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Is it worth investing in a home battery?

It may be worth thinking about if you have photovoltaic panels and want to make the electricity produced by your installation pay for them.

However, there are several technical elements to consider and it is still a major investment and difficult to ensure a return on your investment at the moment.

Home battery + photovoltaic panels: a good idea?

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

  • When you are not at home 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 has set, you pay to draw electricity from the grid.

Installing an electric battery allows you to make the most from the electricity produced by your photovoltaic panels. However, it involves a degree of investment and there are technical constraints.

The benefits of a home battery

1. For the environment

If you generate your own electricity, the battery will allow you to use an average of 60% to 80% of your own electricity. Without a battery, you will only get 30% - 50% (source: Brugel, the Brussels regulator for the gas and electricity markets).

In terms of the supply chain, you can hardly do better! 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 used simply to bridge the gap from one day to the next.

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;
  • you avoid sending electricity back to the grid, which is less financially attractive, and you reduce your electricity consumption from the grid

In addition, with a potential capacity-based pricing system (see below), the home battery can be used to limit consumption peaks on the network and benefit from a lower tariff.

Even without panels, some manufacturers, such as Tesla, maintain that you can buy electricity from the grid when it is cheapest (dual-time rate for example) and use it later. But this requires smart meters and intelligent charge/discharge management. The difference between the day and night rates must also be sufficient so as to generate enough savings 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 of the network.

In the future, some experts even think that home batteries or those from your electric vehicle (this is the vehicle-to-grid principle) could play a buffer role on the smart grid by absorbing renewable production.

What to consider if your meter "runs backwards"

If you live in Wallonia, investing in a home battery right away is not a good option. In fact, the distribution network acts as a sort of "giant battery" and the investment in the battery cannot be paid off. However, the system where the meter runs backwards will disappear by 2030. In addition, new pricing schemes may emerge in the coming years, which could change the situation and make the battery pay for itself.

If you live in Brussels or Flanders, this compensation system has already disappeared.

What to consider before investing

What is a home battery?

  • A lithium-ion or lithium-polymer storage battery;
  • An electronic control system which makes its operation fully automatic;
  • Possibly an inverter to produce alternating current;
  • A cooling system.

Cost: from €5,000

The battery itself is about €600/kWh. Given that the batteries available on the market today 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 installation and about €2,000 for the inverter.

The cost of batteries may fall in the future... thanks to the development of the electric car. In fact, batteries with capacities that have fallen 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: 3 to 20.5 kWh

The available batteries 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 year warranty, 15 to 20 years in service

The batteries currently being manufactured are guaranteed to work for at least 10 years, even with intensive use. This means discharging 90% of their energy every night and recharging them to 100% every day.

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

Payback period

Herein lies the problem... Let's go back to 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 provides 1 kWh at €0.49, which is about twice the price of the grid... at least before the price explosion. Depending on the price you pay for your electricity, the calculation has to be redone.

Note that if you invest in a more expensive battery, but one that can handle more charge/discharge cycles, your cost per kWh will be lower and the battery can be paid off more easily.

Grants and other benefits

Since 2020, Flanders has provided a grant for the purchase of a home battery, increased to €2,550 since 1/1/2021. This grant will be available until 2024, with a decreasing amount each year. With this grant, the investment in a home battery can be paid off.

In addition, you can benefit from 6% VAT instead of 21% for the installation of a domestic battery if your home meets the conditions (over 10 years old).

Intelligent hybrid inverter

If you install photovoltaic panels and a battery at the same time, only one hybrid type inverter will be needed. And the installation costs are lower, because the installation is simpler. 

If you do it in two stages, the battery will have to be equipped with its own bi-directional inverter, as the one in the panels will not be compatible. This leads to a duplicate purchase. However, 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.

Note that some home batteries have a built-in inverter.

How long does it take for a home battery with photovoltaic panels to pay for itself?

The battery allows you to consume an average of 60% of your photovoltaic production instead of 30%. In a typical case, for a household consuming 4,000 kWh/year, this gives:

  • Savings due to annual self-consumption, without battery: €430
  • With battery: €700.
  • Difference: €270
  • Cost of the 5.8 kWh battery, including inverter, minus grant: €2500
  • Return on investment time: 2500/270 = 9 years (to be compared to the lifespan: 10 or even 15 years). This calculation uses 2021 prices. Today, depending on whether you have a fixed rate from before the increase or a variable rate, the situation will be different.

Without this grant, and at the 2021 price of the battery and electricity, the battery cannot pay for itself before the end of its service life. In the new crisis situation, everything has to be reviewed depending on your contract and your rate.

If you invest in a more expensive battery, but one that can handle more charge/discharge cycles, your cost per kWh will be lower and the battery will pay for itself quicker.

What size for your home battery?

It is better to get the largest possible battery. It will not be proportionally more expensive, on the contrary. For a 3 kWh battery the cost is about €4,000. For an 8 kWh model, €8,000 and for 14 kWh, €10,000.

A powerful battery will significantly increase your savings, as your self-consumption will increase (up to 75% instead of 30%) during the sunny months, and the battery will pay for itself more quickly.

Weights and dimensions

Home batteries can weigh over 120 kg. However, they can 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).

And in the future?

With the general evolution of electricity networks towards a system where consumers participate directly in the balance of the grid by adapting their consumption, the home battery could play an important role.

The electricity pricing system will probably move towards a "capacity" system. According to this principle, the more peaks in consumption during the day, the more expensive our electricity will be. It is therefore in our interest to spread out the use of high-consumption appliances as much as possible. The battery will be useful for self-consumption instead of putting too much strain on the grid. The battery also provides another cost-saving feature by allowing the user to benefit from a better electricity rate.

Domestic batteries will probably also play a buffer role on the smart grid by regulating renewable energy flows.

However, they will probably not be alone: 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 an automatic system can provide.

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