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Evaluating Solar Photovoltaic Panels & Modules - Which solar panels to buy?

In this article we offer a broad overview of the types of solar PV panel that are available, the materials they are made from, explain what some of the data on the manufacturer specification sheets means and ask some questions, the answers to which will hopefully make your choice a bit easier.

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What are Solar PV Panels Made of?

The primary material needed to make a Solar PV panel is silicon, how the silicon is engineered and treated gives rise to three different types of Solar Photovoltaic Panel, with different prices, uses and levels of performance.

- Monocrystalline PV Panels:

A Monocrystalline panel consists of a number of solar cells connected together to form a grid. Monocrystalline Solar cells are made using thin wafers of silicon cut from a single crystal. Silicon wafers are expensive to produce but are very efficient conductors. When comparing solar panels a monocrystalline panel is likely to be the most efficient solar panel in terms of energy generation per sqm or per panel.

- Polycrystalline (Multicrystalline) PV Panels:

A Polycrystalline panel is made in much the same way as the Monocrystalline panels mentioned above except the wafer is cut from a block of silicon crystal made up of many crystals. A Polycrystalline Panel will be slightly less efficient and slightly cheaper to buy then Monocrystalline.

Comparing Solar Cells

How to tell the difference

The blue coloured cell on the left hand-side is a Polycrystalline PV Cell. The darker coloured cell is a Monocrystalline PV cell.

- Amorphous Cells (Thin Film):

A Solar PV panel made from Amorphous cells need not necessarily be built into a panel at all Amorphous Cells are sometimes used in roofing materials, Amorphous Cells are less expensive to produce but their efficiency is lower then their crystalline counterparts. Amorphous cells are made by spreading a thin film of silicon onto a material, amorphous cells also have the benefit of being flexible.

Peak Power (kWp):

With Solar panels the maximum output power is measured in watts, generally speaking the larger the panel the more solar cells you can fit into it. An important point that needs to be understood is that UK installations very rarely if ever perform at peak performance.

A very useful tool provided free by the European Commission Joint Research Centre can be used to calculate likely solar yields on PV systems in the UK and around Europe, calculating the likely kWh from quoted kWp figures:

Power Tolerance:

Power tolerances quoted on datasheets (+/‑3%, +/-5%, +/-10% etc) refer to the power difference a manufacturer will allow a solar panel to deviate from the declared kWp figure. For example a 200Wp panel with a tolerance of +/-3% could test from 194Wp to 206Wp. A +/-10% tolerance could mean a panel could test as low as 180Wp and still ship as a 200Wp panel. A low power tolerance is usually a sign of concientous manufacturer and a quality solar panel.

Voltage at Maximum Power (Vmp):

The Voltage at Maximum Power (Vmp) is a unit of measurement that shows us the maximum voltage possible within the solar panel during normal operation (for non electricians, think of the pressure of push being applied behind the current).

When solar panels are wired together in series as illustrated below:


The maximum voltage of the string (the line of solar panels connected together in series) increases for every solar panel that is connected into it. The Vmp figure is then used to specify the correctly matched power inverter. The Vmp of the string needs to be high enough for the inverter to start but low enough so as not to damage it.

Current at Maximum Power (Imp)

If as above Voltage is the push, than the Current (I) is what is being pushed, Current is measured in Amps. The Current at Maximum Power (Imp) is a measurement that tells us the maximum number of Amps in normal operation that an individual panel can generate.

When solar panels are wired together in series than the Current of each string will remain the same, at the level generated by a single solar panel. If solar panels are wired together in parallel (positive - positive) than the Voltage will stay the same whilst the Amps will increase. This may be beneficial in situations such as when using batteries where power can be presented as low Voltage and high Current.

If a number of strings wired in series is to be combined at the inverter than the Amps of each string are added together as below:

Imp (String A) + Imp (String B) = Total Amps

You will need to do this calculation to ensure that the inverter is suitably sized to efficiently handle both the maximum voltage generated by each string and the maximum current (Amps) that the combined strings generate. The more close the match between what the PV strings are generating and what the inverter can hande when operating at it's most efficient, the better your overall power yields will be.

Short Circuit Current (Isc)

The Short Circuit Current (Isc) is the maximum current that could be generated if Voltages were at zero. When specifying inverters and cables close attention needs to be paid to the Short Circuit Current. For safety we use the following calculation to ensure that should a fault occur the inverter and cables can handle the potential Current in fault conditions.

Isc x No. of Strings connected in parallel x 1.25

Open Circuit Voltage (Voc)

The Open Circuit Voltage (Voc) is the maximum voltage that could be generated at zero current. The safety calculation we use is:

Voc x No. of panels in a string (connected in series) x 1.15

This figure tells us the maximum figure that the proposed inverter should be capable of handling and the minimum size of the cables (the diameter/thickness) we need to use, to avoid either being damaged in a fault.

Mounting Area:

If you got the chance to use the Solar PV Calculator we linked to above you will see that factors such as the height above sea level, the tilt inclination of the panels and the location of the installation relative to the equator all have an impact on a panels performance.

You now need to specify what panels are most suitable based on the mounting area you need to work with. You could be limited by factors such as chimneys, shading, skylights or dormer windows. This factor along with the actual space available and your power requirements may well determine the size number, type and style of the panels you need to get.

Framed Solar PV Panels

Framed Solar PV Panels

Framed solar PV panels are best employed when mounting panels on top of an existing roof or when being mounted on a stand alone system.

PV Panels are usually fixed onto either a flat aluminium frame or rails. The frame or rails are then secured to the roof. Please see On-Roof systems for more info.

When mounting PV panels onto existing properties framed solar PV panels are usually cheaper and quicker to install as no major structural roof work needs to be carried out.

If you are installing this type of system we have written a bit more about how we carry out this kind of installation here: Retrofitting Roof Mounted Solar Photovoltaic (PV) Panels.

Frameless Solar Panels

Framess Solar PV Panels & Laminates

Frameless solar panels are best employed when integrating the solar panels into the roof, mounting directly onto roof battens, rafters or purlins replacing normal roof tiles or slates.

We would then frame the panels using matching materials such as tiles or roof slates. Please see Integrated Mounting Systems for more info.

Frameless Solar PV Panels offer a flush finish and form part of the roof. An attractive, cost effective option for new builds and when re-roofing.

Planning Permission:

Installations above a certain size and in certain locations such as in national parks and world heritage sites will need planning permission. Planning permission is not usually required for residential properties unless then installation is over 4m high (unless on a roof, then no bigger than the roof and no higher than 200mm above the roof line). There is more about planning permission for solar PV systems here: Planning Permission for Solar Photovoltaic Systems.

Guarantees and Warranties:

A Solar PV system is likely to be a big investment, if an individual panel doesn't pay for itself within the guarantee period and fails soon after the warranty expires, then financially the investment just wont make sense. In our exprience the guarantees supplied by the manufacturers can be relied upon as a good indication as to the quality and likely lifetime of the panel. It's often worth paying a little extra for a longer warranty.

Free Solar Panel Quote & System design

Free Solar Panel Quote & System Design Service

Now you know what's possible, why not use our free quote and system design service and generate a bespoke solar panel system design and quotation?

Quotations and System Designs are provided Free of charge based on aerial photography and the information that you provide.

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