Features of Solar Panels

If you can't make sense of panel datasheets, this article is for you. It builds on our Solarpedia overview of solar panels, where we categorised panels into three groups, low end, standard and premium. This is good, but it doesn't tell you specifically what features can set panels apart. This is what we'll do in this post.

There are many features that can be considered when deciding which panels you'll choose for your system. Premium panels offer features which differentiates their products from standard panels.

Performance Warranty

As a cell performance will degrade over time, the performance warranty protects against it happening too fast. The cell refers to the actual silicon squares, usually 60 or more, which sit on the panel and convert sunlight to electricity. Almost all performance warranties are 25 years long, start high and taper away with the performance of the cell. Premium warranties start higher, often on or over 97%, drop in a linear fashion, and at a percentage of no greater than 0.7% each year. Some are 0.6% or even less. Obviously the lower the better.

Product Warranty

Sunpower warranty, one of the few 25 year product warranties, not just performance.

Product warranties warrant the structure of the panel and the individual components that form the panel - like the connectors, frame, glass, backsheet etc. Issues or defects from these parts are often highly detrimental or terminal to the panels production. The length of these product warranties vary from as little as a few years to 25 years - the longer the better. Not only are premium warranties longer, they can include things like labour and shipping costs associated with replacement.

Remember, if the panel is dead but the cell is fine, no cover from your performance warranty. Don’t let this scare you, but just be mindful that the product warranty holds much more weight, so make sure you put a greater bearing on this in your decision.

Panel Efficiency

Canadian Solar datasheet showing the efficiency of their panels. The panels are the same size, which is why the higher output panels are more efficient.

Panel efficiency is the amount of sunlight a panel converts into electricity. It’s affected by the size and how much power it produces. For example, if you had a 300W panel and a 270W panel that were both the same size, the 300W would be more efficient. Higher efficiency means you’ll use less space and can capitalise on the more conducive roof areas. It's great for making the most out of those small roofs. Remember that while it’s fine to want more efficient panels, just know they may not necessarily be a need for your project.

Heat tolerance

Temperature coefficient shown on the panel datasheet of a Trina panel.

Panels work on sunlight, not heat. Heat is detrimental to the performance of panels and the temperature co-efficient measures this drop per degree over 25. For this reason, the lower the heat tolerance, the better. So if you had a panel that had a -0.4% temperature co-efficient and the panel was operating at 50 degrees, that’s a 10% drop in output. Remember, the temperature outside is almost always less than the temperature of the panel, so panel temperatures in excess of 50 degrees are not uncommon. Premium panels can have a lower temperature co-efficients.

NOCT

When a panel gets its wattage rating, it gets it from a laboratory test using Standard Test Conditions (STC). This test simulates optimal conditions for panels to produce electricity, but it isn’t a reflection of real world conditions. This is why we have NOCT, which stands for Nominal Operating Cell Temperature. This shows the performance in more realistic conditions, including lower light and higher temperatures. Take more note of a panels NOCT performance on the datasheet, as it’s going to spend much more time in these conditions once it’s installed.

Monocyrstalline or Polycrystalline

Silicon Ingot before its cut into wafers

First of all, this refers to the type of crystalline cells used, not the quality of the panel. Cells are made by creating a large silicon ingot between one a two metres long, which is sliced up into wafers which are turned into cells. Where poly and mono differ is the method used to create the ingot, with benefits either way.

Monocrystalline

Mono panel. Note the darker colour and the little diamond gaps between the cells due to cutting.

Monocrystalline (or single crystalline) was the first type of cell between the two. The ingot is formed using the Czochralski process, which results in a circular, single-crystal structure of high purity. This allows electrons to flow more easily, which makes them more efficient than poly (more power in the same space), and have a more uniform appearance. However, these ingots are more energy intensive to make and they grow in a cylindrical shape, resulting in waste when cutting them into square or near square cells to be placed on a panel. All this makes monocrystalline more expensive per watt.

Polycrystalline

Poly panel. Note the blueish colour and smaller gaps between the cells thanks to the true square shape.

Polycrystalline (or multi-crystalline) has become more popular in recent times. The ingot is formed by melting multiple crystals into a single crystal structure. However as there are multiple crystals in the ingot, electrons can’t move in a poly cell as freely as in mono cells. You can actually see the multiple crystals in the poly cell, resulting in a less uniform appearance. Despite this, it is a simpler and less energy intensive process. The mould is also a square shape, meaning there’s much less wastage in the cutting process. It’s a more efficient method in terms of kW per dollar.

Other Premium Features

Here’s a few more quick things you could find in a premium panel:

Premium aesthetics - mono have a uniform and often darker appearance, which manufacturers can also couple with a black frame. Glass on glass panels are also less noticeable.
PERC cells - Passivated Emitter Rear Contact (PERC) cells have a rear reflective layer and a improved cell design, allowing the cell another chance to capture light that was missed the first time.
Better power tolerance - Power tolerance is the allowance for grouping panels into the power class. So if a panel has a + or - 5% tolerance, you may get a panel producing 5% less. Premium panels don’t usually have a negative tolerance.
Tougher panels - This can include a few things, like higher salt tolerances, copper backed cells, better and higher IP rated junction boxes and connectors, glass on glass design etc.
Rear contact - This is where cell fingers on the front of the cell are moved to the back, allowing greater photon absorption, increasing efficiency. Sunpower and LG are two who use this in their high efficiency products.
Highly automated production facilities - Reduces chance of error and increases consistency from panel to panel. This helps the inverter harvest more and reduces the chance of a weak link.
Manufacturing Origin - Manufacturer outside of China appeals to some people.
Smart Technology - Panels can come with power optimisers or micro inverters built-in. This can have a variety of benefits, like panel level optimisation or monitoring. See our inverter video for more information on panel level optimisation.
Half cut cells - By cutting the cells in two and placing half on the bottom and half on top, shade on the bottom of the panel doesn't impact shade on the top of the panel. The lower cell current also reduces the power loss from resistance in the cell.