that are arranged in a string conforming to the electrical properties of an electrical circuit in series. Poor performance in one part of the circuit will have an effect on the whole circuit.

Most modern panels will actually have “bypass diodes” built in to try and mitigate shading within a panel and therefore across a string. This will counter-act the effect of a certain amount of shading. For instance is a single panel is shaded up to 40%, the “bypass diodes” will work to mitigate this shading, and the overall string performance will be largely unaffected.

However if the entire panel is shaded, then this will definitely affect the whole string. In this situation, if the shading cannot be designed out, then the most effective counter-measure is to use a micro inverter, such as those produced by SolarEdge.

A SolarEdge system optimises the performance of each panel individually, rather than across a string. This means that if one panel in an array is shaded it won’t affect the whole array, helping to maintain a high output.

Tilt angle and orientation

In terms of orientation, this will be governed by your house and by your roof.

Where possible a PV system will be mounted on a south-facing roof to maximise generation. If this isn’t possible an east / west configuration will still give a high output. It is only north-facing roofs which need to be avoided.

Most domestic roofs are pitched between 30 and 40 degrees, which is the optimum panel tilt angle in the UK.

Conclusion

When designing a solar system, all of these factors need to be accounted for in light of your own personal circumstances.

The amount of investment that you wish to make and the size / orientation of your roof are likely to be the fundamental factors which will inform the design of the system.

At Brimstone Energy, we listen to your needs and we do not use sales “tactics”.

Brimstone Energy – Trust, integrity, quality.