The 750W is the maximum Watt Power the power suplly can deliver - in theory.
How much the components of a PC actually consume depends on various factors.
Are they in idle/sleep mode or are they under full load.
One might think it does not make a big difference but, look at the numbers over a year
and you`ll be surprised how much money you could spend.
For example:
Let`s say the pc has a Watt power of 450 W over a 8 h working day.
The price for 1 kWh is about 25 ct..
That makes 4kWh per day = 0.90 €
Per year this is 1314kWh = 328.50 €
This is for 365 days. A little bit too much perhaps, but the workhours of your pc might be much higher
because of rendering over night or during weekends. So this is a quite ok calculation to work with.
This is about what a Gaming computer with a multicore CPU, fast RAM and a graphic card with 8GB VRAM
would need on average it seems when it is under high load. I have read that somewhere when I was building a new computer in 2017.
The Video card is the one with the highest power consumption.
If you get a certified power supply ("Gold" or "Platinum") you can be sure it is efficient under varying circumstances.
The efficiency of a power supply unit should be above 90%, I think. The smaller lower power consumption over the year will show.
Get something that fits your components well, not too small and not too big in terms of Watt power it can deliver
and you are on the safe side. Just look at the numbers of the components, add them up and you have a rough estimate
about the max power consumption.
All the other components like monitors, hdds, your router, printer whatever are not even taken into account...
So, I think it`s fair to say, I guess, that spending a little more time and perhaps money to get well built components
will save you money in the long run.
Get a wall socket power meter. They can withstand a power consumption of about 2000 Watt very often.
Which should be enough for a socket strip where your computer sits in. Over here you can rent them from your power supplier.
Or buy one in a home improvement shop or something like that. Nice gimmick
To get a feel for the money you spend on power there are website tools that let you calculate power consumption.
CHeers, Klaus