Adding a battery to your solar system: when does it pay off?

At midday your solar system produces more power than you need. The surplus goes to the grid for a few cents, and in the evening you buy power back at a much higher price. A battery is meant to close that gap. Whether it pays off on an existing system comes down to three numbers: your evening consumption, your system’s surplus and the feed-in tariff it receives today. Here is how to check them, how to weigh the savings against a quote, and what the conversion involves.

In short

• When it pays off: with high consumption, especially in the evening, and a system that fills the battery on an average day.
• When it usually does not: if your system still receives a high fixed feed-in tariff. Every kilowatt hour you store is a kilowatt hour that no longer earns that tariff.
• What a battery cannot do: carry power from summer into winter. It shifts power from midday into the evening and night.
• What often comes with it: a new inverter and work on the meter cabinet. On older systems the conversion is usually bigger than expected.

Three numbers decide

1. 1. Your consumption in the evening and at night Does the battery cover what you use between evening and morning?
2. 2. Your system’s surplus Only power you have left at midday fills it.
3. 3. Your feed-in tariff The higher your feed-in tariff, the less is left.

1. Your consumption in the evening and at night

A battery only helps if you need power while the sun is not shining. It is well sized when it covers your consumption between evening and morning. The Verbraucherzentrale, Germany’s consumer advice centre, gives a guide value of about 1.5 kilowatt hours of battery per 1,000 kilowatt hours of annual consumption. A larger battery costs more, ages faster and adds hardly any self-consumption.

2. Your system’s surplus

Only the power you do not use during the day fills the battery. The amount fed into the grid tells you. You will find it in your inverter’s app or in the annual statement from your grid operator (Netzbetreiber, the company that runs the local power lines). Small systems with little surplus at midday only fill a battery on sunny days. In winter it often stays half empty, even with larger systems.

3. Your feed-in tariff

Every kilowatt hour that goes into the battery is not fed in straight away. You save the price of grid power but lose the tariff for that kilowatt hour. The higher your tariff, the less is left over. If the battery later does feed power into the grid, it is paid the same tariff, according to the Verbraucherzentrale, as long as the battery only stores solar power.

Power moved per year

1,500 kWh

Your electricity price

32 ct/kWh

Your feed-in tariff

12 ct/kWh

Every kilowatt hour moved saves 20 cents: 32 cents for grid power minus 12 cents tariff.

In one year€300

In 10 years€3,000

In 15 years€4,500

If the battery and the conversion cost more than it saves in 10 to 15 years, it does not pay off.

An example with round assumptions. Battery losses and future electricity prices are not included. According to the Verbraucherzentrale, a battery lasts 10 to 15 years. Source: Verbraucherzentrale

Grant from your municipality? Open the subsidy finder →

The sums look better for systems whose tariff is ending. More on that in our article on systems whose tariff ends.

How long a battery lasts

The Verbraucherzentrale expects a battery to last 10 to 15 years, and solar panels 20 to 30 years. So a battery has to earn its money back within its own, shorter life. Small batteries cost more per kilowatt hour than large ones.

That is why the calculator above shows what a battery saves in 10 to 15 years. Set that against the total price in the quote: the battery, a new inverter if needed, and the work on the meter cabinet. If the price is higher, the battery does not pay off financially.

From our work: A battery added later pays off mainly with high consumption and a solar system large enough to fill it on an average day. But it never reaches the return of the solar system itself. On older systems the conversion is usually bigger than expected: the old inverters are pure solar inverters, so a new hybrid inverter is needed, and today we usually have to fit an APZ field, for which the old meter cabinet often has no space. For someone with a system from around 2015 and a good feed-in tariff, it rarely pays off financially. Some still want the battery, for example for backup power, and that is a different decision.

What the conversion involves

• Inverter: older systems have pure solar inverters that cannot charge a battery. There are two ways round this. An AC-coupled battery brings its own inverter and, according to the Verbraucherzentrale, is easier to add to an existing system. A DC-coupled battery runs on a hybrid inverter that replaces the old one, and loses slightly less power in conversion on average. If your inverter is old anyway, replacing it makes a lot of sense.
• Meter cabinet: under section 14a of the Energy Industry Act (EnWG), the grid operator must be able to control batteries that charge at more than 4.2 kW. This applies to batteries that started running on or after 1 January 2024. If the local grid is at risk of overload, it may limit your draw from the grid to 4.2 kW. Power from your own roof does not count towards that. In return, you pay lower grid fees. The control equipment needs space in the meter cabinet, and today usually an APZ field as well (Abschlusspunkt Zählerplatz, a section for the grid operator’s communication equipment). Old meter cabinets often lack that space.
• Backup power: some batteries supply only one dedicated socket during a power cut, others the whole house. According to the Verbraucherzentrale this often costs extra and is not available with every product. If you want backup power, have it written into the quote explicitly.
• Registration: fixed batteries must be entered in the Federal Network Agency’s market master data register (Marktstammdatenregister), just like your solar system. If we install the battery, we handle the registration for you.

How to check your case

1. Note your annual consumption and the amount fed into the grid from your last statements.
2. Check which tariff your system receives per kilowatt hour, and until when.
3. Note the make and model of your inverter. Both are on its rating plate.

With these details we can tell you honestly whether a battery pays off for you, and also when it does not. Add them to the message in our battery check or give us a call. We advise in English.