[SOLVED] Power supply calculator doesn't match current draw

[mikewinddale]

The current draw being reported by my UPS does not remotely match any online power supply calculator. I suspect that the UPS is probably correct and the power supply calculator is wrong, but I want to see what others think.

Running Prime95, MSI Kombustor (video stress test), and AS-SSD simultaneously, my UPS reports a maximum draw of 603W with 67% usage. But an online power supply calculator estimates a peak load of 758W. My power supply is rated 1000W, while my UPS is rated 1500VA / 900W.

Currently, my system is operating well within the specs of my power supply and UPS, but I want to estimate how much room I have to upgrade in the future.

I suspect that the UPS is more correct than the online power supply calculator. But I want to see if others agree.

And I know my peak power supply wattage (1000W) exceeds my peak UPS wattage (900W). That's part of the reason for this post - to see how close I'm getting to exceeding anything.

If the UPS is correct that I'm only using 67%, then I have plenty of room to upgrade, and I don't have to worry that my 1000W PSU will blow my 900W UPS.

More specific details follow the dotted line:

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Power supply: Phanteks Revolt X PH-P1000PS 80PLUS Platinum 1000W (manufactured by Seasonic)
UPS: Cyberpower 1500PFCLCD, 1500VA / 900W

System:
ThreadRipper Pro 3955WX (16 core)
Supermicro M12SWA-TF motherboard (WRX80)
4x64 GB DDR-3200 ECC RDIMM
AMD Radeon RX 580 8 GB
1x Sabrent Rocket Q4 4 TB PCIe 4.0 M.2 SSD
1x Toshiba XG3 256 GB PCIe 3.0 M.2 SSD (OEM version of OCZ RD400)
1x Seagate BarraCuda 4 TB 5400 RPM 2.5" SATA HDD
7x140mm fan (including CPU)
2x120mm fan
1x40mm PCH fan

According to this power supply calculator, my system should be drawing about 758W - including peripherals like the mouse, keyboard and monitor. (Those peripherals of course draw on the UPS but not the power supply. In any case, their draw is trivial compared to the main system.) I entered the following:
ThreadRipper 3960X (it was the closest to the 3955WX available)
100% TDP
4x64 GB FB DIMM
Radeon RX 580
1x SATA 5.4K RPM
2x M.2 NVMe SSD
1x gaming keyboard
1x gaming mouse
7x 140mm fan
2x120mm fan
1x LCD 24 inches
Always On 24/7
Game On! 24/7

But when I run Prime95 Blend, MSI Kombustor (graphics card stress test), and AS-SSD, all simultaneously, my UPS reports a peak draw of 603W, and it also reports 67% of UPS load.

Incidentally, 603/0.67 = 900, which is the wattage rating of the UPS, so that wattage and that percentage appear to match up.

I suspect the UPS's own reporting is far more accurate. If I'm only using 67% of my UPS's load, then I suspect I still have room to upgrade.

The only upgrades I can think of potentially performing are (1) adding 4x64 GB additional RDIMMs, and (2) upgrading the GPU to something faster than an RX 580, but still not top-of-the-line. For example, a Radeon RX 5700 or a Geforce 3060 - something mid-range.

Going back to the power supply calculator, changing it to 4x64 GB FB DIMM and Radeon RX 5700XT results in a load wattage of 915W, which is pushing the limits of my 1000W PSU, and exceeding my 1500VA / 900W UPS.

But given that my actual reported draw is "only" 603W compared to the calculator's estimate of 758W, I suspect the calculator is over-estimating, and I actually have sufficient headroom.

I have an idea for how I might perform upgrades in the future. Tell me what you think: if I do decide to add RAM or upgrade the GPU, then I can run add more stress tests, to see whether I'm getting close to the UPS's limit. E.g., if I add a GPU, I could start by running MSI Kombustor on its own, then add Prime95 using the "weak" torture test (AVX disabled) with only half my threads, then move up to full Prime95, etc. Each time, running the previous stress tests plus one more stress test on top of it. That way, I can see how close I'm getting to the UPS's limit without immediately running the system at full load at once, with the risk of blowing the system immediately. Does that sound like a reasonable and safe testing procedure?

 

[jay32267]

Put a current meter between the UPS and the PC to know exactly.

 

[Karadjgne]

The ups is accurate. Sort of. What you don't know is if that's the power used on input, power used on output after being modified by AVR, the power covered by battery allowance etc. It could be any of several different power specs.

And no. A 1500VA/900w upc isn't enough to cover a 1000w psu if it's anywhere close to limits.

You won't 'blow it up' as you imply, but if you have too much draw, the psu will simply shut down. If you figure that with all the stress tests you are at @ 600w draw, then upgrading the gpu add another 100w. You are still covered. Throw in a RTX3090 and have it hit its reported 550w (ish) transient spikes, and that 700w draw will vanish, left in a cloud of dust, you'll be 900w+, and tripping protective circuits in the upc. Pc shuts down instantly.

The reason why those psu calculators are garbage is that they only deal in absolutes. 10w per ram stick + 300w+ for gpu + 150w for cpu +35w for hdd +, +, +....
You won't use the full 10w of all sticks at all times. The hdd won't draw the full 35w at all times, cpu, gpu the same etc. The calculator adds every maximum, but you won't ever run all items at maximum simultaneously. Like you can't run ac and heat simultaneously, pick one or the other. Calculators will add both.

Figure your cpu will hit power limits of 280w. The gpu can hit 240w. The motherboard, drives, fans etc can hit another 150w (your setup). I'm figuring you'll max out at somewhere between 650-700w, which is about right if all those simultaneous stress tests show a 600w draw. But you'll never see max, as even all those tests can't run the pc at 100% everything, simultaneously. Normal usage should hit 450-550w. A psu will only output what's needed, regardless of capacity, 1000w means nothing, it could be a 1600w psu. Load is everything and you top out at 700w (ish)

That's within spec of the upc. I'd not push it much higher than another 100w on the load potential.

 

[mikewinddale]

Thanks for explaining what the calculators are doing.

And yeah, there's no way I'm getting a GeForce 3090. I mostly play isometric RTS and RPGs, not FPS. The RX 580 is plenty for me right now. So someday, I'll upgrade to whatever equivalent of a Radeon 6600 or Geforce 3060 is available at the time. With GPU prices the way they are, maybe I'll wait until there's a 4060 or a 5060 and get a used 3060.

I know the PSU is outsized relative to the UPS. I bought the UPS 3 years ago, and I chose 1500VA at the time because above 1500VA, prices quickly double or triple, from around $200 to around $500. So 1500VA was the sweet spot.

When I bought the 1000W PSU, I knew it was too large for the UPS, but I knew that my current system would use less than the UPS's 900W max. So I figured I'd wait until I upgrade the computer before I consider a 2000VA UPS for $500+. For now, the UPS is enough.

 

[mikewinddale]

Actually, my chassis supports dual PSU, so if I did hypothetically get a 3090, I could just get a second PSU and a second UPS for cheaper than buying a 2000 VA UPS.