"Kilo, Watt?" - Distinguishing capacity and output in battery storage and photovoltaic systems

The two terms "kilowatt" and "kilowatt-hour" come up particularly often. Both terms sound similar but denote different things. When two things sound so similar: They are often confused or used incorrectly. The terms and quantities will be explained and why they are also legally important.

Table of contents

• Kilowatts and kilowatt-hours
• Power of electrical appliances and PV systems
• Size of the solar battery storage system
• Summary

Kilowatt and kilowatt-hour

If it helps, imagine a bottle of water. Transferred to the storage tank, the capacity in kilowatt hours (kWh) shows how much water goes in at all or is currently contained. The capacity in kilowatts (kW) shows how much water can go out through the opening at once. If the opening is larger, more water can go in or out at. If it is smaller, it takes longer, and less water is available at one time.

Power of electrical appliances and PV systems

Physically, kilowatt is a measure of electrical power and is abbreviated as “kW”. One kilowatt is 1,000 watts. Most people know this figure from their household electrical appliances, which shows how much energy they need. For example, a modern television set needs 50 – 60 watts, washing machines around 800 – 1,000 watts and hoovers between 1,000 and 1,600 watts, which would be 1.6 kilowatts. The power in watts or kilowatts, therefore, shows how much energy is needed at that moment.

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

But that is not enough. Because the maximum power and thus the size of the PV system is specified in “kWp”, i.e., kilowatt peak.  This is the peak power 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. This is why we speak of 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 reached but is somewhat lower. It is influenced by many factors such as the position of the sun, the temperature, the angle of installation and more.

How much energy comes from the roof is given in kW. This figure fluctuates constantly over the course of a day. If you get 6.5 kW from the roof on a nice summer day, you are doing quite well. How much energy 6.5 kW is, quickly becomes clear when we look at the previous examples: With that, you can do laundry (800 W), vacuum (1.6 kW) and watch TV (50 W). Together, this would be just 2.45 kW, so the rest of the energy would flow into the battery storage unit and, when it is full, into the electricity grid.

Many people also believe that “solar electricity” can only be generated when the sun is shining. It is true that the system reaches its highest output when the sun is shining, but daylight is usually quite sufficient. Even when it rains, a modern, efficient PV system like the one in our example can still generate 1 kW of power, which can then be used to charge the battery during the day. The larger the solar panel system, the more energy it can supply in bad weather.

If the PV system has an output of 1 kW for one hour, it has generated an amount of energy equal to 1 kilowatt hour. The storage unit will be charged after a few hours even in suboptimal weather.

The size of the battery storage unit in kilowatt hours

The size of an energy storage unit is not given in kWp but in kWh, i.e., in kilowatt hours. This storage capacity shows how much energy can be absorbed or released during a certain period. The quantity for this is the hour, i.e., how much energy can be provided in one hour.

A solar storage unit with a capacity of 11 kWh can therefore deliver or store 1 kilowatt of power for 11 hours. Our 11 kWh sonnenBatterie 10 can provide up to 4.6 kW of power at one time, therefore it is full in just under two and a half hours, given that it is charged at full power.

As a rule, the sonnenBatterie is designed so that a household can supply itself with stored energy from evening to morning. Between 5.5 kWh and 11 kWh is the right size for many households.

The household is not always completely supplied by the PV system or the home storage system. In the morning or early evening, this is mixed because, for example, the sun cannot yet supply enough energy. If, for example, 5 kW are needed and the PV system can only supply half of it, the remaining part comes from the sonnenBatterie so that no energy from the grid has to be drawn. The morning coffee is then literally made with the stored solar energy from the previous day.

Summary

• The capacity of an energy storage system is measured in kilowatt hours (kWh), the output in kilowatts (kW).
• The size and thus maximum output of a PV system is measured in kilowatts peak (kWp), the so-called nominal output.
• The capacity of the electricity storage system and the output of the PV system should be well matched.
• The capacity of the electricity storage system should be based on your electricity needs.