1. Buffer tanks for heat pumps cost efficiency and money. There are no advantages at all. (Go to the htd F o r u m - you will be stoned for having a buffer tank) 2. A 300-liter hot water tank is suitable for 4 people. With 150 liters, you won't be able to shower the entire family one after another (let alone one person wants to take a bath and the rest shower). When my three girls come back from the stable and get ready in the shower, you can only use the water that comes afterward to cool boiled eggs. They quickly suck the 300 liters down from 50 degrees to below 30 degrees. Girls, after all... 3. Photovoltaics for a 300-liter hot water tank hardly makes a difference. The heat pump simply uses too little electricity for that. 4. In my opinion, the heat pump is chosen one size too large.