Although they seem similar, it is important to understand the fundamental difference between these two units (kW and kWp) because they measure distinct aspects of solar energy production. And this distinction is crucial for properly assessing the performance and profitability of a photovoltaic installation.
Definition and fundamental difference
Kw (kilowatt) is a unit of measurement for power. It represents the amount of energy an electrical appliance consumes or produces at a given time. In other words, it refers to the rate at which energy is used or generated. For example, a 2 kW clothes dryer consumes 2 kilowatts when operating.
Kwp (kilowatt-peak), on the other hand, is specific to solar. It refers to the maximum power a solar panel can produce under ideal conditions, called STC (Standard Test Conditions). These conditions are standardized to compare panels with each other: sunshine of 1000 W/m², temperature of 25°C and optimal tilt.
In other words, the kWp corresponds to what the panel could produce at best in the laboratory, but not what it provides on average in reality.
The fundamental difference therefore lies in the fact that the kW measures a real instantaneous power, while the kWp measures a theoretical maximum power, determined under standardized conditions.
Note: these two units are sometimes confused with a third, the kWh (kilowatt-hour), which this time represents a quantity of energy produced or consumed over time (for example, 1 kW used for 1 hour = 1 kWh).
Implications for solar energy production
The peak power (expressed in kWp) of a solar panel gives an idea of its maximum production capacity. But in practice, actual production depends on many factors:
- Actual sunshine, which varies depending on the season, time of day, weather, or region;
- Ambient temperature, which can reduce the efficiency of photovoltaic cells;
- Orientation and tilt of the panels;
- Shaded areas (trees, chimneys, neighboring buildings);
- The condition of the installation (dirt, aging, etc.).
This is why a panel with a power of 3 kWp will never produce 3 kW continuously throughout the day. Actual production varies constantly.
The performance factor
To estimate the production of a solar system over the year, we use an indicator called the performance factor (or “load factor”). It corresponds to the ratio between the energy actually produced (in kWh) and the energy that the system could have produced in optimal operation 100% of the time.
This factor is generally between 15% and 25% depending on the quality of the installation, local sunshine and possible losses. In France, a good, well-exposed system is around 18 to 20%.
Concrete example
Let's take the example of a solar installation with a power of 3 kWp. This means that under ideal STC (Standard Test Conditions) this installation can produce a maximum power of 3 kW.
But if, in reality, the sunshine is 500 W/m² and the ambient temperature is 30°C, the actual production will be much lower than this value. Assuming a performance factor of 20%, the average power actually produced at that moment would be around 1.5 kW, and the daily production would be expressed in kWh.
Understanding the difference between kW, kWp, and kWh is therefore essential to properly understand the characteristics of a photovoltaic system:
- Kw measures real instantaneous power (as for an electrical appliance),
- kWp indicates the theoretical maximum power of a solar panel under optimal conditions,
- kWh expresses the energy produced or consumed over a given period.
These distinctions allow you to better estimate production, adjust expectations, and optimize your installation to maximize the return on your solar investment.
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