Photovoltaics for hot water

Why confused? Because it makes no economic sense. What do you want to save with heating costs of 50 € per month? For hot water, you might pay another 15 euros per month... Even if the system only costs 2T € including installation, that means 11 years... 3%... I hope the extra water pipe didn't cost more than 270 €... that's the contribution to energy production in 15 years.

Why are you worrying so much about whether photovoltaics or solar thermal pay off when you are already spending many thousands of euros extra to install a fireplace with heating water heating in addition to the gas condensing boiler?

That's not about efficiency and economy either, is it?!

You can achieve KFW70 without solar thermal, but if that doesn’t work despite the above-mentioned stove, then in my opinion, you should reconsider the whole concept (insulation?).

If you don’t want insulation suitable for KFW70, I would rather stay away from KFW altogether, then it won’t be such a "botch job".

Ok, I’m slowly understanding. I’m an absolute layman. How do I then arrive at the 10% renewable energies in new construction?

more insulation than necessary, heat pump, controlled residential ventilation with heat recovery, ST or similar

But for the building envelope, we are strictly at Kfw 55 standard. Just because we don't have ventilation, we can't achieve Kfw 70.

There are so many parameters with insulation, etc., I wouldn’t rely on the blanket unchecked statement of your [BU], but as a layperson, I really can’t assess it either. Have the calculation for the envelope given to you and look for a forum on the internet that specializes in building services engineering. They are competent and helpful with such things, they can probably evaluate it better.