80 means 80% efficiency of psu.  For example power supply delivers 300W to the system but actually draws 375W from the wall. This means that its efficiency is 375W/300W, which equals 0.8 or 80%. Those additional 75W produce nothing but heat. your 260w 80+ bronze is 260W typical 85% Efficient PSU(80 PLUS Bronze); ENERGY STAR compliant, Active PFC basic online calculator recommends 280w psu for i7-9700, GTX 1650S, sata ssd, no optical drive.  your monitor does not need power from pc psu.  you can run your pc on 260w psu and see how it goes.  260w is good enough for load wattage, which is 50w lower than recommended psu.if you really need psu upgrade, just buy the Dell 360w psu for 5070MT.360W typical 90% Efficient PSU (80 PLUS Gold); ENERGY STAR compliant, Active PFC  
Greetings, I own a 5070 MT, The CPU is an I7-9700, I have recently purchased a Gigabyte Nvidia 1650 super vga.  My power supply has 260W and is also 80 plus bronze, I don’t know what this 80 means. I have a 34 inch Dell monitor, P3421W, I am planning to play sometimes also on this PC, I usually play on a PS console. My question is related to the power supply, do I need to upgrade it to a bigger one? I have been told that the power supply in my case should have at least 400 W.
Dell Optiplex 5070 MT power supply question