Which storage system in combination with a solar system?

Hello!
What recommendations and arguments do you have for which storage system in combination with a solar system?
We have been offered:
- Solarwatt MyReserve with 2, 3 or 4 battery modules (corresponding to 4.8; 7.2 or 9.6 kWh)
- BYD-Box HV 5.1; 7.7 or 10.2 from the Kostal brand.

The solar system is designed for 9.9 kWp. We are also often at home during the day, almost always in the afternoon, so I think we do not need the very large storage. But I would also welcome comments on this!

Thank you very much for your feedback

What funding are you receiving/applying for [für den Speicher]?

I have meanwhile learned that every system (photovoltaic or storage) is too small when you need it and too big when you don’t need it. Of course, this only applies to self-consumption.

We have a system with 8.68 kWp and a 6.4 kW storage (BYD HV). After 4.5 months, the household consumption looks as follows: 65% from photovoltaics, 30% from the battery, and 5% from the grid. These are the very good months without heating. But the above statement also obviously applies to the bad months.

If you manage household consumption sensibly and with understanding, and your wife does not bake whenever she feels like it, you start the next charging cycle with a charge level of 20-30%. Just for classification.

Example: The battery is full on Sunday around 9:30 because there is enough sun. From then on, a lot is fed into the grid. At 3:30 pm, my wife wants to cook and bake. Two ovens, one stove, and the washing machine are on. That uses 7.3 kWh. Since it is cloudy by now, only 1.3 kWh comes from the photovoltaics. As a result, the battery was empty after about 1 hour. At 4:30 pm, it is still cloudy. Only 2 kWh comes from photovoltaics. Basic consumers continue running with 600 watts and the battery is not full until the evening. With evening TV and basic consumers, power is drawn from the grid starting from 2 am.

And that is midsummer. If the photovoltaics produce nothing after 3 pm in winter because it is twilight, even the biggest system with storage is useless. Especially if the heat pump starts with 7 kWh.

So only make the storage big enough to last you overnight for the next 10 years from one time change to the next (that is, from spring to autumn). Modular solutions like BYD also don’t help because you can only retrofit within one year (due to cell drift).

Memory is burning money

I have meanwhile learned that every system (photovoltaic or storage) is too small when you need it and too big when you don't need it. Of course, this only applies to self-consumption.

We have a system with 8.68 kWp and a 6.4 kW storage (BYD HV). After 4.5 months, the household consumption looks as follows: 65% from photovoltaics, 30% from the battery, and 5% from the grid. These are the very good months without heating. But the above statement also naturally applies to the bad months.

If you sensibly and intelligently control household consumption and your wife does not bake whenever she wants, you start the next charging cycle with a battery level of 20-30%. Just for orientation.

Example: The storage is full on Sunday at around 9:30 a.m. due to sufficient sun. From then on, it feeds perfectly into the grid. At 3:30 p.m., my wife wants to cook and bake. Two ovens, one stove, and the washing machine are on. That makes 7.3 kWh. Since it is now cloudy, only 1.3 kWh comes from the photovoltaic system. The storage was empty after just under an hour. At 4:30 p.m., still cloudy. Only 2 kWh comes from photovoltaics. Basic consumers continue to run at 600 watts, and the battery will not be full until evening. With TV and basic consumers in the evening, power is drawn from the grid from 2 a.m.

And this is midsummer. If the photovoltaic system produces nothing after 3 p.m. in winter because it is twilight, the largest system with storage is worth nothing. Especially if the heat pump starts with 7 kWh.

So only make the storage as large as you can get through the night in the next 10 years from time change to time change (i.e., spring to autumn). Modular, like BYD, also brings nothing because you can only retrofit within one year (due to cell drift).

Thank you for the experience report. Have you ever calculated when it will pay off? The question is serious and not meant provocatively, since the solar installer also offered us a BYD storage.

What is cell drift?

when the whole thing pays off

Never. This is serious and not meant provocatively. These things are simply still too expensive. And self-consumption is also not free. You pay for it through your profit and loss statement.