Looking for the Best Solar Panels we have previously identified large global manufacturers and popular UK manufacturers as key selection criteria. However, solar panels do also differ on technical characteristics. Therefore to help consumers find the best solar panel we present 7 Criteria of High Quality Solar Panels. 

1. Poly or Mono

The average performance of Poly or Mono panels is very similar. Mono-crystalline panels are cut from a circular tube of purified silicon material, resulting in rounded corners of each cell. Poly-crystalline silicon cells are covering the entire surface of the panel, but you can see the shiny crystal-like sub-facets of the material. Hence, aesthetic preferences play a role in deciding between poly and mono. Furthermore, Mono panels tend to perform better in low light conditions and at high temperatures (see below), although the actual performance on these criteria differs by brand.

2. Normal (15%) or High (20%) Efficiency

Efficiency is the amount of energy that a solar panel transforms into electricity per square meter. Standard testing conditions state that 1.000 kWh falls on one square meter in a year. Solar panel output given a certain efficiency is expressed in Watt-peak (Wp). This means a normal (15%) efficiency panel panel would produce about 150 Wp per square meter and a high efficiency (20%) panel about 200 Wp per square meter. The difference between these two panels is that the same system would take less space (in square meters) on the roof if high efficiency panels are used. However, it would not produce more electricity since the system size is the same. Popular high efficiency panels are Sanyo (Panasonic) and SunPower.

3. Small (200wp) or Large (250wp)

Small or large panels don’t differ in quality. A 4kWp with 200wp panels requires 20 panels, while a 4kWp with 250wp panels requires 16 panels. However, the total area covered by the solar panels in square meters will be the same for the two systems. Most installers prefer larger panels, as it means fewer panels to carry up and mount on the roof. The preferred panel size is 250wp, which perfectly converts to 16 panels for a 4kWp system. At 300wp you would only get 3.9kWp (13 panels) while many consumers are looking for exactly 4kWp to maximize Feed-in-Tariff returns.

4. Power Tolerance

Power Tolerance refers to the actual power output of the solar panel relative to the watt-peak rating. For example a rating of +/-3% means a 200 Wp panel can actually be between 194 (-3%) and 206 (+3%) watt-peak in real life. At the same time a rating of 0/+5% panel means a 200 Wp panel is between 200 and 210 (+5%) watt-peak in real life. This second panel has ‘positive power tolarance’ of on average 2.5% more electricity. For a 4kWp system at a price per Wp of 2.00 pound/wp this would mean it’s worth paying about 100wp times 2pounds = 200 pounds extra for such a system.

5. Weak Light Performance

Given that panels are rated under standard testing conditions (STC), their actual performance depends on the actual level of sunlight. The impact on strong light (hot days) is explained in the next paragraph. Under weak light, your panels are expected to produce less output – in winter only about 10% of output is generated, while in summer it can be as much as 40%. However, in many European countries the normal light intensity is less than the (STC) standard testing conditions where solar panels are rated at, which makes weak light performance important. Mono and hybrid (HIT) panels are known to perform better under real-life weak light conditions.

6. Strong Light Performance: Temperature Coefficient (Pmax)

Also called the ‘Temperature Coefficient of Pmax’ or simply ‘Pmax’, this is expressed as the percentage of power output drop for each degree increase in panel temperature above 25 degrees Celcius. Since solar panels heat up quickly, on very hot days the temperature of solar panels can easily reach as much as 75 degrees Celcius. If you have a Pmax of -0.5% this means a power loss of no less than 50 times 0.5% which equals 25% of the potential output! Hence when choosing a panel you can limit losses with a low temperature coefficient. As a general guide, a 0.1% improvement will mean 5% extra outputs on hot days. Since not all days are hot days, this tends to be less than 1% extra output on a yearly basis and hence is worth about 80 pounds for a 4kWp system.

7. Test Results

The testing of solar panels is difficult, and strongly depends on the conditions used (e.g. weak light, strong light, orientation, shading etc.). This is why the industry uses the STC rating to compare apples with apples. That this is not the same as reality does not matter, as that reality is very different in California (where the use a so-called PTC rating instead) than in Germany. Hence please be careful when interpreting test results of ‘best solar panels’. A good source for relevant test results to use is Photon Magazine (focused on Germany), although subscription is expensive. In general the guidance is to select a well-priced panel from a large manufacturer to have peace of mind about future warranty. Technical details like power tolerance and Temperature Coefficient do impact your results, but should be considered only after making an initial shortlist based on price and manufacturer brand.