kWp and kWh: Difference, Conversion, Meaning

Kilowatt hour (kWh) and kilowatt peak (kWp) are key terms for photovoltaics. Anyone planning or using a PV system should understand the difference between them. In this article, we explain what the individual units mean and how to convert them. 

First things first

• A kilowatt hour (kWh) measures the amount of energy that a solar system currently consumes or generates.
• Kilowatt peak (kWp) indicates the nominal power of a PV system under ideal conditions.
• In Germany, PV systems generate an average of around 1,000 kWp per year.
• The relationship between nominal power (kWp) and generated yield (kWh) is influenced by the location, orientation and inclination, shading as well as maintenance and cleaning of the PV system.

What does kilowatt hour (kWh) mean? 

A kilowatt hour (kWh) is a unit of energy. In photovoltaics, it is used to indicate the annual electricity yield generated by a solar system. It indicates how much energy in kWh you use in your household or feed into the grid. You can read these amounts of energy on the photovoltaic meter.

What does kilowatt peak (kWp) mean?

Kilowatt peak (kWp) describes the maximum output of a photovoltaic system under standardized test conditions (STC). It indicates the nominal output of solar modules under optimal conditions. In contrast to kWh, kWp refers to the theoretical peak output. 

STC (standard test conditions) are defined conditions for measuring the performance of solar modules. These values serve as a benchmark. However, they are determined under ideal conditions that rarely occur in everyday life:

• Solar radiation: 1,000 W/m²
• Cell temperature: 25°C
• Air mass: 1.5

The NOCT (Nominal Operating Cell Temperature) provides more realistic values. It is calculated under typical operating conditions. For example, the measurement takes place at lower solar radiation and higher cell temperatures:

• Solar radiation: 800 W/m²
• Ambient temperature: 20°C
• Wind speed: 1 m/s
• Cell temperature: approx. 45°C 

How do I convert kWh to kWp? 

In Germany, the conversion from kWp to kWh works with the value 1,000. For each kWp of PV system output, a yield of around 1,000 kilowatt hours (kWh) of solar power is achieved per year. A 10 kWp PV system therefore generates a yield of 10,000 kWh per year.

A PV system with 10 kWp output produces around 10,000 kWh of solar power annually. 

However, this does not cover an electricity demand of 10,000 kWh per year. With a PV system, you can use a maximum of 25 to 35% of the electricity generated directly in your home. The reason for this is that a PV system produces most of its electricity during the day. However, households use most of their electricity during the morning and evening hours.

Photovoltaics therefore generate excess electricity and feed it into the public power grid. If you want to increase your own consumption, you must, for example, shift electricity consumption to midday or install a PV storage system.

What factors influence the relationship?

Various factors influence the ratio between kWp and kWh, including:

• Location: The geographical location has a major influence on the yield of a PV system. For example, a PV system in northern Germany achieves an average of around 900 kWh per kWp per year. In the south of the country, however, the yield can reach up to 1,300 kWh per kWp because the solar radiation there is stronger and more frequent.
• orientation of the solar modules: The optimal orientation of the solar modules is crucial for the yield. PV systems facing south generally achieve the highest yields. They capture the most solar energy throughout the day. Deviations to the east or west reduce the yield.
• tilt angle of the solar modules: The angle of inclination of the modules also influences the yield. In Germany, the optimal angle of inclination is around 30 to 35 degrees. A flat or too steep angle reduces the yield because the light radiation is not ideal and the solar cells therefore generate less electricity.
• shading: Shadows on the solar modules caused by trees, buildings or other obstacles significantly reduce power generation. Even small amounts of shadow significantly reduce the overall yield of the PV system. The solar modules are usually connected in a row, so shading one solar module reduces the output of all modules in the row.
• temperature: A temperature that is too high has a negative effect on the performance of solar modules. Although solar cells need sunlight, they lose performance as the temperature increases. For this reason, PV systems often produce more energy in cooler weather but with good sunlight than on very hot days.
• maintenance and cleaning: Regular maintenance is important to ensure that the solar system functions optimally and delivers the best yield. For example, dirt such as dust, leaves or bird droppings significantly reduce the system's yield. Such problems are identified and corrected during maintenance.
• Technical Equipment: The quality of the solar modules and inverters also has an impact on the yield. High-quality modules with a better efficiency deliver more kWh per installed kWp. 
• age of the system: Solar modules lose efficiency slightly over time. The degradation is about 0.3% per year. After about 20 to 30 years, the loss of performance is about 10 to 20%. An older system will therefore produce fewer kWh per kWp than a new one.
• Professional installation: Incorrect installation reduces PV performance and, in the worst case, can lead to the failure of the entire photovoltaic system. Therefore, only trust qualified installers with references. Fill out the form below and we will help you find a professional photovoltaic company.