ᐅ 9.2 kW system – what size battery storage is recommended?

Good morning everyone,

Two weeks ago, our electrician installed a 9.2 kW (kilowatt peak) system on our roof. The panels face south with a roof pitch of 35° and no shading from neighboring buildings. We also plan to install a battery storage system. Initially, we intended to use the 9.8 kWh (kilowatt-hour) battery from Varta (S4). Unfortunately, this model is no longer available and has been discontinued. The successor series (S5) is now only offered in 6 kWh or 12 kWh sizes. To be honest, the smaller option is too small for our needs, while the 12 kWh battery seems too large and is also more expensive than the originally planned battery. I am concerned that the 12 kWh battery might not be very useful during the weaker months, or that I won’t be able to fully charge it. What are your thoughts? On the other hand, during summer or the stronger generation months, having double the storage capacity would probably be a significant advantage.

Best regards

Let’s take a step back: Why do you want an attic?

Deliverer schrieb:

Let’s take a step back: Why do you want a storage system?

Because we want to use as much electricity as possible ourselves. This includes heating and hot water production via the ground source heat pump, household electricity (family of four) even during off-peak times, as well as the wall charger for the electric car.

Are you aware that this will cost you (significantly) more than simply buying electricity?
As a rule of thumb, you can currently estimate that one kWh from the storage costs about 50 cents.

I have a 13 kWp system and a 7.5 kWh battery. It works perfectly for me. Starting in March, I will be almost completely self-sufficient. Choose your battery size based on your daily consumption. The battery should cover your household’s nighttime consumption even during the summer months. Not more than that. It’s better to have a larger solar array on the roof, as that will make your system pay off faster. More battery capacity is not very useful in summer anyway 😉 and becomes disproportionately expensive.

Yes. Our main focus here is the level of self-sufficiency we can or want to achieve, similar to the heat pump we've chosen. Additionally, the battery has an emergency power function. Whether you actually need this or not is debatable. In any case, we will be using a storage system. The question is the battery capacity based on the system size.

But since we’re calculating anyway: If I conservatively assume the battery can be used for 12.5 years and provides an additional 6 kWh of storage on 200 days each year, costing around €4,000 gross, over its lifetime it’s at least not more expensive than drawing that amount from the grid, right? --> 200 x 6 x 0.35 x 12.5 --> and I doubt electricity will still be available at €0.35 per kWh in 5 to 10 years.