Electricity

Are battery storage for photovoltaic systems worthwhile?

Buy a battery storage system for the new photovoltaic system – is it worth it? Here you can find out what such a store can do and what you should pay attention to when buying.
The most important things in brief:
Purely financially, a battery storage system for private households is often not worthwhile.
The biggest advantage of a storage system: You can increase your own consumption and degree of self-sufficiency.
Do not choose the storage capacity too large.
Table of contents
What does a battery storage do?
What types of battery storage are there?
What parts does a battery storage system consist of?
What size should a well-designed battery storage be?
How many years does a battery storage last?
What is the difference between a DC and an AC-coupled memory?
Does a battery storage bring independence?
Can I store my electricity for several weeks or months?
Can I also use a battery storage as an emergency power system?
Do I need intelligent charging management for battery storage?
How much does a battery storage cost me to purchase?
What operating costs should I expect?
Does a battery storage increase the profitability of my solar system?
Where can I find an overview of the range of battery storage?
Are there funding programs for battery storage?
Is it worth retrofitting my existing solar system with a battery spawner?
What role do home storage play in the energy transition?
Are Stromcloud tariffs an alternative or a good combination to home storage?
How is a battery storage disposed of later?
Are battery storage bad for the environment?
What does a battery storage do?

With a storage system for the home, you can cache part of your self-generated solar power during the day to consume it in the evening and at night until the next morning. If the photovoltaic system generates more electricity than is currently consumed, the storage charges instead of feeding the electricity into the public grid. If there is more electricity demand than the PV system can provide – such as at night or at dusk – the electricity generated on the roof can be used for a time delay by discharging the storage.

What types of battery storage are there?

There are various technical solutions on the market. Lithium-ion batteries are best known. The advantages of lithium batteries are above all the high efficiency, the high energy density and the comparatively long service life even with intensive use. The storage capacity can be used almost completely in continuous operation without the battery suffering. The technology of lithium-ion batteries has prevailed in house storage in recent years. This technology is also used in other fields of application such as electric cars and in large battery storage systems in industry and in power plants. As a result, prices have also fallen in recent years due to the further increase in production figures and constant technical development.
Although they play practically no role on the market, two battery technologies are occasionally advertised: the redox flow battery and the so-called saltwater battery. While the suppliers of home storage systems with redox flow technology have partly withdrawn from the market, the saltwater batteries can actually be purchased. Unlike the compact lithium batteries, these consist mainly of non-flammable liquids and require multiple space and weight due to their low energy density. Disadvantages are the lower power output, higher losses and the open question of the expected service life. As advantages, higher safety (no fire hazard) and very good environmental friendliness are promised.
The lead-acid batteries used years ago no longer play a role as home storage. These batteries have proved to be unsuitable because they require regular maintenance and care and only achieve a significantly shorter service life.
What parts does a battery storage system consist of?

Battery storage systems for photovoltaic systems consist of lithium batteries, a battery management system, electronics for connection to the Internet and for monitoring and also require an inverter. Similar to the photovoltaic system, the stored electricity is converted from direct current into alternating current if necessary and fed into the house network.
What size should a well-designed battery storage be?

The purpose of a battery storage device is to store the solar power during the day for the evening and night. The battery is properly dimensioned if it covers the average power consumption between evening and morning. For example, if you consume 3,600 kilowatt hours of electricity per year in the year, a storage tank of up to 5 kilowatt hours of capacity would be roughly dimensioned correctly: 3,600 kWh / 365 / 2.
In practice, far too large storage is often installed, which causes the state of charge of the battery to fluctuate between half full and full. This high state of charge accelerates the aging of the battery, the unused capacity costs unnecessary money and wastes raw materials and resources spent in the production of the storage. A properly designed storage system leads to a significantly higher self-consumption of solar power. If the memory is significantly larger, self-consumption can only be slightly increased by the enlargement.
Therefore, the consumer center recommends about 1 kilowatt hour of battery capacity per 1,000 kWh annual electricity consumption as a guideline for the storage size. In addition, for small photovoltaic systems, the storage capacity of the battery in kilowatt hours should not be much greater than the power of the photovoltaic system in kilowatts. For a household with a 5 kWp PV system and an annual electricity consumption of 5,000 kWh, a storage tank of around 5 kWh would be ideal.
How many years does a battery storage last?

While solar modules can perform very well for more than 20 to 30 years, battery storage systems have an expected service life of 10 to 15 years.
The reason for this is that chemical processes take place in the battery cells, which lead to an aging of the materials. Some of the aging processes even take place independently of charging and discharging, which is why such batteries have something like an “expiration date”, which cannot be predicted exactly.
Aging leads to a decreasing storage capacity over time, which decreases slowly over a long period of time, but accelerates at the end of service life. A battery ages slowly at the beginning, but later faster.
Lithium batteries in home storage systems are designed for use with a PV system. High or very low ambient temperatures and a long-lasting high state of charge are unfavorable for this type of battery. Therefore, the batteries are best stored in a dry cellar room and should only be fully charged for a short time. It is ideal if the battery fully charges by the afternoon on a sunny day, so that the discharge begins afterwards due to the electricity consumption of the evening. By the following morning, the battery should be largely discharged again.
If the battery is too large or too small, the efficiency and cost-effectiveness of the storage suffers. Before purchasing, you should therefore think about how the memory should be used. If the storage is only operated with a photovoltaic system (without further technology), the size can be determined according to a rule of thumb: In this case, around one kilowatt hour of storage capacity should be installed per 1,000 kilowatt hours of annual electricity consumption. Is it foreseeable that electricity consumption will increase in the future and further technology will be used (e.g. through a planned purchase of an electric heat pump), the storage tank should be designed larger. A higher battery capacity is also useful if the battery storage is to offer additional functions such as emergency or replacement power.
What is the difference between a DC and an AC-coupled memory?

Solar generators generate direct current (DC), which must be converted into alternating current (AC) for use in the household or for fed into the grid. For storage systems, a fundamental distinction is made according to whether it is connected on the DC or AC side.
In an AC-coupled system, the solar power is first fed into the house network by the inverter of the photovoltaic system and removed from there for storage in the battery. Such AC-coupled memories have their own inverter that works in both directions: It can convert the alternating current from the photovoltaic system back into direct current to store it in the battery. And in the opposite direction, it generates alternating current for the household appliances again from the stored direct current.
In a DC-coupled system, the solar power is charged almost directly from the solar generator into the battery. DC-coupled systems do not require their own inverter, but also use those of the photovoltaic system. Such photovoltaic inverters, to which a memory can also be connected, are called hybrid devices. Or the solar modules are connected to the storage system in which the inverter is integrated for both systems.
DC-coupled systems can work more efficiently than AC-coupled storage and are often cheaper to purchase, so DC-coupled systems are often used in new buildings. In contrast, AC-coupled systems can often be combined more flexibly with the photovoltaic system and can also be retrofitted more easily with existing systems.
Does a battery storage bring independence?

A battery storage system reduces the purchase of electricity from the public grid even more than the photovoltaic system alone. With a battery storage system, solar power can also be used yourself, which would be fed into the power grid without storage. Independence from the electricity supplier increases from 25 percent to up to 70 percent (degree of self-sufficiency) in a typical single-family house with photovoltaic system.
As a result, less electricity is fed into the grid and sold. The proportion of solar power used in the house (own consumption) increases significantly.
However, the household does not become completely independent of the power grid by a battery storage system, because in the winter months of November to January photovoltaic systems provide little electricity. The battery storage cannot change that either.
Can I store my electricity for several weeks or months?

No, seasonal storage of electricity, e.g. keeping the energy from summer ready for the winter, cannot be implemented with a battery storage. For this purpose, research is developing hydrogen storage systems in which solar power is split into oxygen and hydrogen by electrolysis in the summer. The hydrogen can then be stored in gas cylinders or large gas storage tanks and converted back into electricity and heat in a fuel cell in winter. A large part of the energy is lost and such systems are not yet widely available for domestic use. However, a first provider launched such a domestic energy system in 2021.

Can I also use a battery storage as an emergency power system?

Yes, an emergency or replacement power supply in the event of a temporary power failure can be useful. However, such functions are only part of the range of functions for some home storage systems and range from an emergency power socket on the memory to the automatic switch to replacement power operation, in which all devices in the household are then also supplied. In this way, important household appliances can be supplied independently of mains power – albeit for a limited time. Important: Often the functions of emergency and replacement power capability are only available for an extra charge, with some providers these functions are not available.

Do I need intelligent charging management for battery storage?

In practice, the full potential of the PV system is often not exploited. Since small PV systems are only allowed to feed into the public power grid with a maximum of 70% of their rated power, it makes sense to consume as much as possible yourself or to charge the battery storage during the sunny midday hours. An intelligent charging control does this fully automatically on the basis of weather forecasts and previous consumption values. At the same time, intelligent control by battery-friendly charging and discharging can contribute to a significantly improved battery life.
The concrete implementation of the intelligent charging and discharge of the storage can vary depending on the system. Intelligent charging control is often already integrated into the storage system. Another option is the external control via an energy management system. This can affect all major production and consumption plants of the household (such as PV system, e-car charging station, heat pump, etc.) centrally control and optimize. Care should be taken to ensure that the individual components are compatible with each other in order to exploit the full potential of intelligent networking and control.
In general, intelligent charging management makes perfect sense due to the increase in efficiency and service life of the storage system.
It is also important how much electricity the power storage system itself needs for functions and electronics. Good storage systems only need a few watts in stand-by mode and are therefore very efficient. The annual electricity storage inspection of HTW Berlin provides a good overview of the efficiency of storage systems.
How much does a battery storage cost me to purchase?

The acquisition costs for home storage have fallen significantly in recent years, as a market analysis also shows. Currently, prices are hardly falling anymore, which can be justified by the current high demand, but also by the shortage, e.g. for control chips. In the medium and long term, storage prices will continue to fall, also because huge production capacities for lithium-ion batteries are currently being built up worldwide.
The prices for battery storage are often compared in euros per kilowatt hour of storage capacity. According to the industry association BVES, a new home storage system had an average capacity of 8.5 kWh in 2020.
Including VAT and installation, these prices in 2021 in North Rhine-Westphalia were about between 750 and 1,200 euros per kilowatt hour for storage facilities greater than 5 kWh. Smaller storage costs significantly more per kilowatt hour than larger systems. Buyers of a home storage system invested an average of 11,000 euros in the battery in 2020. The costs depend decisively on the storage system (AC or DC system) and the range of functions. Storage systems that also offer an emergency or replacement power function, for example, are usually more expensive.
What operating costs should I expect?

Lithium battery storage can be operated largely maintenance-free. The devices are usually connected to the Internet and can thus be conveniently monitored via smartphone app. In the course of the system check of the photovoltaic system, which should be carried out approximately every 4 to 5 years, the consumer center also recommends checking the battery storage.
The manufacturer’s warranty conditions do not always cover all costs even during the up to 10-year warranty period. Some guarantees only cover the battery itself, but not the extensive electronics of the storage system. Sometimes cost risks for operators are hidden in the small print of the warranty conditions.
Does a battery storage increase the profitability of my solar system?

Battery systems have become more efficient and cost-effective in recent years. Nevertheless, in most cases, a storage system rather worsens the profitability of the photovoltaic system, as the systems are still expensive and the electricity can alternatively be fed into the grid for remuneration. This is often more worthwhile than self-consumption after storage. But the decisive factor is how the future electricity price develops: If this continues to rise, electricity storage can pay off in the future. Often, the financial aspects play only a minor role for buyers. The higher independence from the electricity supplier, more self-consumption of your own solar power and a personal contribution to the energy transition are often the main reasons for the purchase decision.
This is how a battery storage system affects economic efficiency:

Solar power from the roof is significantly cheaper than the purchase of electricity from the grid. While the kilowatt hour of own solar power costs around 10 cents, it is currently around three times as much for mains power. The more solar power is consumed directly from the photovoltaic system during the day, the better this is for the profitability of the photovoltaic system.
According to the same logic, providers of battery storage argue with the stored solar power consumed overnight. But the cost of the storage is often not estimated and some optimistic assumptions are made: The service life of the battery storage is often equated with the photovoltaic system instead of setting a more realistic, shorter service life (10-15 years). The price of electricity is unrealistically high increases of three to seven percent per year. From 2013 to 2020, however, the household electricity price rose on average by only one percent per year. And currently, the policy to promote the energy transition wants to stabilize the price of electricity even further and has on the 1st. July 2022 abolished the EEG surcharge.
The question of the profitability of a battery system depends heavily on what expectations are set for the development of the electricity price in the coming years. A forecast is currently very difficult.
The calculation usually also forgets the power consumption of the battery storage itself. In a typical household with an average large photovoltaic system, for example, 1,500 kilowatt hours of solar power per year flow into the storage system. About 1,200 kilowatt hours of this can be used in the household. The remaining 20 percent and thus around 300 kilowatt hours fall by the wayside during storage as losses, standby consumption and protection against deep discharge in the winter months.
Where can I find an overview of the range of battery storage?

In general, it is advisable to seek advice directly from a specialist company. If you are looking for technical information on different systems and want to compare the functionality of home storage on the market, here are some freely accessible overviews:
C.A.R.M.E.N. – Market Overview
Pv magazine – Market overview
HTW Berlin – Electricity storage inspection
Are there funding programs for battery storage?

In some federal states, there are subsidies for the purchase of a battery storage system, usually in connection with the purchase of a new photovoltaic system. These funding programs are often quickly exhausted and are then sometimes resumed at a time interval. Many municipalities or regions also offer subsidies, partly as part of regional solar campaigns. A call to your own municipal administration and a look at the website of the state government can help here.
At the KfW development bank, photovoltaic systems can be financed cheaply through loans, but a special loan program for electricity storage was discontinued some time ago. As part of an energy-efficient new building or an energetic renovation of an existing building, photovoltaic systems with battery storage systems are supported, including as part of the newly introduced BEG funding at KfW in 2021 – but only if no feed-in tariff according to EEG is then claimed. Up-to-date information about all funding programs can be found in our article “Funding for the home”.
Is it worth retrofitting my existing solar system with a battery spawner?

Purely financially, retrofitting battery storage systems in existing photovoltaic systems is usually not yet worthwhile. However, it depends on the consideration: If you assume sharply rising electricity prices in the long term, economic efficiency is getting closer. The purchase of a storage facility is also often considered for the over 20 photovoltaic systems, where the high EEG remuneration of the early years ends.
You have to take a close look at it: First, there are costs for the memory itself and the installation. It may also be necessary to make changes to the meter cabinet that incur additional costs. Above all, the decisive factor is how high the feed-in tariff is and how long it will still be paid. For solar systems that are a few years old, which receive a comparatively low feed-in tariff and can still be operated for a long time, retrofitting at a later date can be financially worthwhile with future falling battery storage prices – even if it was different when the system was put into operation. Here, too, the following applies: Decisive for a possible economic efficiency is the question of how the price of electricity will develop in the future.

What role do home storage play in the energy transition?

Decentralized battery storage can occupy an important place in building technology for prosumers in the future. They serve as important short-term storage for the day and night compensation of photovoltaic generation.
Battery storage systems can also have an important function for the power grid for buffering the generation power and consumption peaks and thus relieve the power grid. For this purpose, the first field tests and investigations are underway, in width this function is not yet offered on the market.
Electricity storage systems can also support the traffic turnaround. Grid expansion can be significantly reduced if electricity storage, solar energy and charging infrastructure of electric cars are intelligently interlinked. Battery storage also plays an important role in setting up the fast charging network for electric cars, they assume a buffer function so as not to overload the local power grid.
If the right framework conditions are set, battery storage in houses with photovoltaic systems can also help to reduce the expensive grid expansion necessary for the energy transition and at the same time to connect more photovoltaic power to the grid.
Are Stromcloud tariffs an alternative or a good combination to home storage?

By the term “cloud” the providers mean a kind of “electricity account” into which excess electricity is fed into the grid and obtained from the grid again at other times. It is often claimed that the power grid works as a virtual power storage system. But: The power purchase “from the cloud” is technically quite normal power purchase from the grid. So the concept is fundamentally different from the storage of solar power in the physical battery storage. However, it is not uncommon for both PV systems and battery storage to be offered in combination with such tariffs.
The cost and contract conditions are often complex and difficult to understand. So far, such tariffs are not worthwhile for solar system operators, as feeding in surplus electricity and purchasing the residual electricity from a green electricity provider are cheaper than the special electricity cloud tariff. Further information on Stromclouds can be found in our article “Stromclouds: Special tariffs for prosumers have their price”.
How is a battery storage disposed of later?

The batteries of a battery storage system also reach their end of their lives at some point and then have to be disposed of. The Battery Act in Germany stipulates that the disposal costs must be borne by the manufacturer or the seller. This is free of charge for consumers, so as the owner of a battery, you are not at risk of expensive disposal coming later.
Are battery storage bad for the environment?

With all the technical advantages that lithium-ion batteries offer in home storage, the question of the environmental compatibility of the materials – especially lithium and cobalt – also arises again and again. In fact, both the environmental impact and social ills in the extraction of these materials must be assessed in some cases critically. Even against the background of the increasing demand in electric cars, improvements through stricter production legislation, improved recycling and research on raw material alternatives are urgently needed and are already being implemented in part. Research is also currently trying to simply reduce the required amounts of lithium and cobalt material for future batteries. And the recycling of batteries is also already being practiced, but currently (due to the small quantities of defective batteries) in the test stage and not yet in a large-scale implementation. However, it can be assumed that batteries purchased today will definitely be recycled according to their service life.

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