What is it and why should I buy an energy storage unit for domestic use?

Electricity storage batteries are used to store the excess energy produced by a renewable energy system for later use.

In this way, the energy generated, for example, by a photovoltaic system during the day can also be used in the evening or at night, when there is no sun and the system is unable to produce.

With home energy storage, a family can:

• Make the most of self-generation from renewable sources

• Achieve greater energy independence

• Increase the power available for home consumption

• Ensure the continuity of the power supply in the event of a temporary outage.

The other important aspect of storage systems is the possibility to use the energy stored in the batteries at times when the electricity grid tariffs are more expensive, thus reducing the cost of the bill.

Combining a storage system with a photovoltaic system also contributes to making your smart home more sustainable.

What types of battery are available for domestic use?

The most widely used domestic energy storage batteries on the market are electrochemical batteries of one of the following types:

• Lead-acid: each cell consists of two electrodes (one lead, the other lead dioxide) immersed in a dilute electrolyte solution of sulphuric acid. The greatest advantage of lead-acid batteries is their cost-effectiveness, due to the low cost of raw materials, but on the other hand, they do have a rather low energy density. Moreover, lead is a toxic material and pollutant.

• Sodium-sulphur: the two electrodes are in a molten state, physically isolated from each other by a ceramic separator that acts as an electrolyte. The ceramic separator only allows ions to pass through at temperatures close to 300 °C, and this, together with the need for the two electrodes to remain in a molten state, forces the cell to be kept at a high temperature (300-350 °C). On the other hand, sodium-sulphur batteries have very high efficiency and their components are low toxicity.

• Nickel-cadmium: the positive electrode consists of hydrated nickel oxide, the negative electrode of cadmium and the electrolyte is a dilute solution of potassium hydroxide. This type of battery allows a high number of charging cycles, but at the expense of energy density. Nickel-cadmium batteries suffer from the “memory effect”, which progressively reduces cell voltage, although in more modern versions this drawback has been considerably reduced. In addition, cadmium is a toxic metal.

• Lithium ion: the cell consists of thin layers (making up the electrodes and the separator) immersed in an electrolyte based on lithium salts. Lithium-ion batteries offer a very high charge density and do not suffer from memory effect, which is why they are by far the most widely used in storage systems. On the other hand, they are more expensive than the others and their solvent is potentially flammable, so they require some construction precautions.

A storage system has an average service life of 10 years, after which it is often necessary to replace the battery module.

There are various incentives for the purchase and installation of an energy storage system. Anyone wishing to install a photovoltaic system with an accumulator or to integrate a storage system into an existing photovoltaic system can claim a tax deduction of 50% of the expenses incurred until 31 December 2024 (as established by the Budget Law 2022, which extended the terms of the renovation Ecobonus).

The tax deduction is to be divided into 10 equal annual instalments. Alternatively, it is possible to opt for an invoice discount of the same amount or to assign the tax credit to a third party.

The Budget Law 2022 also stipulates, in Art. 1, paragraph 812, the recognition of a bonus for storage systems, in the form of a tax credit, for expenses incurred between 1 January and 31 December 2022 for the purchase of batteries connected to electricity production systems powered by renewable sources (i.e. not only photovoltaic systems), even if already existing and benefiting from incentives for on-site exchange (Decree-Law no. 91/2014). The bonus cannot be used with other tax benefits for the same expenses (e.g. the renovation Ecobonus mentioned above). If the credit is not exhausted in the tax period in which the expenses were incurred, it will be possible to use the advanced credit in subsequent years. The application for the bonus must be submitted from 1 to 30 March 2023 exclusively online, either directly or through an intermediary. Within 5 days of sending the application, a receipt is issued stating that the application has been accepted or rejected, with reasons. The credit can be used in the tax return for the 2022 tax year, for a reduction in taxes due.

The size of the bonus is not known in advance, because it will be determined by the ratio between the EUR 3 million ceiling allocated by the government and the number of applications received.

Lastly, when combined with a photovoltaic system installed in conjunction with one of the driving interventions provided for in the DL Relaunch for energy efficiency or the seismic safety of buildings, the purchase of a home energy storage unit entitles the purchaser to a tax deduction of 110% of the expenditure incurred (Superbonus), up to a limit of EUR 1,000 per kWh of storage capacity and for a maximum of EUR 48,000. From 1 January 2022, the tax deduction is to be split into four equal annual instalments. Alternatively, the deduction can be used as a credit assignment or invoice discount of the same amount.