With storages, Photovoltaics offers more

DC coupling versus AC coupling

Store system or storage battery?

For a new installation, we recommend a DC storage system. DC-coupled battery storages are integrated before the PV inverter.

The E3/DC home power station is a compact DC system solution with an inverter, a charge controller, energy management and an storage battery. The All In One solution is very efficient and economical, due to the high degree of independence it offers.

AC-coupled systems are integrated into the circuit after the photovoltaic system inverter. The storage runs independently of the PV system. The double transformation or conversion between alternating current and direct current results in conversion losses. AC systems are recommended for subsequent installation, because an inverter is already available or one can be selected independently of the storage system.

A hybrid power storage combines AC and DC-coupled systems, so that no additional inverter is required, and the storage can be operated as a source of both AC and DC current. New PV installations or existing systems can be connected to the storage.

Battery technology of the storage

Number of cycles and useful life of solar batteries

In terms of home storages for solar power, lithium-ion-based storages have almost completely replaced lead batteries, because they have many decisive advantages.

The number of cycles: this indicates the number of complete charges and discharges before the storage capacity falls below a limit that is no longer relevant. Lithium-ion storages achieve between 4,000 and 7,000 charging cycles, whereas lead batteries often only achieve between 1,200 and 1,500 cycles. With E3/DC home power stations, the number of charging cycles is unlimited during the 10-year warranty period.

Service life: a power storage ages with time, independently of the number of cycles it completes – in other words, performance is lost over the course of time alone. Lead batteries have a service life of about 10 years; the estimated service life of a lithium-ion storage is about 20 years. No long-term studies are available yet. However, intelligent energy management is important to ensure the long service life of an storage, because they are very sensitive to overuse.

Battery technology of the storage

Depth of Discharge (DoD) and efficiency of electricity storages

Removing too much of the storage capacity can shorten the life of a storage. The Depth of Discharge (DoD) refers to the percentage of the capacity that can be removed. The higher the DoD, the better use can be made of the stored capacity. Lithium-ion storages achieve a DoD of 70-100 % – with the E3/DC products, 90-100 %.

The loss of charge in the storage is determined by the efficiency. The efficiency of the energy transfer or energy conversion of lithium-ion storage units is between 93 and 98 %, and the efficiency of lead batteries is only between 70 and 85 %.

Safety of the PV storage system: with lithium-ion storages, a highly functional energy management system must be in place, in order to prevent damage caused by overcharging. Furthermore, energy management is the only form of maintenance for lithium-ion storages, while lead storages must be inspected once a year.

Upgrade or new installation

When is the storage added to the PV system?

One of the most important questions you need to ask before installing a power storage is whether the solar battery can be retrofitted or installed as part of a new PV system. This is not only important for the choice of the storage system, but also in terms of whether the installation of a storage is worthwhile.

If one still receives a high feed-in tariff according to the EEG, then it is too early to purchase a solar storage. Depending on when the solar installation is put into operation, the electricity produced autonomously is worth more than the cost of electricity from the grid. Feeding-in is then more profitable than using the electricity yourself, until the compensation expires after 20 years.

However, for several years now, the feed-in tariff per kWh has been significantly lower than the price of electricity purchased from the grid. Self-consumption is therefore worthwhile, but without storages, only 30 % of the electricity produced can be consumed directly. With an electricity storage, 80 % and more of the household electricity requirements can be covered, and a high degree of independence can be attained. For new systems, the integration of a solar storage, preferably in the form of an efficient DC storage system, is thus always recommended.

How economical is a storage?

Maximum reduction of electricity costs

The amortisation of an electricity storage is realised through savings in energy costs. The economic efficiency of a photovoltaic storage therefore also depends on the future development of electricity prices and feed-in tariffs. According to a study by the Federal Statistical Office, at an increase of 92 %, the price of electricity for private households has almost doubled from the beginning of 2000 to mid-2014; a reduction in electricity prices is most unlikely in the future. The feed-in tariff for PV systems is falling constantly, but it continues to contribute to the economic viability of the PV system, because a high level of self-supply (independence) generates larger surpluses during the warm months.

Every kilowatt hour of self-generated electricity lowers the household electricity bill. In order to roughly calculate the economic efficiency of a PV storage, the number of full cycles per year (max. 300) achievable in practice must be multiplied by the usable capacity in kWh and the battery efficiency over the service life. In relation to the purchase price, the storage price is calculated in €-Ct/kWh. Besides the savings in electricity costs, a PV system with a storage provides tax advantages, and the possibility of a 100 % depreciation of the entire system over a 20 year period. Furthermore, the investment increases the value of your property in the long term.

Assistance for investing in the solar batteries

PV storage grants and financing

Investing in an energy storage not only reduces dependency on energy providers and ever-increasing energy costs for electricity, heat and mobility, it also supports the energy revolution, i.e. the switch from conventional electricity production to the generation of decentralised renewable energy. Therefore, many public institutions are promoting the use of electricity storages in conjunction with PV systems. After the termination of the KfW Grant Programme 275 for storages, many federal states and municipalities set up their own support programmes, which work on fixed grants per kWh or on a percentage subsidy to the eligible investment costs. As a rule, a 10-year warranty is required – and E3/DC provides exactly that for the entire storage system. In any event, it is worth seeking a local support programme, and E3/DC’s certified installation partners will be happy to assist. The long warranty and the high level of investment security of a PV system with electricity storages make the private contribution to the energy revolution very attractive for investors as well, and the conditions for this are currently more favourable than ever.