What to look for in a solar panel

Solar Panel Efficiency

When people mention solar panel efficiency, they are referring to the ratio of the energy in sunlight that hits the surface of a panel to the electrical energy produced by a module.  This ratio is expressed as a percentage and can be found in the panels data sheets.  As a guide, all quality monocrystalline panels will have an efficiency of 18% plus.  The most efficient panel in todays market has a 22.8% efficiency. 

General Rules of Efficiency

  • The higher the efficiency the high the cost of the panel.
  • Efficiency of a panel increase with the wattage of the panel, i.e. a 315w panel has a higher efficiency than a 300w panel and a 330w panel has a higher efficiency than a 315w panel.
  • You don’t need a 15 x 400w panels if you have enough room on you roof for 20 x 330w panels.  It’s all about the total amount of electricity you produce in an array, not the individual output of a panel.

Solar Panel Warranty

There are two forms of warranties with solar panels; The Manufacturer’s Warranty (Product Warranty) and The Performance Guarantee. The manufactures warranty varies between 10 years, 12 years, 15 years, 20 years and 25 years depending on the panel brand and wattage.  The power warranty is either 25 or 30 years depending on the panel brand and wattage. All things deteriorate as they get older. The power warranty guarantees that a panel’s power output will only deteriorate by a specific percentage over 25 years. Most panels guarantee a power deterioration of no more than 16% to 20% over 25 years.  The best performance guarantee in the market at the moment is no more than an 8% deterioration of power output over 25 years.

General Rules of Warranties

  • The longer the warranty the higher the price of the panel
  • The less the power deterioration over time, the higher the price.
  • A small panel manufacture can be as honorable and ethical as a large well-known brand, however, well known brands have spent a fortune on their brand and its image and are more likely to honor warranties
  • Does the panel manufacturer have an office in Australia? If your installer is no longer in business do you really want to deal with an entity in China, Singapore, Korea etc.?

Power Tolerance

Power tolerance is a measure of how much electrical power a solar panel can produce above or below its rated capacity at any time. For example, a power tolerance of -5%/+5% on a 100-watt (W) panel would mean the panel could produce 95 W to 105 W under real-world conditions. A 0% negative power tolerance means that the panel should always produce power equal to or greater than its rated power.

Power Temperature Coefficient

All solar cells have a temperature coefficient. As a solar panel increases in temperature, the power output of the solar panel decreases. Generally, monocrystalline solar cells have a temperature coefficient of -0.5%/degC. This means a mono solar panel will lose half of one percent of its power for every degree the temperature rises.  Solar panels are all rated at 25deg C, however, when solar panels are installed on a roof, they generally reach much higher temperatures.


Let’s say a 330W monocrystalline solar panel installed on a roof is at 65deg C

65degc – 25deg c = 40deg c

40degc x -0.5% = 20%

Therefore, the panel power loss = 20% x 330w = 66w

Therefore, the panel power = 264w.

Obviously, the lower temperature coefficient the more electricity is generate. The best Power Temperature Coefficient of any panel today is -0.258%.  Most top panel’s Power Temperature Coefficient ranges from -0.40 to -0.35. Again, the better the coefficient, the higher the price.