Why Solar Panels Don't Always Reach Kilowatt-Peak (kWp)

Created by Bryonie Simon, Modified on Fri, 8 May at 9:04 PM by Bryonie Simon

What is Kilowatt-Peak?

Used to measure or describe the maximum theoretical output of a solar panel or solar array under standard test conditions (STC) and is typically used as a benchmark to compare size/power of arrays. It is almost impossible to reach this figure as there are many factors which do not meet the STCs when a system is installed on site.

Standard test conditions are based upon

25°C ambient temperature

1000 W/m² irradiance

1.5 sea level air mass (AM) spectral distribution (i.e. the distribution of wavelength and light intensity caused by the amount of atmosphere that the sunlight passes through reach the panels)

Factors Which Contribute to Performance Reduction from kWp

Temperature

Cell temperature will fluctuate throughout the day

Efficiency of solar panels decreases as the temperature rises due to increased resistance

Stronger/colder winds will dissipate the heat more effectively

Intensity of light

Tilt of panels may not be optimal on roof-mounted arrays for direct sunlight

Even during sunny weather there will be intermittent cloud coverage which reduces intensity of light hitting the panels

Dirt on the panels will reduce the amount of light reaching the cells until they are cleaned by rainfall

Shading Factor (SF)

Many popular design software packages provide the kWp but also provide another value in the performance estimate, which is amended based on SF, giving a more realistic performance estimate when the panels will be shaded by trees, buildings etc.

Design Software

Many popular design software packages will take things such as geographical location into account when calculating kWp, but some may purely use the values from the manufacturer specifications

Deterioration

Panels reduce in efficiency by roughly 1% per year they are in use

Inverter Power Loss

There is some power loss as the inverter works to convert the DC power into AC

Most modern inverters have an efficiency of over 95% but not possible to reach 100% efficiency