Kilowatt hour (kWh) and kilowatt peak (kWp) are crucial units of measurement for measuring the yield of a PV system. They indicate how much electrical energy is generated or consumed in a certain period of time. In this article, I will explain in detail the meaning of kWh, as well as the power measurement in kilowatt peak (kWp) and its conversion to kWh. (kWp and kWh)
Kilowatt-hour (kWh): measure of the yield of a PV system
The abbreviation kWh stands for kilowatt hour. The kWh describes the energy consumed or generated by an electrical system with an output of one kilowatt (kW) in one hour.
For example, the electrical system can be your household or a PV system. That is why you will find the unit kWh on your electricity bill and in the calculations of our zolar online configurator for the profit from your PV system.
The kWh is a composition of two more basic units of measurement :
kW: the unit watt (W) or kW (1,000 W is 1 kW) describes an electrical system’s power. This is about whether the energy is strong or weak. For example, solar modules have 300 W or 400 W power.
Hour: The “hour” in kWh is used to compare the energy consumption or energy production of various electrical devices by always measuring the amount over the same period of time.
You can understand the difference between kW and kWh using a water tap:
- The power in kW describes the strength of the water jet.
- The amount in kWh measures how much water has flowed out of the tap after one hour.
In relation to PV systems, this means: With 1 kWh of solar power, you can:
- 1 electrical device with a power of 1 kW
- Operate for 1 hour.
How much is a kWh of electricity?
To give you an idea of how much 1 kWh of electricity is, we have put together a few everyday examples for you:
- Lighting: With 1 kWh of electricity, you can operate a modern LED lamp with an output of 10 W or 0.01 kW for 100 hours (1 kWh / 0.01 kW = 100 hours). Assuming you leave the LED switched on for 4 hours a day, it would consume 1 kWh of electricity in 25 days.
- Cooking: An average electric stove has a power of 8 kW. Therefore, with 1 kWh of electricity, you can cook for 0.12 hours or 7 minutes at full power (1 kWh / 8.5 kW = 0.12 hours).
- Washing clothes: An average washing machine has about 2,400 W or 2.4 kW. At full power, 1 kWh can wash clothes for 0.42 hours or 25 minutes (1 kWh / 2.4 kW = 0.42 hours).
| Device | Performance | Operation with 1 kWh of electricity |
| Laptop | 30 W or 0.03 kW | 33.3 hours |
| Refrigerator | 130 W or 0.13 kW | 7.7 hours |
| Smartphone | 20 W or 0.02 kW | 50 hours |
| LCD TV | 100 W or 0.1 kW | 10 hours |
| Electric toothbrush | 2 W or 0.002 kW | 500 hours |
| Heat pump | 4,250 W or 4.25 kW | 0.24 hours or 14 minutes |
By the way, the price for 1 kWh of electricity varies depending on location and energy provider. Currently, 1 kWh of electricity costs an average of 35 cents per kWh across Germany, and this price is shown on your electricity bill.
So, we have clarified what a kWh of electricity is. But what is meant by kWp—the unit that you encounter in connection with the output of PV systems?
Kilowatt Peak (kWp): Measure of the power of a PV system
The abbreviation kWp stands for kilowatt peak. This unit of measurement indicates the maximum power that a photovoltaic system can generate. So imagine the output of a PV system in kWp as its peak output at the most favorable solar radiation.
It is important to note that the actual output of a PV system varies constantly, depending on the amount of sunlight. The peak power of the system in kWp therefore serves as a reference point to compare the performance of different PV systems.
You can find out the actual yield of a PV system on your roof using our zolar online configurator. Our solar calculator calculates how much kWp of PV power you can accommodate on your roof based on your roof size:
How the kWp of a PV system is measured
The output of a PV system in kWp is used to compare different systems . In order to meet this requirement, the output of all PV manufacturers must be measured under the same conditions (sunlight, etc.) .
These conditions are known as “ Standard Test Conditions” (STC).
According to STC, the performance of solar modules is measured at:
- A solar radiation of 1,000 W per square meter
- A cell temperature of 25 degrees Celsius (i.e. the temperature inside the solar modules)
- And an air mass ratio of 1.5. This means that below STC, sunlight has traveled a path through the atmosphere that is 1.5 times longer than the direct path from the Sun to Earth.
You can probably imagine that a cell temperature of 25 degrees in direct sunlight is a considerable understatement. However, the STC works with this value because measurements at lower temperatures are less prone to error.
Nevertheless, the STCs were criticized for this unrealistic assumption and a second test standard was developed: the Nominal Operating Cell Temperature (NOCT)
According to NOCT, the peak power of PV modules in kWp is measured at:
- An irradiation power of 800 W per square meter,
- An ambient temperature of 20 degrees Celsius,
- And a wind speed of 1 meter per second.
Although STCs are still widely used and important to compare different solar modules, NOCTs are increasingly used as complementary information to better predict performance under real-world conditions.
Conversion from kWp to kWh
To calculate the annual yield (in kWh) of a PV system from its maximum output (in kWp), you can multiply the output by a factor of 1,000 . For example:
- System output = 8.9 kWp
- Annual yield of the system = 8.9 kWp kWp x 1,000 kWh per kWp = 8,900 kWh
Please note: this is only an approximation of the yield. A more precise calculation of the yield in kWh at a specific location usually produces a slightly different result. For example, the yield of the same 8.9 kWp system without approximation/with exact calculation is 10,569 kWh in the first year.
You see: the difference is not that big. At the same time, the exact calculation is somewhat complex, because you need the following data:
- The system output in kW
- The irradiance at the location in watts per square meter
- The plant size in square meters
- The duration of irradiation in hours
You then calculate the annual yield using the following formula:
Yield of the PV system (kWh) = irradiation power (W/m²) x system size (m²) x system power (kW) x irradiation duration (hours)
I recommend:
- Use the factor 1,000 kWh / kWp to quickly calculate the approximate annual yield of a PV system .
- Use an online configurator to calculate your exact yield. To set it up for you, our PV experts only need a few details about your house.
Summary
- The unit kWh describes the amount of energy consumed or generated by an electrical system with an output of one kilowatt (kW) in one hour.
- With 1 kWh of electricity you can operate an electrical device with an output of 1 kW for 1 hour.
- The unit of measurement kWp indicates the maximum power that a photovoltaic system can generate under the most favorable solar radiation.
- The output in kWp serves as a reference point to compare the performance of different PV systems.
- To estimate the annual yield (in kWh) of a PV system from its maximum output (in kWp), you can multiply the output by a factor of 1,000.