What is a battery storage system?
A battery storage system stores electrical energy and releases it when needed. The basic principle is simple: if more electricity is available than is currently being consumed, the storage system charges. If energy is needed later, it discharges again and supplies the connected consumers. In practice, this is primarily known from photovoltaic systems, emergency and backup power solutions, or mobile energy systems on construction sites and at events.
For many beginners, this initially sounds as if a battery storage system is simply a power tank. While not entirely wrong, the technical aspect of the topic needs to be looked at more precisely. Because a storage system is not only judged by its size, but also by how much power it can deliver or absorb at any given moment. This is precisely where the difference between storage capacity and power lies.
Key Takeaways
- Battery storage systems store electricity with a time delay: Excess energy is absorbed and later released as needed.
- Capacity and power are two different things: kWh stands for the amount of energy that can be stored, kW for the simultaneously available charging or discharging power.
- Charging power and discharging power are crucial in practice: They determine how quickly a storage system can be filled and which consumers can actually be supplied.
- The application determines the correct design: A house, a construction site, an event, or an emergency power solution all have very different requirements.
- Practical Tip: Never compare battery storage systems solely by the kWh figure, but always evaluate capacity, power, charging power, discharging power, and intended use together.
1) Simply Explained: How a Battery Storage System Works
A battery storage system consists of four main components:
the actual battery with its cells
a battery management system
an inverter or power electronics
the control system for charging, discharging, and protection functions
The battery stores energy chemically. During charging, electrical energy is converted into chemical energy. During discharging, this process is reversed. The battery management system monitors voltage, temperature, state of charge, and cell balance. The power electronics ensure that the stored electricity is available in the appropriate form.
In a building, this usually works as follows: The PV system produces electricity during the day. Part of it is consumed immediately, the rest charges the storage system. In the evening or at night, when the sun is no longer shining, the stored electricity is used again. With mobile battery storage systems, the principle is similar, except that the storage system is often recharged from the grid, via generators, or sometimes via PV.
A battery storage system is not just a power tank, but a system consisting of a battery, battery management, power electronics, and control.
2) The Most Important Difference: Capacity is Not Power
Many confuse these two values. However, it is crucial to separate them clearly for the correct selection.
✓ Storage Capacity kWh
- Describes how much total energy is stored in the system.
- 5 kWh typically represents a smaller storage system.
- 20 kWh is significantly larger.
- 100 kWh or more falls within the typical range for larger commercial or mobile applications.
✕ Power kW
- Describes how much energy can be charged or discharged at any given time.
- Determines how quickly the stored energy can be accessed.
- Dictates which consumers can be supplied simultaneously.
- Is often the decisive factor for peak loads and larger machinery.
Capacity can be thought of as a tank: a small tank stores little, a large tank stores a lot. However, capacity does not yet indicate how quickly this energy can be delivered.
A simple example: Storage system A has 10 kWh capacity and 2 kW power. Storage system B also has 10 kWh capacity, but 10 kW power. Both therefore store the same amount of energy. However, storage system B can supply significantly more consumers simultaneously or charge and discharge significantly faster.
3) kWh and kW Simply Explained
kW = Power
Kilowatt describes instantaneous power.
Examples:
Kettle: approx. 2 kW
Fan heater: approx. 2 kW
Smaller circular saw: approx. 1 to 2 kW
Larger construction machine: significantly more
kWh = Amount of Energy
Kilowatt-hours describe the energy stored or consumed over time.
Example:
A consumer with 2 kW runs for 3 hours:
2 kW × 3 h = 6 kWh
This is precisely why a 10 kWh storage system cannot supply a 2 kW consumer indefinitely, but theoretically for about 5 hours – ignoring losses and real operating conditions.
When comparing battery storage systems, always consider kWh together with kW. Only then does it become clear how long a storage system will last and what it can deliver simultaneously.
4) What do charging power and discharging power mean?
In addition to capacity and overall power, there are two other important terms.
Charging Power
Absorption- Describes how quickly a storage system can absorb energy.
- A storage system with 10 kWh capacity and 5 kW charging power can be charged with up to 5 kW under ideal conditions.
- Is important when energy sources temporarily supply a lot of power, such as PV or generators.
Discharging Power
Output- Describes how quickly the storage system can release energy again.
- Is particularly important for emergency power, larger individual consumers, construction sites, and events.
- Determines whether peak loads in commercial settings or starting currents of machines are manageable.
Practical Relevance
Design- A storage unit can supply small loads for a long time.
- However, this does not automatically mean that it can also start large machines.
- The combination of capacity and available power is always crucial.
5) Why this distinction is so important in practice
In practice, battery storage systems are often considered too simply. Many first ask: How many kWh does the storage system have?
This is understandable, but not enough. A storage system must always fit two things: the desired runtime and the required power.
✓ Example 1: Single-Family Home Home System
- A 10 kWh storage system can be useful for evening consumption.
- However, if a heat pump, stove, and other larger consumers are to run simultaneously, the discharge power can become a bottleneck.
- For home applications, therefore, not only the runtime but also the simultaneously available power is important.
! Example 2: Construction Site Mobile
- A mobile battery storage system with 30 kWh initially sounds large.
- If a machine draws 15 or 20 kW for a short time, the power electronics must be designed precisely for this.
- Without adequate power, capacity alone is of little use.
Example 3: Emergency Power
For emergency power, it's not just how long a storage system lasts, but also which consumers are allowed to start up at all. Lighting and routers are technically completely different from pumps, compressors, or heating systems.
A battery storage system must always match both the desired runtime and the required power. Only then is it meaningfully designed for practical use.
6) How long does a battery storage system last?
This always depends on two factors:
the available capacity in kWh
the actual consumption in kW
An example:
A storage system has 20 kWh capacity. The connected consumers draw a total of 4 kW.
This results in a calculated runtime of:
20 kWh ÷ 4 kW = 5 hours
In practice, losses, reserves, and control limits are also added. Furthermore, a storage system is usually not discharged to the last drop. The real runtime is therefore somewhat shorter.
7) What else is important for beginners regarding battery storage systems?
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1
A storage system does not automatically replace an emergency power generator. It is very well suited for quiet energy supply, optimizing self-consumption, short-term supply, peak load management, and consumption optimization. For long outages or very high power requirements, a classic power generator is often still the more robust solution.
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2
Large capacity alone is not everything. A storage unit with a high kWh but low kW cannot cover every application.
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3
The power electronics are just as important as the battery. The battery stores the energy, but the electronics determine how well this energy can be used in practice.
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4
The intended use is decisive. A storage unit for a single-family home, for a commercial building, for a construction site or for an event has completely different requirements.
8) Where battery storage systems are used today
Battery storage systems are used today in very different areas:
Buildings and Commercial
Stationary- Single-family homes for using PV power
- Commercial buildings for reducing peak loads
- Emergency power and backup power solutions
Mobile Power Supply
Construction Site & Event- Construction sites as mobile, quiet energy sources
- Events as an alternative or supplement to a generator
- Hybrid systems together with photovoltaics and power generators
Hybrid Systems
Combination- Collaboration with photovoltaics
- Combination with power generators
- Flexible adaptation to changing load profiles
9) Frequently Asked Questions (FAQ)
What is a battery storage system simply explained?
A battery storage system stores electrical energy and releases it again when needed. It charges when more electricity is available than currently required, and discharges when energy is needed later.
What is the difference between kWh and kW?
kWh describes the amount of energy stored or consumed. kW describes the power, i.e., how much energy can be charged or discharged simultaneously.
Why isn't it enough to only look at the kWh number?
Because capacity alone does not say anything about how quickly the stored energy is available. In practice, capacity, charging power, discharging power, and intended use must always be considered together.
Can a battery storage system replace an emergency power generator?
Not automatically. A battery storage system is very well suited for quiet and short-term supply. For long outages or very high power requirements, a classic power generator is often still the more robust solution. A hybrid solution is often sensible.
10) Conclusion
A battery storage system stores electrical energy and makes it available again when needed. Two values are particularly important for the correct selection:
kWh = how much energy can be stored
kW = how much power can be charged or discharged simultaneously
Capacity thus determines the possible operating time, power determines which consumers can be supplied. Only when both values match the application is a storage system sensibly designed.
Those who compare battery storage systems only by their kWh number often fall short. In practice, capacity, charging power, discharging power, and intended use must always be considered together.
If you would like to check which battery storage system suits your application, we at SEV will be happy to advise you – whether for buildings, emergency power, construction sites, events, or hybrid energy solutions.