NVIDIA recommends an 850 Watt PSU for the Founder version. However, ASUS already recommends at least 1000 watts for the 3090, if you connect it to one of the newer i9 or Ryzen 9 and it is generally assumed that some versions of the 4090 will need more power than the Founders version. In short: For the absolute best killer gamer PC of the year 2023, you might rather spend an extra 50 francs for a 1200 watt PSU. But if you just want to build a good gamer PC with a Ryzen 7 or something and then put a Founders Edition 4090 in it, that's completely enough.