There are hundreds of different

solar PV panels

on the market – you can see this if you go to the MCS website and search for the certified products under

Solar PV

. Not each panel is the same, but in the same manner as you have different types of cars, they all do very much the same thing!


There are two main types of make of PV cells on the market:

Mono: called as such as the panels are made from Monocrystalline Silicon. These are the most efficient types of cells but are more expensive.

Poly: called as such as they are made from Polycrystalline Silicon. Cheaper to produce but are also less efficient.

There are also PV cells made from Amorphous Silicon, but these make thin-film cells which are not used for solar PV installations on properties.

The cells are added together to make up each panel. A typical Solar cell will be no bigger than an A5 sheet of paper. Putting a few cells together creates a PV panel (putting panels together creates a PV Array – but more on this later).

When we talk of cell efficiencies, we mean how much energy they actually capture from the sun. The way this is measured is by testing every cell under what are called “Peak Test Conditions”. These conditions are 25 degrees temperature of pure sunlight. The amount of energy captured from 1 square meter of sunlight (i.e. light put onto 1 square meter of cells) is then measured for each square meter of cells. Once this is known you get the Wp (Watt Peak) of a square meter of a cells. This is the number of Watts produced under Peak conditions.

However, panels not only differ in make (i.e. Mono and Poly) but they also differ in size. Below are the typical panel sizes available on the market:

993mm * 1685mm * 42mm

1001mm * 1495mm * 42mm

994mm * 1318mm * 46mm

Thus, the number of cells and their Wp in each panel will differ, as the area/size of each panel differs (note none of the above panels are one meter squared!). To overcome this the manufacturers give each panel a W (Watt) rating – i.e. the number of Watts the PANEL will produce under peak test conditions. Typical panels come in the following W sizes:

170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250

As you can see, the “weakest” panels are 175 with the most powerful being 250W.

Different manufacturers make different types of panel. Some manufacturers include:

  • Mitsubishi
  • Romag
  • Sanyo
  • Schott
  • Schuco
  • Sharp
  • Yingli

Thus you may hear people talk about the Sanyo 245’s or Mitsubishi 210’s. All this means is that the Panel is made by Sanyo or Mitsubishi and is 245W or 210W.

To create a system – or something we call an “array” – you simply decide how many panels you want / can afford / can fit on the roof. The weaker panels (170) are much cheaper than the strongest (250W). Some panels are smaller in size, so you can use more of them if you are limited to the amount of space you have. Choosing the right panel depends largely on three factors:

–          How much roof space is there?

–          How much are you willing to spend?

–          How much energy you need/want to produce?

Some clients will be more limited by the amount of roof space, and thus will be focused on the highest power panel that will fit in a certain area. Others will be more concerned about cost and will therefore look towards the cheaper, less powerful panels. Those with unlimited space will be more concerned with using the most powerful panel on a cost efficiency basis. Cernunnos considers all these factors when selecting the right panel for a client. We work with over 100 panels and select the best on a cost/performance ratio. We work out the cost per panel per Watt per meter squared – giving us the best efficiency panels on the market.

Let us assume you want the strongest array you can get. Therefore, we work out you can fit 10 250W panels on a roof (Cernunnos do this on the site survey by measuring the roof). This would give a Wp system size of 2500Wp – or 2.5kWp (Kilo Watt Peak). That is, under peak conditions, the system would produce 2,500 Kilo Watt Hours per annum. However, the UK weather does not allow for Peak conditions. Thus we have to adjust for this and we do this by estimating how much the system will produce. There is an industry standard method for estimating how much a system will actually produce and it is called SAP 2005 (SAP = Standard Assessment Procedure). SAP is used all over the building industry to measure all sorts of things, and one of them is how well a Solar PV system will perform. It is a country wide standard, with the base case city being Sheffield. Thus, in places that get more sun than Sheffield (i.e. the South of UK) SAP underestimates a system performance, and vice versa for northern UK. However, under the MCS regulations, we MUST produce a SAP figure when selling a system to a client. We can inform clients that we think the system will outperform, and give them a figure of how much – but we must always give the SAP figure and explain what it is.

SAP takes into account the azimuth of the system (i.e. which way it faces: South, East or West etc); the angle of the system (i.e. the angle of which it is on the roof – an ideal angle being between 30 and 45 degrees); and how much shading there is (shading can affect a systems performance significantly). In general, SAP estimates that the actual output for a system will be approximately 86% of its kWp size. Thus a 1kWp system will produce 860kWh of electricity in any year in the UK.

A Solar PV system is not only made up of PV panels. There are also Roof Mounting Rails, Inverters and Generation Meters. The Roof Mounting Rails are exactly what they imply: rails that fix the panels to the roof. The rails fix onto the roof rafters, and sit on top of roof anchors which fit underneath the tiles, meaning that no roof tiles need to be cut or removed (this means the roof keeps its water tightness after the panels are fitted). They are aluminium frames that are cut to size for the system.

The generation meter is a small meter that measures how much electricity is actually produced by the system. It is a simple bit of kit and is generally installed next to the “consumer unit” (the electricity box that has all the trip switches). It only measures how much electricity is produced – it doesn’t measure how much electricity is used or exported – you need a Smart Meter for this.

The Inverter is probably the most important bit of kit after the Panels. Solar PV Panels produce DC electricity – which is a very dangerous type of electricity. The UK uses AC electricity and thus the Inverter converts the Solar PV electricity from DC into usable AC electricity. The inverters must be sized to the system size – so for a 3kWp system you must get a 3,000 inverter (they come in all sizes). It will normally be installed in the loft of the client – out of the way, and is about the size of a shoebox.