If I buy a 750w psu that is way overkill so that I can upgrade in the future, will it crank up my electric bill, or do psus only suck up the power they need. How does this work?
The rest of the computer only draws the power it needs, and in turn the PSU only draws the necessary power from the wall.
No, it doesn't draw 750w all the time. That's just the maximum power the PSU can deliver safely. if the computer is idling at say 50w, the PSU will be drawing 50+ watts from the wall. (The + being whatever inefficiency the PSU has, the excess power being turned into heat.)
As for upgrading, though, I think the time when successive GPU generations used increasingly more power are over. Outside of dual GPU configurations and HEDT builds (threadripper and the like), I don't see an upgrade needing 750w. Maybe a Radeon VII and 9900K, which are the two biggest GPU/CPU power sucks available today, and even then you could get by with 650w short of extreme overclocking of the CPU.
Note that for "80+" certification, efficiency is only measured down to 20% of the rated power draw. So if you are idling along at 50 Watts, only a "500W 80+ titanium" (not recommended) would be forced use no more than 5W by itself.
In general, power supplies (and a lot of electronics in general) work best at half their rated capacity, so if "nominal" (which is rather high in gaming, rather low if surfing the web) power is 375W then your 750W should be most efficient. Just make sure all the "maximum power draws" don't add up to more than 750W (presumably under the effects of a "power virus").
And as hammer182 points out, additional capacity is often worth the efficiency hit.
correct. when i tested my set up. it recommend i think a 800watt psu. or something like that.(you can find it as red devil) yet i was able to see how much am pulling. which is 550watts under load.
oddly cpu have very detail power spec. now a video card on the other hand. every manf power spec will vary some.
This was the PRECISE mindset I built my rig with in late 2016. 650W SHOULD have been sufficient, but then when Vega came out, the Sapphire Nitro+ cards needed 3 8-pin connectors. For the line of PSU I chose (EVGA SuperNova Platinum), the 650W had only 2 6+2 pin connectors (the 750W had 3). That ruled out upgrading my Nitro+ RX 480 to a Nitro+ Vega 56 or 64 (until more recently when Sapphire released a throttled-down Vega 64 with just 2 8-pin connectors, but with Navi 10 just months away, that's too little too late).
I might have engaged with the Vega series if not for being short a spare 6+2 connector on my PSU. Other OEMs made Vegas with just 2 8-pin connectors, but I wanted to stick with Sapphire if I was staying with an AMD GPU. It's a minor nit-pick, but if the prices between your desired brand's 650W and 750W PSU are negligible, you might want to check this feature just in case AMD, nVidia, or even Intel make a triple 8-pin GPU ever again.
I'll gladly bow to actual experience in this.