Storing surplus electricity produced by your photovoltaic panels for later use is an idea worth studying.

• When you are not there and the sun is shining, your panels produce 'free' electricity that you do not use because it goes back to the grid.
• Conversely, in the evening, when the sun is down, you pay to draw electricity from the grid.

Installing an electric battery can allow you to take advantage of this lost energy. However, it involves a degree of investment and technical constraints..

On the other hand, you may be entitled to certain compensations. Furthermore, you should also take into account future developments such as vehicle-to-grid.

Therefore, although it's a good idea from an environmental perspective, investing in a home battery is not yet profitable, especially given the current price of batteries and taking into account the current price of electricity from the grid.

Advantages of a home battery

1. For the environment

In terms of the supply chain, you can hardly do better than producing your own electricity.

However, you need to know that your home battery will not allow you to get through the whole winter on your own reserves. A home battery is only used to bridge from one day to the next.

With a battery, you will consume on average 60% to 80% of your own electricity, compared to 50% without (according to Brugel, the regulatory authority for the Brussels' gas and electricity market). 

2. For your wallet

With a home battery, you can optimise your electricity needs and purchases. As a producer:

• you store self-produced electricity – which is therefore free – to use it later;
• if your meter no longer runs backwards, the energy fed back into the grid is lost or valued at a low rate. It is therefore in your best interest to use your electricity locally, or to store it;
• you avoid paying a fee for energy fed back to the grid;

Even without panels, some manufacturers, such as Tesla, maintain that you can buy electricity from the grid when it is cheapest (dual hourly rate for example) and use it later. However, this requires the use of smart meters as well as a smart load balancing. The difference between day and night rates must also be sufficient to generate a sufficient saving to finance the purchase of the battery.

3. For the electricity grid

Consuming locally generated electricity rather than feeding it back into the grid can help manage the balance.

In the future, some experts even think domestic batteries, or the batteries in your electric car, could play a buffer role on the smart grid by absorbing renewable production.

4. To ensure a secure supply for you

In the event of a power failure, the home battery can be used as backup power. Be careful, though. This use has technical constraints, such as the installation of a specific inverter (see below).

To consider before investing

Cost: from € 5,000

For the battery itself, count on spending around € 600/kWh. Given that the batteries currently available on the market can store between 3 and 20.5 kWh (with 5 to 6 kW of power), this represents an investment of €1,800 to €12,000.

Add to that € 500 to € 1,000 for the installation and around € 2,000 for the inverter.

This price may fall in the future... thanks to the development of the electric car. In fact, batteries whose capacities fall to 80% could be reused in our homes. According to the Blackrock Investment Institute, the price per kWh of batteries should fall to € 420/kWh in 2025.

Storage capacity: from 3 to 20.5 kWh

The batteries available can store between 3 and 20.5 kWh with 5 to 6 kW of power. As an indication, the average consumption of a household (in Brussels with 4 people) is 9.5 kWh/day.

Lifespan: 10 years minimum

The batteries currently manufactured are guaranteed to work for at least 10 years, even with intensive use, i.e. by discharging 90% of their energy each night to recharge them 100% each day.

A 3 kWh battery can therefore supply 2.7 kWh for 3,650 days, or 9,855 kWh.

Offset time too long

Let's take the example of our 3 kWh battery capable of supplying at least a total of 9,855 kWh over 10 years. With a minimum cost of €4,800, this battery therefore supplies 1 kWh at €0.49, which is about double the price of the grid... so no savings are made and the investment is not profitable in this case.

Weights and dimensions

Domestic batteries can weigh more than 120 kg. They can, however, be installed in a service room or discreetly hung on the wall because their design makes them quite flat (about 15 cm against about 1 m high).

Technical constraints

Before investing in a home battery, check that it has a built-in inverter, suitable for the use you want to make of it. If it does not, you will need to buy and install an inverter in addition to your battery.

In fact, the inverter from your photovoltaic installation is one-way: it transforms the direct current from the panels into alternating current usable for your devices. However, a home battery needs a two-way inverter, as it both charges and discharges.

But if you want to use the battery as a back-up power supply in the event of a power failure on the grid, you will need a grid-forming inverter.

Home batteries and vehicle-to-grid

In future, domestic batteries will probably also play a buffer role on the smart grid by regulating renewable energy flows,

What's more, electric car batteries, which remain unused during the day in car parks, could also be used. This is called vehicle-to-grid.

Electric cars could also be used to power the home during the evening, recharging at night at low prices, etc.

All this, of course, requires technical and financial management at all times which only a fully automatic system can provide.