How is a battery storage charged

Battery storage - send the sun into prolongation?

Economy of a battery storage

Battery storage is currently rather uneconomical, but the attractiveness of battery storage can increase with rising electricity prices and falling feed-in tariffs for solar power. This could be particularly interesting for system operators who no longer receive any feed-in tariffs after 20 years.

Storage technologies and the market for battery systems continue to develop: prices are falling and demand is rising. They are attractive to many consumers because they can use a much higher proportion of solar power themselves and the household becomes more independent of the energy supplier. A battery storage system is even more worthwhile:
• the cheaper it is to buy and install,
• if the feed-in tariff is low and the electricity purchase price is high,
• if the self-consumption share of solar power increases significantly through the storage,
• if you take advantage of subsidy programs and tax advantages when purchasing.

Choosing the right memory

The decisive factor in a battery storage system is always the usable net capacity. This indicates the percentage of the memory that can be discharged without being damaged. Some manufacturers only give the gross capacity - ask again in this case.

We recommend choosing the battery storage so that the storage capacity per 1,000 kWh of annual electricity consumption in the household plus the electricity consumption of any commuter vehicle that may be used is a maximum of 1 kWh. For example, with an annual electricity consumption of 3,500 kWh plus 2,500 kWh of electricity for an electric car that is not parked at home during the day, the storage capacity should not exceed 6 kWh. Without a commuter vehicle, the maximum recommended storage capacity would be 3.5 kWh. For a battery-powered second car that is parked at home during the day, we recommend that you do not provide for additional battery storage capacity, as the use of the battery storage is then too little.

In order for the storage to be used sensibly, the capacity in kWh should not be greater than the output of the PV system in kWp.

Lead or lithium?

This question rarely arises today. Because nowadays there are hardly any lead storage systems available for home storage. These systems have not caught on due to high maintenance intensity, short service life and the associated high costs. Today's standard are lithium ion batteries. They are much more durable and efficient than lead storage.

Lifespan and number of cycles

A distinction must be made between two aspects regarding the service life of the batteries: The number of cycles and the aging in the course of operation ("calendar aging").

Manufacturers often advertise 4,000 and more charge and discharge cycles. A cycle here means to fully charge the battery once and then to discharge it completely. The number of cycles indicates how often the battery capacity can be used. An average household reaches around 250 cycles per year. Mathematically, this would result in a service life of over 16 years.

However, battery storage systems are also subject to calendar aging. Chemical processes in the battery cells are responsible for this. These are less dependent on the use, but rather on the design and quality of the cell and the ambient temperature. The storage tank should be in a room with a maximum of 20 ° C to 25 ° C - the basement is therefore better suited than an attic, which gets very warm in summer. Experts assume a realistic service life of 10 to 15 years. However, there is still no practical experience with today's systems over such a long period of time.

Presumably, the battery storage will reach the end of its calendar life long before it has run through the technically possible number of cycles. Therefore, anything that increases the number of charges and discharges improves the economy of the battery storage system. More intensive use of the battery should therefore not reduce the service life. From this point of view, it also makes sense to buy a battery with a smaller than too large capacity, because this also increases the number of cycles actually used.

The manufacturer's guarantees extend to up to 10 years, whereby the guarantee conditions can be very different. For example, there are guarantee conditions that name additional costs if the guarantee occurs.

The right charging strategy for home storage

In the simplest case, the storage system charges its battery as soon as electricity from the PV system is not needed for other household applications at the same time and would otherwise be fed into the grid. In practice, this means that the battery is often fully charged early in the morning and the solar power of the photovoltaic system has to be cut at lunchtime due to technical requirements. Because small PV systems are usually operated in such a way that they can only feed into the grid with a maximum of 70% of their nominal power. To avoid this, the memory should be loaded intelligently, i. H. based on the forecast and observation of the weather and electricity consumption in the respective household.

Forecast-based storage regulations also support the grid-friendly operation of photovoltaic systems and battery storage systems. Grid-friendly here means that the battery storage system is primarily charged with the PV system's high generation power, for example at noon. This supports the more even use of the power grid and can reduce the grid costs that all electricity customers have to bear.

Additional electricity consumption

Storage devices themselves consume electricity, for example for charging electronics. This results in additional energy consumption that can amount to several hundred kilowatt hours per year. This corresponds to the consumption of a larger household appliance. However, there are devices with monitoring electronics that switch the memory to a "sleep mode" in times of low sunshine, for example in winter or at night. This reduces the storage's own consumption.

There are single-phase or three-phase connected storage systems. Three-phase systems deliver a higher electrical output, but they also have a higher self-consumption.

Electromobility and battery storage

If the solar power share of the e-car is to be increased with the help of the battery storage, there are other technical aspects. Because e-cars charge with at least 1.4 kWt of power, the battery storage should achieve at least this output power; more would be better. The higher the output power of the battery storage system, the more stored solar power ends up in the electric car. However, a high output is only worthwhile to a certain extent. Because the disadvantage of a battery storage system is that its efficiency usually drops if it is not used or not used sufficiently. At times when the e-car is not being charged, a battery storage system with a higher output power cannot call it up at all. So it gives off significantly less energy than it could under optimal circumstances. More energy is lost unused. In addition, memories with more output usually have larger storage capacities than are recommended for private households. The costs are higher, while utilization and efficiency are lower.