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How To Compare Solar Panels

The nominal power output of a solar panel is expressed in Watt peak (Wp). It refers to the maximum power output under standardized test conditions and makes it possible to compare different solar panels. Solar panel efficiency tells us how much power output a solar panel has under certain conditions.
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When talking about solar panel efficiency, we are generally looking at output per given area. To determine solar panel efficiency, we have to sum up the nominal power output listed on the nameplates on the backsides of the PV modules in units of watts. This factor has to be divided by 1000 to convert to kilowatts (kW). In the context of domestic installations kWp is the most common unit encountered.

To determine solar panel efficiency, we need to know the nominal power output of a solar panel.

Nominal Power Output of Photovoltaic Systems

The nominal output of a solar panel is measured as maximum power output in Watts-peak (Wp) under standard test conditions (STC) in a laboratory. The STC light intensity is 1000W/m2. The spectrum is similar to the sunlight in summer that reaches the surface of the earth at latitude 35°N and temperature of the cells at 25°C.

The power is measured while the resistive load on the module is varied between open and closed circuit. The maximum power measured is the nominal power of the module in Watts-peak (Wp). Manufacturers advertise and sell modules based on this value for solar panel efficiency, which is known as “Nameplate Rating” of solar panels or arrays. Costs of solar panels are often described in costs per nominal watt output.Solar Panel EfficiencyThin Film Solar Cell. Source: NREL

Performance Test Conditions

STC should not be confused with PTC, Performance Test Conditions. PTC is also a testing standard for solar panel efficiency and was developed to represent more real conditions. Keep in mind to use the correct power rating when calculating solar panel efficiency.

Actual Power Output - Solar Panel Efficiency in Real Conditions

The actual power output of solar panel efficiency in real conditions is usually less than the nominal power output. Conditions are never as perfect as in a laboratory. Factors like weather and climate or the angle in which the sunlight falls on the panels influences solar panel efficiency.

Panel array capacity or power capacity

The solar panel array capacity describes the amount of solar insolation that is actually converted into solar power, in relation to the solar insolation that reaches the panel. Solar panel capacity usually lies between 12.5 and 15 %. While solar panel efficiency tells us the possible output of a system, panel array capacity informs us about the amount of solar energy that a solar panel converts into electricity.

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Written by: Christian Märtel, Editor
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