Check the prices. At the current prices, they won't pay for themselves. I did the math on a 100kWH and a 10kWh battery assuming 60 days of negative energy prices (5 hours per each such day) and even assuming you can charge them fully within that window, you won't recover their cost in the 10 years they are guaranteed to work. Not even 1/3rd of their price. With abundant energy during sunny and windy days in Northern Europe, investing in panels or batteries is questionable at this point. Chances are, your neighbours are already subsidizing your energy consumption.