How should I understand the power supply? The power consumption is 300 W but the minimum recommended power to use the product is 850W ? Do I really need an 850 W power supply?
The 850W definitely refers to the entire system, but is probably significantly oversized. For comparison, a 750W power supply is recommended for an R9 390 (max. power consumption 375W). However, I only have a 550W power supply, which is completely sufficient. Therefore, I recommend searching Google for "power supply calculator". There you enter the components and in the end you know what kind of power supply is sufficient.
The recommendation (850W power supply) probably refers to the entire computer, not just the graphics card. CPU, mainboard, hard disks etc. also need power and 850W is definitely enough for the graphics card + system. You can certainly also run the graphics card with a less powerful power supply, but that probably depends on the system configuration.
Nowadays, power supplies are often very efficient (> 85%) even at low load (~30%) and therefore you can also use a too "strong" power supply without increasing the electricity costs.