Electricity storage for photovoltaics: Costs & Funding

Lithium storage makes the self-consumption of solar power easier and is becoming cheaper and cheaper. We will show you what photovoltaic batteries cost, which models perform well in the test and what you should look out for when buying a P.V. storage system.

What are the advantages of a photovoltaic system with storage?

With a solar system with storage, homeowners can generate solar power themselves and use it in the evenings, for example. The advantage is that with battery storage, you increase the self-consumption of your solar power and thus save electricity costs. However, this is offset by the costs of the memory.

Are there other P.V. storage systems besides lithium storage?

Lithium storage has now established itself on the market for photovoltaic systems. They are efficient, durable, almost maintenance-free, and are always becoming cheaper. There are other battery technologies as well. According to an overview of battery storage power box systems by the association C.A.R.M.E.N., the following different cell types are currently on the market for detached and semi-detached houses:

  • Storage systems made of lead cells: the predecessors of lithium storage are still available from numerous suppliers
  • (Vanadium) Redox Flow Battery (V.R.F.B.) from VoltStorage
  • Saltwater batteries (Aqueous Ion Exchange Battery/A.I.B.) from Autarxia and BlueSky Energy
  • Innovenergy molten salt battery

Photovoltaic with lead-acid battery: advantages & disadvantages

Battery storage with lead is still suitable for apartment buildings with high electricity requirements. If a larger storage system with 50 or more kilowatt-hours is planned, information can still be advantageous for cost reasons. Lead storage is also less sensitive to heat than lithium storage. The location of a lead storage tank should also be as cool as possible: better in the basement than under the roof.

However, lead-acid batteries take up a lot of space. Lead-liquid acid batteries also require venting. Hydrogen can form in the cells and must be removed. Otherwise, very explosive oxyhydrogen will be produced. We must check the degree of acidity occasionally and top up distilled water. You can do this yourself or leave it to a specialist company, for example, during annual maintenance.

How do redox flow batteries work?

A third storage type for photovoltaics is the redox flow battery. Two liquid electrolytes with metal ions flow from tanks through a cell, which generates electricity in a chemical process. This principle is reversible: if solar power is left, the redox flow battery converts it back into chemical energy and stores it in the tanks.

One advantage of redox flow systems is that we can repeat the charging process 10,000 times (number of cycles).

What are the advantages of lithium batteries?

Lithium R.V. batteries use 80 to 100 percent of their capacity; in contrast, lead batteries are only ever discharged up to half. With the same volume, lithium storage stores twice as much energy as lead-acid batteries and thus requires significantly less space than lead-acid storage. In addition, there are now many suppliers of lithium batteries, and there is still a lot of research and development on this type of battery. That is why the costs for solar storage systems with lithium are continuously falling.

Lithium batteries also have low energy losses compared to other storage devices. Based on the battery, the efficiency is between 95 and 98 percent. Lead storage has about 10 percent higher losses.

A good lithium battery lasts between 6,000 and 8,000 charge cycles. Regarding the usable storage capacity, however, it is still somewhat more expensive than a comparable lead-acid battery.

Photovoltaic & Battery: Costs

Compact lithium storage costs around 700 to 1,200 dollars per kilowatt hour for average detached and semi-detached houses. A lithium storage unit with 4 to 8 kilowatt hours (kWh) for an average single-family house costs around 3,000 to 10,000 dollars.

Lead storage costs between 250 and 350 dollars per gross kilowatt hour. However, since they can only be discharged halfway, this costs between 500 and 700 dollars per usable kilowatt hour (net kilowatt hour). A lead storage tank with 4 to 8 net kilowatt hours for an average single-family house costs around 2,000 to 6,000 dollars.

Is an electricity storage system for photovoltaics worthwhile?

Lithium storage costs fell sharply a few years ago. Nevertheless, according to the North Rhine-Westphalia consumer advice center, battery storage for solar systems is still “rather uneconomical” (as of February 2021). Many manufacturers had been counting on the fact that electricity prices would continue to rise and that the self-consumption of solar power would become increasingly worthwhile. High increases in electricity prices are by no means certain, however.

“The first suppliers are announcing electricity price reductions, and we will reduce the E.E.G. surcharge in 2021 and 2022. This contradicts many of the providers’ economic forecasts,” explains Thomas Seltmann from the consumer advice center in North Rhine-Westphalia.

Solar power storage: Promotion

Various solar-powered generator subsidies from states, municipalities, and energy suppliers exist. The funding check shows you which grants are available in your region. We can also find further information on subsidy programs for battery storage in the list of current subsidies for photovoltaic storage.

Another form of funding for photovoltaic storage is a low-interest loan from KfW number 270 (renewable energies – standard). It also promotes the retrofitting of a battery storage system. However, KfW no longer pays a repayment grant. The corresponding subsidy program 275 (renewable energies – storage) was discontinued.

Buying P.V. storage: You should pay attention to this

When you purchase solar storage or solar backup generator, a few details determine the quality of lithium storage:

  1. High efficiency

Good lithium storage is characterized above all by the fact that little energy is lost when storing and withdrawing:

Good lithium batteries achieve a battery efficiency of 95 to 98 percent.

The battery inverter causes further losses. Good inverter efficiency is between 90 and a little over 97 percent.

Both combined result in the system efficiency that an installed battery, including an inverter, can achieve. Lithium systems come up to 94 percent. Six percent of solar power is lost in heat when stored and withdrawn.

  1. Low standby losses

Like any other electrical device, a battery storage system also needs electricity on standby. When discharged, the power consumption for standby should not exceed 2 to 5 watts. The best solar batteries consume only 1 to 3 watts on standby when fully charged, according to the H.T.W. Berlin electricity storage inspection.

3. Fast, accurate regulation

In real operation, temporary, undesirable deviations from the target battery performance are inevitable. The result is an unpleasant energy exchange between the storage system and the power grid. To keep these deviations as small as possible, the so-called settling time should be less than 2 seconds, and the steady-state control deviations should be less than 5 watts.

The values should be apparent from the manufacturer’s data sheets. If not, ask specifically about it before buying a photovoltaic storage system!

How big does a storage battery have to be?

In a single-family home, a photovoltaic storage system should absorb between 4 and 8 kilowatt hours (kWh) of solar power. Such storage is about the size of a refrigerator, so we can easily place it in the house.

The energy a battery can store is called its energy content or storage capacity. Photovoltaic storage systems in residential buildings are designed according to their storage capacity in kilowatt-hours. They must cover the electricity requirement in the house even when there is no sun.

In addition to the energy content, the performance of a storage unit in kilowatts (kW) can also be important. If there are powerful electricity consumers in the building, the power must match them. It is important, for example, if an electric car is to be charged from the battery in the house. Electric cars sometimes require high currents for short periods. Photovoltaic storage systems with an emergency power function also need sufficient power to bridge the power failure and supply several devices with electricity simultaneously.

Photovoltaic storage: calculate the size

The size of the photovoltaic storage system or solar power station depends on the power consumption in the building and the performance of the photovoltaic system. The rule of thumb is: per 1,000-kilowatt hours of power consumption

the power of the P.V. system should be at least 1 kilowatt and

the storage capacity of the solar storage tank should not exceed 1-kilowatt hour.

The power of the P.V. system tends to be higher, and the storage capacity tends to be lower. That’s because most homes only use their storage for the electricity they use in the evenings and at night. On the other hand, the P.V. system should still supply enough electricity in the evening when it is only running at the partial output.

Example: A 4-person household has an annual electricity consumption of 5,000-kilowatt hours. Then the memory should have 4 to 5-kilowatt hours of usable storage capacity. The photovoltaic system should have a nominal output of 5 to the 8-kilowatt peak.

Which providers for photovoltaic storage are there?

The association C.A.R.M.E.N. offers a good, annually updated market overview of P.V. storage. Interested parties can use the online market overview to sort the available batteries according to cell type, emergency power capability, and storage capacity in kilowatt-hours, for example. List prices are also given for some solar storage tanks.

How long service life can a solar power storage unit achieve?

The so-called cycle is decisive for the service life of a solar power storage unit, i.e., the one-off charging and discharging. One cycle is calculated per sunny day in a year with about 150 to 230 cycles. Lithium-ion batteries achieve a service life of 6,000 to 8,000 processes; lead storage earns 3,000 to 4,000. Guideline values are at least ten years for a lithium phosphate battery and seven to ten years for a lead battery – even if more is mathematically possible.

On the data sheets of the solar power storage, the number of cycles is the theoretically achievable value. They are often used as a basis for the manufacturer’s guarantees. The aging condition of a battery is called the “State of Health” (SoH).

What should be considered during installation and connection?

The increased safety and fire protection requirements are important for designing and installing photovoltaic storage systems. The building should be left to professionals, as with any major electrical system. Some storage systems are connected to the power grid in three phases with 400 volts.

Photovoltaic storage systems with lithium cells like it cool up to a maximum of 25 degrees Celsius. The warmer the location of the battery, the faster it ages. When the temperature drops below zero, the charging capacity of the 12v lithium battery decreases. Lead-acid batteries are more robust in terms of installation location.

In any case, the photovoltaic storage system should be easily accessible, for example, for inspection and maintenance purposes. It should also have an isolating switch, similar to photovoltaics. The fire brigade can quickly disconnect the system from the house network if a fire is in the house.

What is a storage log?

The P.V. storage log is an essential part of the documentation for a photovoltaic system. The values recorded therein describe the technical condition of the system when it was commissioned. If there are problems later, We can find the error more quickly with the values from the memory log. Insurance companies also value the protocol because it proves that the installation company installed the photovoltaic system according to state of the art.

The predecessor of the P.V. storage protocol was the P.V. storage pass until 2020. There is the P.V. combination protocol if a storage unit is installed with a photovoltaic system.

Photovoltaic storage: direct current or alternating current?

Storage for photovoltaics with batteries made of lithium or lead is D.C. voltage systems, just like photovoltaics. The inverter converts the direct current from the storage into an alternating current for household appliances. Storage systems have the lowest losses when the solar power flows directly into the battery and only via the 12v inverter into the home network. It is usually the case with new installations of solar systems with storage. A separate battery inverter is generally installed when retrofitting an existing P.V. system.

Retrofit photovoltaic storage?

It is also possible to retrofit a power storage unit if a photovoltaic system is already in place. Electricity storage can be retrofitted anywhere and at any time. Adjustments to the house electrics and their protection may be necessary. It is one of the reasons why retrofitting an electricity storage system is a case for experts. When buying, ensure the solar storage tank fits your photovoltaic system.

Are photovoltaic storage systems safe?

We may charge lithium storage with 300 or 500 volts. However, such systems and connections are secured accordingly.

Lead storage tanks with liquid acid need degassing that leads to the outside. It is important because they give off hydrogen, which can ignite. Regular inspection is important, usually once a year. It costs about 100 to 200 dollars.

Can lithium storage burn?

Lithium batteries contain solvents that outgas when overheated. Lithium storage systems get hot when discharged too deeply or when a short circuit develops in the storage cell. However, before the cells release too deeply, the battery switches off automatically.

If the safety control of the battery fails, the affected cells form a gas flare, just like overheating smartphone batteries. Pressure-resistant metal housings, therefore, protect the lithium cells. In the event of a fire, the storage tank must also be able to be switched off from the outside via a safety switch.

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