Sizing a new Back-Ups

[Anonymous user]

I need to buy a UPS for a home PC, and have come up with a question I hope someone here can answer.

The PC is pretty basic, nothing particularly high power. And I turn it off when I'm not using it, so I don't need tons of battery power time; 5-10 minutes would be plenty.

I was considering buying a Back-UPS ES, either a 550VA or a 650VA model, either of which appeared to meet my needs according to APC's online sizing tool.

But then I noticed that the 650VA model specs include a load capacity of 390W (and the 550VA model has a load capacity of 330W). Now I've just replaced that PCs OEM power supply with a nice sturdy 550W model, so I'm wondering whether attaching it to a UPS with just a 390W capacity is asking for trouble.

I know that the power supply will generally not draw its max power, but I am confused that PSU rating is not given as a factor in sizing a UPS. Or perhaps I am misunderstanding the "load capacity" spec.

Can anyone help me with this?

Thanks,

Jeremy

[Anonymous user]

Jeremy,

In order to realize the full capability of your PC's power supply you would have to be running multiple hard drives, high end processing, etc.

I'm not sure what the main use of the computer is, but if you find yourself utilizing high resource applications then in order to be safe you might want to go for a slightly larger unit.

Even though your computer has the 550W power supply you won't be pulling any more wattage than you were with the last power supply unless you increase the equipment inside the case.

Hope this helps you out. If you want to get a definite answer the best thing would be to hook a kill-a-watt type meter up to the input on the PC power supply and find out what the PC draws when your running a resource heavy application.

[Anonymous user · Accepted solution]

TheBackup wrote:
In order to realize the full capability of your PC's power supply you would have to be running multiple hard drives, high end processing, etc.

I'm not sure what the main use of the computer is, but if you find yourself utilizing high resource applications then in order to be safe you might want to go for a slightly larger unit.

Even though your computer has the 550W power supply you won't be pulling any more wattage than you were with the last power supply unless you increase the equipment inside the case.

While I do plan to install a bit of extra hardware, I do not expect to reach the new 550W limit. But I was advised that the cheap OEM 300W PSU that came with my PC might be a bit underpowered if I were to install stuff like a video card (it's currently using integrated video). So I decided to find a good deal on a PSU in the 500W range, which is by no means huge for mid-level PCs.

TheBackup wrote:
Hope this helps you out. If you want to get a definite answer the best thing would be to hook a kill-a-watt type meter up to the input on the PC power supply and find out what the PC draws when your running a resource heavy application.

Right. I guess what I was hoping to learn from this forum was how closely you need to pay attention to PSU capacity when sizing a UPS. In other words, is it considered bad practice to buy a UPS with a max output of less than your PSU's rating, or do you ignore the PSU rating and instead pay attention just to the power-consuming components inside the PC?

For the record, the PC is a PowerSpec B645, the specs for which are online here: <>. Core2 Duo CPU, 2 GB RAM, 2 SATA hard drives, and one IDE optical drive for now. I'll be adding a floppy drive and a second optical drive right away, and may or may not eventually replace the integrated video and sound.

The only other items that I expect to plug into the UPS would be my 20" LCD monitor and maybe my desk telephone.

Thanks for all assistance,

Jeremy