How to Find the kWp and Efficiency Rating for any Solar Panel

Each manufacturer of PV panels provides a data sheet, which will specify the kWP or "rated" amount of power the solar panel will produce.

Different manufacturers in different countries use different terminology, for example in the Sun Power 315 data sheet ((manufactured in the USA) that you can download by clicking here , you will note that under the section labeled "Electrical Data", the top number is lableled Peak Power in the first column and Pmax in the second column.

In Germany, you might see the same information referred to as Nominal Power or Pmpp. In the solar industry, the peak power rating of a panel is frequently abbreviated as kWp, which I also use in this website.

It's sort of looking at the horsepower rating for a car ... in that they both give you the basis for understanding the amount of power that can be produced under ideal conditions, but in general the higher the number the more power the panel will produce.

Standard Testing Conditions

What does this number mean and how was it calculated? The nominal power ( Peak Power or Pmax) of a photovoltaic module or solar panel is determined by measuring current and voltage while varying resistance under defined illumination.

The specific testing conditions are specified in standards such as IEC 61215, IEC 61646 and UL 1703; specifically the insolation level is 1,000W/m2, with a spectrum similar to sunlight hitting the earth's surface at latitude 35°N in the summer (i.e., perpendicular to the solar panel being tested). A standard airmass of 1.5 and temperature of the cells at 25°C is also assumed.

The power is measured while varying the resistive load on the module between open and closed circuit. The maximum power measured is the nominal power of the module in "Wp".

Most testing laboratories around the world such as UL, NREL, TÜV, FSEC, CSA, KIER, Intervac, and Bodycote all use the SPI-Sun Simulators (Spire Solar the developers of the world's most efficient solar cells).

In other words, the independent groups that test and certify the amount of power produced by a specific type of panel from that manufacturer use the most sophisticated equipment, and standardized conditions to enable you the consumer to be able to analyze and compare the performance of that manufacturer's solar panels against any other you might want to consider.

Note: Just like its possible to boost the horsepower of a car above the nominal rating, it's also possible to produce more electricity than the so-called Peak Power or kWp rating. All you have to do is use the solar panel in an area where the insolation is higher than 1,000 W / m2. In other words - in parts of the world that are less than 35 degrees latitude above or below the equator.

To Calculate The Efficiency Rating of the Panel

While the kWp rating of the panels is clearly indicated on the manufacturer's data sheet. It takes a bit of calculating to find out what the efficiency rating is. Don't worry - it's easy.

To calculate the efficiency rating of a particular first look at the manufacturer's data sheet. Using the Sun Power 315 module as an example we see that:

Peak Power or Pmax rating = 315 Watts = 0.315 kWp (1,000 watts in a kilowatt)

Look at the dimensions of the module (in meters). In this case the module is 1.559 meters by 1.046 meters or has a surface area of 1.631 square meters.

The efficiency rating = kWp / surface area of the panel or in the case of Sun Power 315 panel:

efficiency rating = .315 / 1.631 = 0.1933 … or 19.3% efficient (which is the same as listed in their data sheet).

It's good to know how to make this calculation - because not all manufacturers publish their efficiency ratings in their modules.

Tips to Keep in Mind

When looking at or comparing kWp ratings you should look at the following additional information:

1. You will notice that next to the term "Peak Power" is a figure giving the accuracy … in this case the actual peak power of the panel could range to 5% higher than 315 watts or 3% lower. This is actually a pretty tight spread. Many manufacturers have a + or - of 10% … which suggests a greater variability in performance.

2. The second thing to look at … which is not usually found on the manufacturer's data sheet is the performance warranty. Typically, the manufacturer will guarantee that the kWp output of the panels will not be less than 90% of the rated peak value (under standard testing conditions) for the first 10 years, and not less than 80% for the next 15 years.

3. The third question to consider is who is making the guarantee. If it's a major company that's been around for a long time -- then you can pretty much expect them to continue to be around. However, in the case of Sun Power … they are a new company. Like most young companies, it is difficult to predict if they will still be around in 25 years and be able to honor the performance warranty. So the question to ask is who stands behind them.

Note: Some non-EU (i.e., Chinese manufacturers) are using major re-insurance companies to strengthen their guarantees. In the event the company is not able to honor its guarantee, the insurance company would step in and do so.

While the industry is booming along right now and most PV manufacturers are prospering … it had hard times in 2009, and could face hard times again. Also, as is typical in fast growing industries there will be acquisitions, consolidations, new technologies … in short a shakeout, and not every supplier, installer, manufacturer, etc. will be around.

Note: If performance guarantees are important to you (and they certainly are if you are using bank financing to pay for your installation), then having re-insurance gives you a bit of extra comfort on the performance side, and should be something you should ask about.

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