Calculating PV power: kWh & kWp + optimal size - SHIELDEN

Calculating PV power: kWh & kWp + optimal size

There is no doubt about it: photovoltaic power is the future. With a photovoltaic system, you can escape the effects of rising electricity prices and contribute to climate protection. If you are interested in this topic, you may be asking yourself: What performance should the system provide in the best case scenario? How do you calculate this? PV performance can be determined using a simple formula.

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What key figures describe the performance of a PV system?

From kWh to kW peak - in order to calculate the optimal PV output, we must first clarify a few terms:

kWh - kilowatt hour

The abbreviation kWh stands for kilowatt hour and means that one kilowatt of energy is produced in one hour. Therefore, the unit kWh is used as a measure of the amount of electricity generated or the power produced by the PV system. 1 kWh equals 1,000 times one simple watt-hour (Wh).

To help you visualize this, here are three examples from everyday life: With one kWh of energy, you can generate approximately one kilowatt-hour of energy.

Perform a 60-degree wash cycle
50 hours of work with a laptop
Electric shave 2,800 times.

kWp - kilowatt peak

The kilowatt peak, also known as nominal power, is an important unit of measurement in photovoltaics. The kWp describes the maximum output power (kW) that a PV system can provide. This value allows comparisons to be made between PV systems. In order to provide accurate results, manufacturers must ensure that PV systems are tested under standardized conditions.

The performance of a PV system depends on many factors, such as weather, solar radiation and location. The term kWp reflects performance under ideal (laboratory) conditions. In fact, these are almost never realized, which is why the performance of PV systems always fluctuates.

Ideal conditions are also known as Standard Test Conditions (STC). These are uniform conditions under which the performance of PV modules is tested. This allows you to compare solar modules from different manufacturers. The STC conditions are:

1,000 watts of solar radiation per square meter
Air quality (AM) of 1.5
Cell temperature of 25 °C

This is how kWp is converted into kWh:

1 kWp is equivalent to 1,000 kWh per year. The average 1 kWp PV system in Germany generates 1,000 kWh per year. With a 7 kWp PV system, 7,000 kWh can be realized.

These values vary by location. You can expect higher yields in southern Germany than in the Far North, where global radiation is higher. The table below shows a rough estimate.

Federal State Specific Yield
Baden-Württemberg 1,070 kWh/kWp
Bavaria 1,060 kWh/kWp
Berlin 910 kWh/kWp
Brandenburg 990 kWh/kWp
Bremen 930 kWh/kWp
Hamburg 959 kWh/kWp
Hesse 1,018 kWh/kWp
Mecklenburg-Vorpommern 1,014 kWh/kWp
Lower Saxony 985 kWh/kWp
North Rhine-Westphalia 987 kWh/kWp
Rhineland-Palatinate 1,037 kWh/kWp
Saarland 1,063 kWh/kWp
Saxony 1,035 kWh/kWp
Saxony-Anhalt 1,004 kWh/kWp
Schleswig-Holstein 1,002 kWh/kWp
Thuringia 1,021 kWh/kWp

What does instantaneous power mean for a PV system?

Instantaneous power describes the power produced by a PV system at a specific time. In order to determine this value, three parameters are required: the nominal power, the number of PV modules and the current intensity of solar radiation.

What does the specific power of a solar system mean?

Specific output relates the amount of power generated by a solar system in kilowatt hours (kWh) to the nominal output of the system (kWp). A period of one year is usually considered. Different system sizes allow comparison of specific performance.

Invoice:

Specific power: kWh (annual generation)/kWp (nominal power)
Example: 7000 kWh (annual generation) / 7 kWp (nominal power)
Specific power = 1000 kWh / kWp

What is the average power output of a PV system?

The average output of a PV system for single-family and multi-family dwellings is approximately 5 to 10 kWp. This corresponds to 800 to 1,200 kWh per kW peak. The amount of solar energy generated by PV depends on a number of factors, such as the location of the PV system and the performance and orientation of the PV modules.

Calculating the optimal PV performance: this is how it works

In order to calculate the optimal PV performance, you need to know how much electricity you use. A four-person household consumes an average of about 4,000 kWh per year. With an average PV output of 1,000 kWh per kW, this corresponds to a peak output of 4 to 5 kW of PV.

PV system sizing: an example

In our example, we assume an annual consumption of 4,000 kWh. You can determine this average by looking at your electric bill for the past few years. The average solar radiation at the house location is 1,000 kWh per kWh.

To make the system economically worthwhile, you should use as much solar energy as possible yourself. Due to the reduced feed-in tariff, it is no longer worthwhile to supply the public grid.

For a 4 kWp photovoltaic system, you need 12-13 photovoltaic modules with a peak output of almost 320 watts. The invoice for this:

4,000 kW / 320 Wp = 12.5 solar modules = 13 solar modules

The size of a solar module is typically 1.7 square meters. If there are 13 modules, the roof area is 22.10 square meters:

13 solar modules * 1.7 square meters = 22.1 square meters

Single-family houses usually offer this size of free area.

What factors affect the performance of photovoltaic power generation?

The efficiency with which a solar system delivers energy depends on various aspects. We would like to take a closer look at these factors below.

Solar module performance

The performance of the solar module is an important factor in the amount of PV power generated. A 300 Wp solar module will also produce 300 W of electricity when exposed to 1,000 W of sunlight. This performance is highly dependent on the size of the solar module and the type of solar cell. Monocrystalline solar cells are the most efficient, followed by polycrystalline solar cells and finally thin film solar cells.

How many kilowatt peaks per square meter are possible?

Monocrystalline solar cells have efficiencies of 18% to 26% and a standard module has an output of about 350 Wp. With a module size of 1700 mm x 1000 mm (i.e., 1.7 square meters), the peak power per square meter is about 0.2 kilowatts.

Solar module sizes are not standardized. However, most manufacturers use 60 standard cells or 120 half-cells per module. As a result, peak kW output per square meter varies between module types.

Module Type Length x Width kWp per m²
48 Cells 1350mm x 1000mm 0.185
60 Cells 1650mm x 1000mm 0.21
72 Cells 2000mm x 1000mm 0.205
120 Half Cells 1700mm x 1000mm 0.208

Weather and global radiation

Global radiation is higher in the South than in the North. Therefore, the location of the PV system also plays an important role when planning the size of the system. If you want to achieve the same power generation in Hamburg as in Munich, you need more or more powerful PV modules.

The weather also affects the performance of the system. In principle, photovoltaics can also work in the shade. However, less electricity is generated on cloudy days than on sunny days. But be careful: If the temperature is too high, the system can also lose performance. As soon as solar cells reach temperatures higher than 25 °C above STC conditions, performance drops by 0.3 - 0.4% per degree.

In very hot weather, solar modules can reach temperatures of up to 65 °C. This corresponds to a performance loss of 10.5% to 14%. In Germany, photovoltaic power generation is highest from May to July. August is often too hot.

The table below gives a rough indication of the global radiation and the associated power depending on the weather conditions:

Weather Global Radiation Power
Sunny 1000 W/sqm 1000 Watts
Partly Cloudy 400 W/sqm 400 Watts
Mostly Cloudy 150 W/sqm 150 Watts
Rainy 50 W/sqm 50 Watts

Orientation of the solar module

The orientation of the solar module also has a significant impact on performance. In Germany, the best yields are obtained when the modules are oriented directly south. However, a bias to the southeast/west also produces very good yields. Due to the completely east-west orientation, the yield is slightly lower. Both sides of the roof can be covered with PV modules.

If you use most of your solar energy in the morning and in the evening (precisely when the sun shines on both surfaces), an east-west orientation is advantageous. This means that you can increase your own consumption and have to consume less electricity.

Roof slope

In Germany, a slope angle of between 30 and 45 degrees is ideal for south-facing roofs. In the case of an east-west orientation, the modules should have a flatter inclination angle because the sun is also lower.

Shadows

Permanent shading or partial shading of individual PV modules should be avoided at all costs. This leads to a significant reduction in PV power generation. When planning your system, take into account possible shading from neighboring buildings, trees or self-shading from tiger windows and chimneys. If shading cannot be avoided, a PV optimizer can help.

Remember also that the sun not only moves throughout the day, but throughout the year. It is much lower in winter than in summer. This means that shadow casting is also significantly longer.

On the other hand, modern solar modules cope well with temporary shadows thanks to bypass diodes. These simply divert power away from the shaded portion of the solar cell, thus reducing power loss.

Annual photovoltaic power generation

The average photovoltaic capacity in Germany is 1,000 kWh per installed kWp per year. A 6 kWp system therefore generates 6,000 kWh of solar energy per year. Southern Germany generates more than 1,000 kWh. In the north, the output of the solar system is lower.

In our table you can see the average annual output in different cities.

City Yield in kWh per kWp
Freiburg 1.048
Munich 1.041
Augsburg 1.029
Stuttgart 1.025
Würzburg 1.001
Frankfurt 980
Nuremberg 975
Mainz 973
Erfurt 970
Leipzig 955
Aachen 946
Rostock 945
Düsseldorf 907
Hannover 901
Kiel 895
Bielefeld 880
Bremen Airport 879

Photovoltaic output throughout the year

The photovoltaic output throughout the year depends on the global radiation. In Germany, the highest irradiance and therefore the highest possible PV output is found from April to August. Good yields can still be achieved in March and September. However, power generation is significantly lower in winter.

In the table below you can see the average monthly PV output in Germany.

City Yield in kWh per kWp
Freiburg 1.048
Munich 1.041
Augsburg 1.029
Stuttgart 1.025
Würzburg 1.001
Frankfurt 980
Nuremberg 975
Mainz 973
Erfurt 970
Leipzig 955
Aachen 946
Rostock 945
Düsseldorf 907
Hannover 901
Kiel 895
Bielefeld 880
Bremen Airport 879

FAQ

How much PV power is generated per square meter?

The average photovoltaic capacity per square meter is slightly less than 0.2 kWp. 200 watts can be produced annually. In principle, about 300 to 350 watts of PV power can be generated per 1.5 square meters. Depending on the location and type of PV, this value may deviate.

What is the PV output per module?

Modern modules have a PV output of between 300 and 500 Wp per module. PV systems for single-family and multi-family homes typically use PV modules with an output of 300 Wp. These modules are less expensive than high-performance modules of 400 Wp or higher.

How much electricity does PV generate per day?

On average, PV generates 2.7 kWh per kWp per day. this is the total annual value, as the actual amount of electricity generated by PV per day depends on the season and the weather. Therefore, the annual value is more meaningful.

How much electricity does PV generate in winter?

Winter PV generation averages 350 to 400 kWh per installed kWp. In this example, winter is defined as the period from October to March. These are the months with the lowest PV generation in Germany.

What is the output of photovoltaic power generation on cloudy days?

On cloudy days, the output of a photovoltaic system decreases to between 100 and 300 watts per square meter. Depending on the thickness of the cloud cover, the output is also reduced because only diffuse light hits the solar modules. PV performance under cloudy conditions also depends on the type of solar cells and modules. Some models have particularly good low-light performance.

What is the maximum output of a private sector PV system?

The recommended maximum output for private sector PV is 10 kWp. For systems larger than 10 kWp, registration with the tax office becomes slightly more complicated. From a legal point of view, there is no official maximum output for private sector PV.

What is the PV output per square meter on a flat roof?

The PV output per square meter on flat roofs is also 0.2 kWp. This is because the solar modules on flat roofs are optimally aligned with the sun due to their height. This means that there is no loss of performance due to lower tilt angles.

How much power does a 20 square meter PV system generate?

A 20 square meter PV system can achieve an output of approximately 4 kWp. This applies to systems with module outputs in the 300 Wp range. PV systems with high performance modules in the 400 to 500 Wp range can achieve an output of 5 to 6 kWp on 20 square meters.

What is the power output of a 30 square meter PV system?

A 30 square meter PV system can achieve an output of approximately 6 kWp. Assuming an average output of 0.2 kWp per square meter, a 6 kWp PV system is sufficient for a single-family home.

What is the power output of a 100 square meter PV system?

Based on an average PV output of 0.2 kWp per square meter, a 100 square meter PV system would produce 20 kWp of output.

What is the PV output of a single-family dwelling?

The PV output of a single-family home is typically between 5 and 10 kWp. How many square meters do you need? For 5-10 kW peak systems

Single-family homes require 25 to 50 square meters of roof space.

Conclusion

In order to calculate the performance of your PV system, you should know your personal electricity consumption. This means that your own PV system can be perfectly sized to meet your own consumption. The output of photovoltaic power is measured in kWp (peak kilowatts). Annual electricity production is measured in kWh (kilowatt hours). One kilowatt of peak photovoltaic power generates nearly 1,000 kilowatt-hours of electricity per year.

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