Calculate photovoltaic power: kWh & kWp + optimal size

There's no question about it: photovoltaics is the future. With a photovoltaic system, you can free yourself from rising electricity prices and make a contribution to climate protection. If you are interested in the topic, you are probably asking yourself: what output should a system ideally produce? And how do you calculate this? The photovoltaic output can be determined using a simple formula.

Which key figures describe the performance of a photovoltaic system?

From kilowatt hours to kilowatt peak – to calculate the optimal photovoltaic performance, we first need to clarify a few terms: 

kWh – The kilowatt hour

The abbreviation kWh stands for a kilowatt hour and means that one kilowatt of energy in one hour The unit kWh is therefore used as a measure for the amount of electricity generated or the electricity production of a photovoltaic system. One kWh corresponds to the 1,000 times one simple watt hour (Wh).

To help you better understand this, here are three Examples from everyday life: With the energy of one kWh you can power about one 

• run through a 60-degree wash cycle,
• 50 hours of work on a laptop, 
• shave 2,800 times with an electric shaver. 

kWp – The kilowatt peak

The kilowatt peak, also rated power is an important unit of measurement in the photovoltaic sector. The kWp describes the maximum power in kilowatts (kW) that a photovoltaic system can produce. This value enables PV systems to be compared. In order to provide accurate results, manufacturers must ensure that the photovoltaic systems are standard conditions be tested. 

The performance of photovoltaic systems depends on many factors such as weather, solar radiation and location. The term kWp reflects the performance under ideal (laboratory) conditions These are almost never achieved in reality, which is why PV systems always achieve fluctuating performance.

The ideal conditions are also known as standard test conditions (STC) are standardized conditions under which the performance of photovoltaic modules is tested. solar modules different manufacturers. The STC conditions are:

• a solar radiation of 1,000 watts per m²
• an air mass (AM) of 1.5
• a cell temperature of 25 °C

And this is how you calculate kWp in kWh about:  

One kWp corresponds to 1,000 kWh of electricity generation per year. 1 kWp photovoltaic system In Germany, an average of 1,000 kWh per year. With a PV system with 7 kWp, a yield of 7,000 kWh can be achieved.

The values vary depending on the location. In the south of Germany you can expect a higher yield than in the far north, as the global radiation is higher there. The following table shows a rough estimate. 

What does instantaneous power mean for PV systems?

The instantaneous power describes the power generated by a photovoltaic system at a specific point in time. To determine the value, three parameters required: the nominal power, the number of PV modules and the current strength of the solar radiation. 

What does specific power of solar systems mean? 

The specific power relates the electricity production of the solar system in kilowatt hours (kWh) to the nominal power of the system (kWp). As a rule, the period of one year Different system sizes can be compared using the specific performance. 

Invoice:

• specific power: kWh (annual electricity production) / kWp (nominal power) 
• Example: 7000 kWh (annual electricity production) / 7 kWp (nominal power)
• specific power = 1000 kWh / kWp

What is the average output of a photovoltaic system? 

The average output of a photovoltaic system is single-family and multi-family houses approximately 5 to 10 kWpThis corresponds to electricity production from 800 to 1,200 kWh per kilowatt peak. How much solar power your photovoltaic system produces depends on various factors such as the location of the photovoltaic system and the performance and orientation of the PV modules.

Calculating optimal photovoltaic performance: How it works 

To calculate the optimal photovoltaic performance, you need to know your own electricity consumption. A 4-person household consumes on average around 4,000 kWh per year. With an average PV yield of 1,000 kWh per kWp, this corresponds to a photovoltaic system with 4 to 5 kilowatt peak output.

Size of the photovoltaic system: Example

In our example, we assume an annual electricity consumption of 4,000 kWh. You can determine this average value from the electricity bills of the last few years. The location of the house has an average solar radiation of 1,000 kWh per kWp. 

You can find out the performance for your location from the radiation map of the German Weather Service.

In order for the system to be economically viable, as much solar power as possible should be used by the user. Feeding it into the public power grid is no longer worthwhile due to the reduced feed-in tariff. 

For a PV system with 4 kWp you need 12 to 13 PV modules with a performance of almost 320 watt peak. The bill for this:

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

A solar module usually has a size of 1.7 m². With 13 modules, this would correspond to a roof area of 22.10 m²:

13 solar modules * 1.7 m² = 22.1 m²

A free area of this size is usually available on single-family homes. 

Which factors influence the performance of photovoltaics? 

How efficiently a solar system generates energy depends on various aspects. We will take a closer look at these factors below.

performance of the solar modules

The performance of the solar modules is an important factor for the photovoltaic yieldA 300 Wp solar module produces 300 W of electricity when exposed to 1,000 W of sunlight. This output is largely determined by the size of the solar modules and the type of solar cells. Monocrystalline solar cells have the highest efficiency, followed by polycrystalline and finally thin-film solar cells. 

How many kilowatt peak per m² are possible?

Monocrystalline solar cells with an efficiency of 18 to 261 TP3T have an output of around 350 Wp in a standard module. With a module size of 1700 mm x 1000 mm, i.e. 1.7 m², approximately 0.2 kilowatt peak per m² is possible.

The size of solar modules is not uniform. However, most manufacturers use 60 standard cells or 120 half cells per module. The output in kilowatt peak per m² therefore varies for different module types.

Weather and Global Radiation

The global radiation is higher in the south than in the north. The location of the photovoltaic system therefore also plays an important role when planning the size of the system. If you want to achieve the same electricity yield in Hamburg as in Munich, you need more or more powerful PV modules. 

The weather also influences the performance of the system. Basically photovoltaics also work in the shade. If it is cloudy, however, less electricity is produced than when the sun is shining. But be careful: if the temperature is too high, the system also loses power. As soon as the solar cells reach a higher temperature than 25 °C in the STC conditions, performance drops by 0.3 to 0.4% per degree. 

On very hot days, solar modules can heat up to 65 °C. This would correspond to a loss of power of 10.5 to 14%. In Germany, from May to July The highest energy yields are generated with photovoltaics. In August it is often too hot. 

The following table roughly shows the global radiation and the associated power depending on the weather situation: 

orientation of the solar modules

The orientation of the solar modules also has a significant impact on performance. In Germany, the best yields are achieved when the modules are aligned directly south Deviations according to South-East/West but also produce very good yields. With a complete east-west orientation, the yields are somewhat lower. However, both sides of the roof can be covered with photovoltaic modules. 

One east-west orientation It is advantageous if you use most of the solar power in the morning and evening - exactly when the sun shines on the two surfaces. This allows you to increase your own consumption and have to feed less electricity into the grid.

roof pitch

In Germany, a south-facing roof requires a tilt angle between 30 and 45 degrees ideal. With an east-west orientation, the modules should have a flatter angle of inclination, as the sun is also lower. 

In the following table you can see the dependence on orientation and inclination. 

shading 

Permanent shading or partial shading of individual PV modules should be avoided at all costs. This leads to significantly lower PV yields. When planning the system, therefore, please take into account possible shadows from neighboring buildings, trees or self-shading from dormers and chimneys. If shadows cannot be avoided, PV optimizers can help. 

Also remember that the sun moves throughout the year, not just during the day. In winter it is much lower than in summer. As a result, the shadow cast is also much longer. 

Modern solar modules, however, can cope very well with temporary shading thanks to bypass diodes. These simply redirect the current from the shaded part of the solar cells, thereby less power loss arise.   

photovoltaic output per year 

The average photovoltaic output in Germany per year is 1,000 kWh per installed kWp power. A 6 kWp system therefore generates 6,000 kWh of solar power per year. The yield in the south of Germany tends to be over 1,000 kWh. In the north, the output of the solar system is less. 

In our table you can see the average yield per year for different cities read.

photovoltaic output over the course of the year 

The photovoltaic output over the course of the year is global radiation In Germany, the radiation from April to August highest – and the potential photovoltaic yield is accordingly also high. Good yields can still be achieved in March and September. In winter, however, the electricity yield is significantly lower.

In the table below you can see the average photovoltaic yield per month in Germany. 

FAQ

What is the photovoltaic output per square meter? 

The photovoltaic output per m² is on average just under 0.2 kWpThis means that an output of 200 watts can be achieved per year. Basically, around 300 to 350 watts of photovoltaic output are possible per 1.5 m². Deviations from this value are possible depending on the location and type of photovoltaic system.

What is the photovoltaic output per module? 

The photovoltaic power per module of modern modules is between 300 and 500 WpPhotovoltaic systems on single-family and multi-family homes generally use PV modules with an output of 300 Wp. These are cheaper than high-performance modules with 400 Wp or more. 

How much electricity does photovoltaics generate per day? 

On average, the photovoltaic output per day is 2.7 kWh per kWp. This is the total annual value, since the actual electricity production per day with photovoltaics depends on the season and the weather. An annual value is therefore more meaningful.

How much power does photovoltaics produce in winter? 

The photovoltaic output in winter is on average 350 to 400 kilowatt hours per installed kWp of photovoltaics. In this case, winter is defined as the period from October to March. In Germany, these are the months with the lowest photovoltaic electricity yields.

How much power does photovoltaics produce when it is cloudy? 

A PV system achieves 100 to 300 watts per m² lower performance when it is cloudy. Depending on the level of cloud cover, the yield is also reduced, as only diffuse light reaches the solar modules. The photovoltaic performance when it is cloudy also depends on the type of solar cells and modules. There are models with particularly good low-light performance. 

What is the maximum output for private photovoltaic systems?  

For photovoltaics in the private sector, a maximum output of 10 kWp recommended. For systems larger than 10 kWp, registration with the tax office becomes a little more complicated. Legally speaking, there is no official maximum permitted photovoltaic output in the private sector.

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

The photovoltaic output per m² on a flat roof is also 0.2 kWpThis is because the solar modules on a flat roof can be optimally aligned towards the sun thanks to their mounting. This means there is no loss of performance due to a lower angle of inclination.

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

A 20 square meter photovoltaic system achieves an output of around 4 kWpThis applies to systems with a module output in the 300 Wp range. PV systems with high-performance modules of 400 to 500 Wp achieve outputs of 5 to 6 kWp on 20 square meters.

How much power does a 30 square meter photovoltaic system generate?

A 30 square meter photovoltaic system achieves an output of around 6 kWpThis is based on an average output of 0.2 kWp per m². A 6 kWp PV system is sufficient to cover the energy needs of a single-family home.

How much power does a 100 square meter photovoltaic system generate? 

Based on an average photovoltaic power of 0.2 kWp per m², a 100 m² photovoltaic system generates a power of 20 kWp. 

Which photovoltaic power for a single-family home? 

The output of photovoltaics on a single-family home is usually between 5 and 10 kWp. How many m² do you need for this? For a 5 to 10 kWp system, 25 to 50 m² of roof area are required on the single-family home. 

Conclusion

To calculate the performance of a photovoltaic system, you should individual electricity consumption This means that your own PV system can be perfectly dimensioned for your own consumption. The output of the photovoltaic system is kWp (kilowatt peak). The annual electricity yield is given in kWh (kilowatt hours). One kilowatt peak of PV power generates almost 1000 kilowatt hours per year.