What you need to know about battery storage

• What is an electricity storage system?
• Why is an electricity storage system worthwhile?
• How does an electricity storage system work in the home?
• What battery technologies are available for electricity storage?
• Why does sonnen only work with lithium iron phosphate technology?
• How economical is a storage system?
• How long is the service life of an electricity storage system?
• What is the difference between an AC and DC system?
• How independent does a storage system make me?

What is an electricity storage system?

An electricity storage system stores surplus electricity for a short time for later use. For example, a battery storage system such as the sonnenBatterie stores the surplus electricity produced by a PV system on a sunny day for the evening and night hours when energy is needed in the household, e.g. for cooking or for the washing machine, but the sun is no longer shining. In this way, homeowners can consume more self-generated electricity. Additionally, a higher self-consumption reduces electricity costs.

Why is an electricity storage system worthwhile?

If you produce green energy on your own roof with a photovoltaic system, you are doing something good for the environment. For many years, the system was always the same: either the electricity was consumed directly by the household itself, or it was fed into the national energy grid. Feed-in tariffs provided a return for producing green energy.

The self-produced solar energy, however, can only be consumed by the household in the hours in which it is also produced by the PV system - just in time, so to speak. This means that washing machines and refrigerators can only be supplied with free electricity from the roof during set times. The rest of the electricity is fed into the grid. But these payments have been falling for years.

The solution to being able to use one's own solar power independently of time and thus increase one's own self-consumption is called battery storage. This smart device stores the electricity generated by the PV system and keeps it available without loss until it is needed. Only when the home storage unit is fully charged is the surplus electricity fed into the grid.

By keeping a certain amount for self-consumption, the owners of an electricity storage system noticeably optimise their electricity costs, because, with ever lower acquisition costs and simultaneously rising electricity prices, self-supply is the most economical solution. Every kilowatt hour that is generated by the owner does not have to be paid for.

How does an electricity storage unit in the housework?

In the case of the sonnenBatterie, a battery storage unit is a compact, slim device about 1.80 m high that is placed in a suitable location in the house, usually in a utility room or the garage. The energy storage unit is connected to the PV system, the household, the grid, and the inverter. The sonnenBatterie stores surplus energy from the PV system that cannot currently be consumed in the house. In return, the battery storage system can also supply energy if the PV system is not able to provide enough energy to fulfil the demand in the house. In this way, the storage system avoids the otherwise necessary need to draw power from the grid and saves electricity costs. To control these processes, the battery storage unit needs an intelligent control system. With the sonnenBatterie, this is the energy manager. The energy manager recognises how much electricity the PV system generates, how much of it is consumed in the house and how much energy the battery must provide. This intelligent control is designed to consume as much free in-house electricity as possible and to keep grid consumption as low as possible, thus actively saving electricity costs. In addition, the energy manager can specifically wait to charge the storage unit and only start doing so when PV production is at its highest. Since this behaviour relieves the electricity grids, it is also referred to as grid-serving storage. Grid-serving because a lot of electricity is fed into the grid on sunny days, which can overload the grid. The sonnenBatterie counteracts this potential overload by storing solar energy.

What happens in the event of a power failure?

In the event of a power black out, the PV system and thus also the storage unit switch off. This means that the stored electricity cannot be used to continue supplying the house during the power blackout. If you want to continue to be supplied with electricity in the event of a power blackout, you have various options with battery storage systems. Only individual consumers can continue to be supplied in the event of a power failure, or the entire house can be protected against power failure as a "full home backup". With sonnen, you can achieve "full home backup" with sonnenProtect, an additional piece of hardware that also enables you to operate your sonnen battery and PV system as an island solution.

What battery technologies are available for battery storage?

The predominant battery technology for residential storage in private households is the lithium-ion battery. In the early days of home storage, lead-acid batteries were still predominant, but this technology is now obsolete and no longer plays a role. There are other niche technologies such as saltwater batteries or redox flow batteries, but almost all market-relevant manufacturers of home storage systems use lithium-ion batteries. However, the lithium-ion battery is only a generic term for numerous subgroups that differ significantly both chemically and in their properties. Two main subgroups have become established in home storage: lithium iron phosphate and nickel-manganese-cobalt (NMC) or nickel-cobalt-aluminium (NCA). These nickel-cobalt batteries are usually used in electric cars due to their high energy density.

sonnen does not manufacture any battery cells itself and is therefore independent of individual technologies. This means that sonnen can offer its customers the best available battery technology in each case. sonnen has its own battery laboratory for this exact purpose and has been intensively testing all the various battery technologies currently on the market to identify whether they can be used in a home storage system. Based on the results gathered there, we have only used lithium iron phosphate batteries since the company was founded.

Why does sonnen only work with lithium iron phosphate technology?

Since the company was founded, sonnen has exclusively used lithium iron phosphate batteries (LFP batteries). In our opinion, this battery technology is the best currently available technology for residential storage systems. It has clear advantages over other battery technologies such as nickel-manganese-cobalt (NMC) and nickel-cobalt-aluminium (NCA) in terms of safety, longevity and cost-effectiveness. Lithium iron phosphate also performs better in terms of environmental compatibility than batteries with nickel or cobalt as electrode materials. 

How economical is a storage system?

A PV system can save about 30% of a household's annual electricity costs. Why so little? On a sunny day, a PV system generates the highest amount of energy in the middle of the day:

In the household, however, the energy demand tends to be highest in the mornings and evenings when, for example, cooking is taking place or various consumers are switched on. The electricity that the PV system produces too much during the day is fed into the grid - for which it currently receives 8 cents per kilowatt hour feed-in tariff (source: Federal Network Agency). But the electricity that is missing in the evening, when the sun is no longer shining, must then be purchased for an average of 31 cents/kWh. This means that with a PV system alone, electricity costs can be reduced, but most of the electricity must still be drawn from the grid.

This is where home storage comes into play. It stores the excess energy produced during the day so that it can be used in the evening or at night. This maximises self-consumption and homeowners are able to cover the vast majority of their electricity consumption with self-generated energy. As a result, they are only affected by rising electricity prices to a relatively small extent.

The lifespan of the storage unit also plays a special role in the topic of economic efficiency. The longer a product can be used, the later it must be replaced and the more economical it is. sonnen guarantees 10,000 charging cycles for the sonnenBatterie and thus a very long service life - a mobile phone battery, for example, can only be charged about 300 - 400 times.

Most customers, however, do not answer the question of whether an energy storage system is worthwhile for them with a detailed calculation of profitability. For many, the purchase is a matter of personal conviction. Only with storage can one use one's own self-generated energy around the clock and thus advance the personal energy transition at home a great deal. Towards a clean, decentralised, and sustainable energy future.

How long is the service life of a battery storage unit?

Durability and safety are the top priorities at sonnen. After all, a residential battery storage system must perform reliably for many years, only then is it truly sustainable and economically viable.

That's why we not only apply the highest quality standards to our technical products, but also continuously maintain and improve their performance and service. The lithium iron phosphate technology used by sonnen is considered to be particularly safe and is independent of individual suppliers on the market, so performance is constantly being optimised. All storage systems are also protected against possible faults several times over and exceed all currently valid legal requirements.

Every sonnen product comes with a 10-year warranty on all components. The lithium iron phosphate battery of the sonnenBatterie can be charged and discharged more than 10,000 times and even then still has 80% of its initial capacity. After 15,000 charging cycles, the value is still 60% capacity on average - a peak value in the industry.

The battery's sensitive cells are regularly maintained remotely by sonnen, which has a positive effect on the service life and ensures smooth use.

What is the difference between an AC and DC system?

Every electricity storage system needs a battery inverter or voltage converter to charge and discharge the battery. This allows you to adjust the voltage individually, but also to regulate the power. This is the only way to ensure efficient, safe, and economical charging and discharging.

When purchasing a battery storage unit, it is important to decide whether you want to use an AC (alternating current) or DC (direct current) system. AC systems are ideal if you already have a solar system with a PV inverter and want to add the electricity storage system later. Whether it is a new purchase or an expansion with e.g. wind energy or a combined heat and power unit: retrofitting is possible at any time with AC systems.

In a DC system, the PV inverter and battery inverter are combined and are therefore usually only suitable for new installations. A DC system takes generated energy almost directly from the PV system, which is why the maximum PV system size for direct charging of the battery is limited. The advantage of DC systems is that they often take up less space and require less installation work.

How independent does a storage system make me?

Anyone who decides in favour of a photovoltaic system with integrated energy storage takes their energy future into their own hands. In the past large consumers such as washing machines, dryers, car chargers or dishwashers had to be used during the sunniest time of day, to directly convert the generated energy, with battery storage it is possible to use it day or night.

The size of the photovoltaic system and storage unit can be individually adjusted depending on the energy household needs. Consumers tend to increase their self-consumption by up to 75%, comparatively to solar panels alone.