Which PSU should I get?

https://www.newegg.com/Product/Product.aspx?Item=N82E16817139147

CX550M , 34.99 after rebate

for future builds, a couple of "rules of thumb" an electrical engineer gave me that have helped me:

1) average current draw (watts) should be 50-55% of the PSU's max output
2) peak draw should not exceed 75-80% max of the PSU's max output

You can simply add the cpu's TDP plus the TDP of the video card, + 20-25 watts for mobo, 5 watts per HDD
and go from there

If you read the spec charts on the PSUs, you'll find their "sweet spot" in terms of efficiency (ie 100 watts in = 94 watts out), distortion etc are at their best at 50-55% of the PSU's max output. So the closer you get to the PSU's max, the dirtier the current gets, and the less efficient that PSU is

fwiw

So where did you hear this? I'm kind of curious.

answer to your question is in the first line of my post that you quoted

The 2 rule of thumbs were his, paragraphs 2 & 3 were my deductions once i started looking at the graphs seasonic offers on their PSUs

Electrical circuits are designed for constant 80% load. They peak at their rated capacity. That includes well built PSU's. It also includes the circuits in your home.

Well built PSU's offer clean power entirely throughout their range,they don't exceed specifications with more load (right up to the shutdown point).

Sizing PSU's - I agree, leave some headroom for the unexpected.

a 50 percent rule of thumb is insane however. What am I going to do with a trifire setup? size a 2K PSU? nope. I'm going to size a 1250 and it will work and keep working because that's what they're designed for.

The concepts mean well and undersizing something doesn't result in injury or death when it comes to electricity, but it's not accurate.

well, in that case help me reconcile the PSU you recommended with what you just stated.

i couldn't find the TDP on his GIG 1060 3 GB, so i pulled up the TDP for a Zotac mini gpu w/3 GB (https://www.zotac.com/product/graphics_card/zotac-geforce-gtx-1060-mini#spec) and it showed 120 watts, plus the 65 watts for his CPU is 185 watts. FIgure 25 watts for mobo = 210 watts. 210/550 = 38% draw when he's running that cpu & GPU at 100% ?? I figured you were allowing for some extra headroom

You stated "Electrical circuits are designed for constant 80% load." I'm not electrically literate i have known about the 50-55% avg load for power supplies for a long time before the engineer gave his rules of thumb. What i have also known for a long time was that 80% was the max load recommended for a power supply in ordinary non-electronic scenarios. i looked and apparently seasonic is no longer posting the data graphs on their PSUs, but notice 2nd line down on the very seasonic PSU i'm running on this computer (different rig than the one in my sig below), they state 90% efficiency at 50% load - it falls off dramatically above 80% load, and i mean down to 75 or 77% at 90% load. Same with harmonic distortion and the other electronic values, they go even further south. The further above 50% of the PSU's max output, the worse it gets.

Running a triple SLI, I don't know what GPUs you'd be running in a triple SLI setup, so I'll assume GTX 1080s at 180 watts each = 540 watts, - i don't know what cpu you'd run, so i'll assume 100 watts that brings us to 625 watts, + 25 watts for mobo means 650 watts. So you wouldn't be crowding a 1250 watt cpu at all - 650/1250 = 52% of the PSU's max output at 100% load. The very PSU you tell me you'd run is exactly within the rule of thumb that electrical engineer suggested. Not sure what your dispute is about??? The PSU you recommended for the OP's application is actually more conservative than my rule of thumb.

If you ran GTX 1080 Ti GPUs, it'd be 250 watts per, or 750 total + 100 watts for the CPU + 25 watts for the mobo = 875 watts / 1250 = 70% - sorry bud, but you would be stressing that PSU in that scenario - i wouldn't give it the greatest life expectancy. I suspect you'd run a 1500W PSU which would mean a 58% draw at 100% load

If my math is wrong, tell me

again, i'm not electrically literate - this engineer did a lot of work for us (we fab a lot of custom equipment/machinery for customers and we used him for the electrical requirements. He may have been conservative but that fell in line
with our attitude.

all's well, have a good night