# Is not the highest achievement

## “Kilo, watt?” - Correctly differentiate between capacity and performance in electricity storage systems and photovoltaic systems

From a physical point of view, kilowatts are a measure of electrical power and are abbreviated to “kW”. One kilowatt is 1,000 watts. Most people are familiar with this information from their electrical appliances in the household, which shows how much energy they need. For example, a modern television needs 50 - 60 watts, washing machines around 800 - 1,000 watts and vacuum cleaners between 1,000 and 1,600 watts, which would be 1.6 kilowatts. The power in watts or kilowatts shows how much energy is required at that moment.

It is exactly the other way around with a photovoltaic system. There, the kilowatt figure shows how much energy it can generate from the incident sunlight. A solar system with an output of 7 kW can therefore provide 7 kW at once.

But that is not enough. Because actually the maximum power and thus the size of the PV system is given in "kWp", i.e. kilowatts peak. This is the top performance that the PV system can mathematically achieve. If you buy a 7 kWp system, you can nominally generate energy with an output of 7 kW. That is why one speaks of the nominal power. In our example, the household would buy 28 photovoltaic modules with an output of 250 watts each and would then have a total output of 7 kWp.

In practice, this theoretical value is actually never achieved, but is somewhat lower. It is influenced by many factors such as the position of the sun, the temperature, the installation angle and many more.

How much energy actually comes from the roof is given in kW. Of course, this number fluctuates constantly, depending on the course of the day. Anyone who gets 6.5 kW off the roof on a beautiful summer's day is well served. How much energy 6.5 kW is quickly becomes clear when we look at the previous examples: You can use it to do laundry (800 W), vacuum clean (1.6 kW) and watch TV (50 W). Together that would be just 2.45 kW, so the rest of the energy would flow into the storage unit and, when it is full, into the power grid.

Many people also believe that “solar power” can only be generated when the sun is shining. Although the system achieves its maximum output when the sun is shining, daylight is usually sufficient. Even when it rains, a modern, efficient PV system as in our example can still generate 1 kW of power, which can then be used to charge the battery during the day. Here, the bigger the solar power system, the more energy it can deliver in bad weather.

If the PV system has an output of 1 kW for one hour, it has generated an amount of energy of 1 kilowatt hour. The memory would therefore be charged after a few hours even if the weather was not optimal.