As an example: My father (living in Germany) installed a 10 kWp photovoltaic system in spring last year along with a 10 kWh battery. In the summer months he was

As an example: My father (living in Germany) installed a 10 kWp photovoltaic system in spring last year along with a 10 kWh battery. In the summer months he was almost self-sufficient with that, as he could use his own solar power at night or on rainy days. The forecast for the whole year is ca. 65% self-sufficiency. So local storage reduces the amount of both the power taken from and provided to the grid and therefore is beneficial for grid stability. The problems start, when the storage is not local anymore, that is when the power surplus is not at the same location as the storage itself. A few examples: When the sun is shining your electric car is probably at the parking lot of your workplace and you can't store the output of your PV system. Or when it's a rainy day, you and also all other households in a wide area around you, wont' produce much power. Then the power has to come from somewhere else, e.g. a sunnier place, offshore wind or gas power plants. So to use all the distributed storage effectively, you have to route the power around a lot, which probably makes changes in the power grid inevitable. How much one has to invest in the grid and how that relates to the overall power price is probably a very complex topic depending on a lot of variables including geography, power mix and cost of storage.