Measuring PC's Wattage?

Today I was upgrading a computer for someone, just changing the mobo, cpu and ram, and they didn't feel like shelling out an extra $50 for a new power supply to go along with it. Their original psu was a 350W, which I wasn't sure if it'd be enough... So I built it, fired it up, and everything was going swell until I plugged in another harddrive to clone their data in case the repair install went south. With two harddrives plugged in it would only post maybe one out of every two times I tried to boot it... I wasn't sure if this was caused by lack of power, so I figured I'd calculate exactly how many watts it was drawing... I cut open a spare power cord, cut one of the hot wires, and attached my multimeter between them. I set my dmm to AC amperage, though I couldn't get it to power up... My multimeter is rated up to 10A, and I've used the amperage function before just fine, so I'm pretty sure the fuse isn't blown, yet for some reason it just wouldn't start up.

SO... I was wondering if there's an easier way of figuring out the total power draw of a computer, like say software based.. Either that or can anyone point out something I was doing wrong in trying to measure the amperage?

http://extreme.outervision.com/psucalculatorlite.jsp

^ Try this.


Also, NEVER buy a computer and use it with a crappy power supply. It's like getting a Ferrari and using plastic lego wheels for it.

I don't think software is capable of measuring the power usage of the entire computer. It would have to communicate with the power supply and the only leads from the power supply are power/ground/activation.

I use a kill-a-watt. $20 on the bay if I remember correctly. It can properly measure wattage, voltage, current, KWhrs, frequency, and volt-amps of anything that runs on ~120V up to 15A. Current multiplied by voltage does not always equal wattage in an AC system unless the load is purely resistive (ie. incandescent bulb). Google the difference between Watts and Volt-Amps.

pseudolobster said:

Today I was upgrading a computer for someone, just changing the mobo, cpu and ram, and they didn't feel like shelling out an extra $50 for a new power supply to go along with it. Their original psu was a 350W, which I wasn't sure if it'd be enough... So I built it, fired it up, and everything was going swell until I plugged in another harddrive to clone their data in case the repair install went south.?

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Ok... 350Watt power supply was the original PSU. The machine you built, was it old or new hardware? Be as specific as you can with the specs of the machine you built. The reason I say this is you may have to upgrade to a bigger psu. If a single hard drive is enough to make your computer start faulting out because of power issues, you are running your PSU near it's max output. That decreases the PSU's life, increases the risk of fire dramatically, and puts all your other parts at risk of a line spike when the PSU dies (Dying PSU's love to surge).

Minimum rated power supply I'd use for a normal build today would be a 500Watt unit. Anything less is asking for trouble unless it is a barebones unit or e-machine type computer.

measuring current

SO... I was wondering if there's an easier way of figuring out the total power draw of a computer, like say software based.. Either that or can anyone point out something I was doing wrong in trying to measure the amperage?[/QUOTE]






as mentioned the kil-a-watt is very good I use one, they can be had at your local hardware store or ebay. the other choice you have is a clamp on amp meter. you can use the power cord you modified with it.

to bad but it sounds like trying to read the supply current of a switching power supply with your hand held DMM just wont work.

Best regards Pyro

pseudolobster said:

I wasn't sure if this was caused by lack of power, so I figured I'd calculate exactly how many watts it was drawing... I cut open a spare power cord, cut one of the hot wires, and attached my multimeter between them. I set my dmm to AC amperage, though I couldn't get it to power up...

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Couldn't get the Power Supply or your Digital Multimeter to power up? If you are having problems with your power supply starting up when it isn't attached to your motherboard, you may want to look into short-out pins 14 and 15 on the power supply main power connector. A simple paper clip, straightened out and then bent at the right angle will do the trick.

My multimeter is rated up to 10A, and I've used the amperage function before just fine, so I'm pretty sure the fuse isn't blown, yet for some reason it just wouldn't start up.

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Oh so it's the DMM that won't start up. Well batteries could be an issue, but beyond that I can't help you...sorry.

SO... I was wondering if there's an easier way of figuring out the total power draw of a computer, like say software based.. Either that or can anyone point out something I was doing wrong in trying to measure the amperage?

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Ok, the Kill-a-watt will tell you how much power your computer is pulling from the wall. If you want to know what your components require and how much they are drawing from the PSU (so you know how big of a power supply you may need) then you will need to do some old fashioned research.

Find out the exact make and model (and version number) of each component. ANYTHING on the computer that plugs into the PSU. Motherboard, CPU, RAM (and remember if you have more than one chip, do add up the total usage), Hard Drives, CD-ROMs, Floppy (if you have one), Addon Cards (video, audio, network). Find out what all the components need for power, add it up, then add an extra 25-33% of the total power usage on top of that (this gives you wiggle room if you add more later and allows your machine to run everything at once without melting down the PSU).

If you just want to know how much your computer is costing you to run, then a Kill-a-watt is perfect for your needs. If you want to know how much power your computer is using off the power supply, you have to just do the old "scratch and search" (write all the component info on a scratch pad and do a google search for the specs).

If you told us which hardware you were using we may be able to help.

bschott said:

Ok... 350Watt power supply was the original PSU. The machine you built, was it old or new hardware? Be as specific as you can with the specs of the machine you built.

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MSI G31 mobo, Intel E5200 CPU, 2GB DDR2. Onboard everything.

I stressed to the customer this might not be enough power but they were willing to risk it in order to save fifty bucks.

For what it's worth though at home I'm running a 120W cpu, a 100W video card, 6x 1TB harddrives, 2 optical drives, a really power hungry RAID card, a dedicated sound card, and 4 extra fans... All on a 500W PSU. Been running steady for two years now, have even had it up to 8 harddrives at one point.

bschott said:

Couldn't get the Power Supply or your Digital Multimeter to power up? If you are having problems with your power supply starting up when it isn't attached to your motherboard, you may want to look into short-out pins 14 and 15 on the power...

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The computer. The computer wouldn't turn on. It's a fully functioning computer that was working with another cord, until I cut a wire on a power cord, stuck my DMM in between, and tried to measure amperage. It won't turn on with that cord attached, going through my meter. It works perfectly fine with other cords, it's a complete system that's working fine, I just wanted to get a rough estimate of how close to 350W I was pushing it.

Have you checked the new cord's continuity? Possibly there's a loose connection there? If not, buy a proper unit (the kill-a-watt)?

@ flaminpyro. fluke makes a clamp on amp meter. if you put that around the 120vac side of the power supply it would tell you the total draw.

pseudolobster said:

MSI G31 mobo, Intel E5200 CPU, 2GB DDR2. Onboard everything.

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350W Will be enough for that configuration. My guess is that the 350W PSU wasn't even 250W ==> Overheated something ==> Insta-Problems!

nikokapo said:

350W Will be enough for that configuration. My guess is that the 350W PSU wasn't even 250W ==> Overheated something ==> Insta-Problems!

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Yea, I was running a very similar setup with only 250W. His should be able to handle it

charliebruce said:

Have you checked the new cord's continuity? Possibly there's a loose connection there? If not, buy a proper unit (the kill-a-watt)?

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Yep, I checked that... I'm guessing my meter probably has a secondary fuse specifically for 10A, which come to think of it, I've never actually tested before. I'm guessing a fuse has been blown.

In either case I think I'll buy a kill-a-watt for future use, they're $20 with free shipping on newegg right now. Here's the URL if anyone wants it, and use coupon code EMCLSMN42 for free shipping.

I'm guessing a fuse has been blown.

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Wait, have you tried the setup with another PSU?

post the power supply model number and the full pc spec.

Just replace the PSU. They aren't expensive.

My computer at home draws over 350W. But it has a ton more power than that one.