Solar Batteries

Use your self-produced energy, whether the sun shines or not.

A home battery system stores surplus solar power for later use. The solar battery extends the use of a PV-system’s generated energy and will provide free, sustainable power even when the sun is not shining and the panels don’t produce energy.

Therefore, with a solar battery little to no electricity is fed into the grid while also avoiding having to buy expensive electricity from the grid later. Due to the increased self-consumption, the electricity costs decrease significantly. You will pay less per used kWh.

What is the benefit of a solar battery system?

Photovoltaic systems without a storage opportunity produce electricity to be used immediately. This is rarely effective, since solar energy is mainly generated during the day, when the power demand of most households is low. However, electricity demand increases significantly in the evening. With a battery system the excess solar power produced during the day can be used when it is actually needed. Without having to change your habits, you:

  • permanently reduce your electricity bills
  • personally contribute to a sustainable future
  • optimize your self-consumption of your PV system’s energy
  • declare your independence from large energy suppliers

Lead or lithium-ion batteries – a fundamental decision

Battery storage systems for residential use are equipped either with lead-acid, lead-gel, or lithium-ion batteries. Lead-acid batteries were used mostly in the past, amongst other things, as car batteries. Given their considerable advantages, lithium-ion batteries are becoming increasingly accepted over lead-acid batteries. They have come to be regarded as the standard in photovoltaics in recent years.

Lifetime: The expected lifetime of lithium-ion batteries is around 15 years, whereas the maximum for lead batteries lies between 5 and 10 years.

Safety: Lead batteries must be housed in a ventilated room since they emit gases. This is not necessary with lithium-ion batteries.

Depth of discharge: Lithium-ion batteries discharge up to 100% of stored power. Lead-acid batteries, however, can normally only be discharged up to 50%, since deeper discharge reduces their overall lifespan.

System efficiency: The system efficiency of lithium-ion batteries can exceed 90%, where only approx. 70% is possible with lead batteries. Photovoltaic systems with lead batteries therefore have to be larger, leading to unnecessary extra costs.

Our tip: Choose a photovoltaic storage system with on lithium-ion batteries.

Which solar battery is best for you?

After the fundamental decision between lithium-ion and lead batteries has been made, further criteria can help you decide which battery system to buy:

PV system with storage

Storage capacity

The size or capacity of the storage system should be appropriate to meet residential consumption demand. In principle, the storage system should be large enough to supply a household with solar power all through the night. A battery capacity of 4 to 8 kWh is usually sufficient for an average four-person home consuming around 4,500 kilowatt-hours (kWh) annually. A poorly calculated battery system usually results in unnecessary extra costs. If the storage system is too small, not enough energy can be stored to cover the needs. The purchase of additional power from the grid becomes necessary. If the storage system is too large on the other hand, unneeded power is permanently stored, reducing the battery’s service life and not saving you any cost.

Depth of discharge (DoD)

Depth of discharge means what percent of the stored energy can actually be withdrawn from the device. In order to avoid damage, the majority of battery systems need a residual charge. The market norm for depth of discharge lies between 50% for lead batteries and up to 100% for lithium-ion batteries such as the SOLARWATT MyReserve storage system. According to the Bundesverband Solarwirtschaft e. V. (German Federal Association of the Solar Industry), a battery system should never be discharged below its specified depth of discharge, because this shortens the life of the battery.

Solar battery

Individual electricity requirements

For the choice of the right solar battery size, we recommend you determine your household’s electricity demand as accurately as possible. You can look at your annual electricity consumption of the last five years, or obtain an estimate from an expert. As households are becoming very individual in terms of their demands, with e-mobility, heat pumps and other electrical consumers, an individual configuration is crucial. Modular battery systems can help you prolong your system’s life by giving you the opportunity to extend your capacity over the years. The SOLARWATT MyReserve battery is a great option if the future’s household size is unclear.

Efficiency of the system

Conversion losses occur during the storage process; i.e. part of the energy is converted into heat. The total efficiency indicates what percentage of the energy stored in the system can then actually be used. Fluctuations between 70 and 95% are the market norm. Our tip: Choose a storage system with a total efficiency of over 90%.

Please note that battery efficiency and total efficiency are different specifications. Total efficiency depends not just on the battery but on the complete storage system, i.e. all the way from your roof to your sockets. Due to conversion losses, total efficiency is usually lower than the battery’s own efficiency and is not disclosed by most vendors. SOLARWATT MyReserve is connected on the DC-side of the inverter. This means a first conversion from DC to AC is omitted, the solar energy can flow directly into the battery and only very small conversion losses occur. The current is only converted when it is fed into the household network. MyReserve thus achieves a battery efficiency of 99.2% and a total efficiency of 93%. This excellent efficiency is also helped by the system components, which are matched perfectly with one another.

Lifetime

The expected lifetime of storage systems varies between 5 to 15 years, depending on the model and manufacturer. The lifetime of the MyReserve is around 15 years. SOLARWATT guarantees an output of at least 80% of the battery module’s capacity for 10 years. Our tip: Choose a solar storage system with a 10-year guarantee.

How solar batteries work

The purpose of storage systems is clear: They make solar energy available when you need it – usually in the evening and at night. The principle behind this is: The energy generated in a photovoltaic system is initially used for self-consumption. This means that active electricity consumers such as freezers or other household appliances are immediately supplied with solar power. However, when more energy is available than needed the excess energy flows into the battery, thereby charging it. Power  flows into the grid when the battery is not loaded (see intelligent charging strategy) and there is no demand from the household. If the demand during the day is higher than the amount of solar power produced the stored energy is also available – regardless of whether the battery is fully or only partly charged. Only when the battery is fully discharged, will power be drawn from the grid. In this way it is possible to cover a large part of your energy needs with the power generated by a photovoltaic system.

How can I integrate a battery into my existing PV system?

Most battery systems available on the market cannot be integrated into an existing photovoltaic system without further ado. Technical changes, such as inverter replacement, or additional work are often necessary. The heavy weight and, in some cases, gigantic dimensions of many battery systems limit where they can be installed, leading to a great deal of work. Usually several installers are kept busy for at least one day. The MyReserve, conversely, was designed for a simple Plug&Play installation. Its light weight and compact dimensions make a one-man assembly possible. The battery system is installed on the DC-side of the inverter and stores the direct current directly from the PV modules. Conversion to alternating current only takes place when power is fed into your home’s mains electricity network. This reduces conversion losses and enables a total efficiency of 93% for the entire system. Additionally, the MyReserve is compatible with all common photovoltaic inverters and can therefore be easily retrofitted. Our tip: Stay on the safe side and only have your photovoltaic storage system installed by a certified specialist.

Is a solar battery storage worthwhile for me?

Solar battery reduces the cost per kWh

The investment in a solar battery is always worthwhile when purchasing a new photovoltaic system. It enables you to use a large part of your solar power yourself, thus considerably reducing the amount of electricity purchased from an energy supplier. Retrofitting a storage system is worthwhile in a market where the price for electricity is higher than the feed-in revenue – where feeding power into the national grid no longer makes economic sense. System operators receiving extra remuneration for self-consumption of solar power should also expand their solar system with a solar battery. Our tip: Please take future developments into account when choosing a battery. If it is already foreseeable that legal boundary conditions will change in the near future or that your solar system will yield a lower profit for some other reason, you should plan ahead and install the battery storage directly.

Costs for a PV storage system – what you should expect

There are numerous manufacturers on the storage system market worldwide. The price of a solar battery depends, amongst other things, on the capacity of the storage systems and which components are installed. There is also immense variation between different states in terms of electricity prices and between installers. On account of large fluctuations in price, we advise you examine the technical data or characteristics of storage systems and determine what is most important to you.

Use your solar power yourself with a battery system

With a battery storage system, you can use the solar power from your system when you really need it and permanently lower your electricity bills. Sophisticated technology, a long service life, and recently reduced prices make PV storage systems a worthwhile investment – for both new and existing solar systems. We’ll be glad to help you with any questions about topics discussed above or about our MyReserve storage system.