How can I tell how many watts my system eats?

[Ken B.]

I'm totally green in this area, so please forgive.

I want to find out exactly how much wattage my system eats at full load and at idle. I'd like to use a multimeter and would appreciate any advice on a good cheap one and how to use it.

I know there are sites using a kind of calculator with generic figures for various components, but I want to be more exact than that.

Thanks.

[Adak]

I'm totally green in this area, so please forgive.

I want to find out exactly how much wattage my system eats at full load and at idle. I'd like to use a multimeter and would appreciate any advice on a good cheap one and how to use it.

I know there are sites using a kind of calculator with generic figures for various components, but I want to be more exact than that.

Thanks.

It's not quite as easy as that, unfortunately. To measure "load", you need to actually have either 1) the current passing right through the measuring meter, or 2) a "clamp" type meter that you put around ONE (let's call it the "hot" wire for simplicity) wire.

But #1 will likely burn your meter up, because building something to accept that much power isn't cheap. Cheap multimeters I've seen specifically say NEVER do that with more than X number of watts or say don't do that, at all.

That's a good warning, by the way, as they can and will short "explosively", if you try to pass power they're not made to handle, through them. :(

#2 is a great idea, but those types of meters, aren't usually cheap. Far from it. To use one however, you also must have the wires separated on an extension cord or the actual power cord leading to the device. It won't detect anything if it's clamped over both the HOT and the Neutral.

I use a special extension cord for this, with a clamp type meter (no, it's not cheap), which I've separated the wires at in one spot about 7 inches long.

A cheaper alternative may be picking up one of the consumer-oriented "watt minder" kind of meters. They're easy to use, built to handle light loads going through them, etc. They're not really cheap, like pocket multimeters, but that's because they're built to a higher standard, and have to cost more than a simple multimeter.

Hope that helps.

Adak

[Enablingwolf]

While not perfect. This link will give you an alright guess-timation of what your going to be using.

http://www.extreme.outervision.com/psucalculator.jsp

[iceage]

You can buy a Kill-A-Watt device that you plug into the wall and then plug your rig into the Kill-A-Watt. It'll measure your total wattage coming from the rig. Kinda hard to find but a nice investment.

http://www.supermediastore.com/kilwateldet1.html

PS- I've never bought from this store before but they're in stock there atm. There was a time when they were hard to find.

[bchur83]

The easiest way would be to buy a cheap UPS and use the monitoring software to check the load your system is putting on the UPS. Make sure you get a UPS rated at least 500VA. I have my main system in my sig hooked up to an 1100VA (850W) UPS and it says that I have a load of about 46% with the monitor on, playing a game. So roughly it is using 400W.

[four4875]

well, most multimeters i see are rated at 10 amps.. so 10 amps at 117 volts.. 1170 watts, dont think the comp will be pulling that much. you'd have to cut the hot wire (black) in the cord and put one probe to each side, and plug the probes into the current plugs on the multimeter. set MM to AC current, measure that, write it down. then measure the volts on the power, probably right around 115 or 120, then multiply amps by volts (p=iv) and thats how many watts. but be careful, 110 can hurt if you touch it, so be sure everything is properly insulated and the meter can handle the load.

[dogbert_2001]

BTW, if you're measuring current and voltage in AC (rms), multiplying them gives you Apparent Power in volt-amperes (VA). Your power factor determines how much Real Power you consume in Watts (W). Power factor is the cosine of the difference in phase of current to voltage (for pure sinusoids).


If you want to be real cheap, go outside and read your power meter. You'll probably need long test times to get decent accuracy, and you can't have things like your fridge or HVAC turning off an on during your tests.

[Ken B.]

well, most multimeters i see are rated at 10 amps.. so 10 amps at 117 volts.. 1170 watts, dont think the comp will be pulling that much. you'd have to cut the hot wire (black) in the cord and put one probe to each side, and plug the probes into the current plugs on the multimeter. set MM to AC current, measure that, write it down. then measure the volts on the power, probably right around 115 or 120, then multiply amps by volts (p=iv) and thats how many watts. but be careful, 110 can hurt if you touch it, so be sure everything is properly insulated and the meter can handle the load.

Would any of you care to confirm if the above sounds reasonable?

Also, iceage, that Kill A Watt sounds really cool. Thanks! I might consider it.

[Adak]

Would any of you care to confirm if the above sounds reasonable?

Also, iceage, that Kill A Watt sounds really cool. Thanks! I might consider it.

Sounds about right. I don't recommend it for anyone who has to ask how to do it, however. He notes you need to cut power cords, etc.

BEFORE you try anything like that, be SURE you read the directions and power limits of using YOUR particular multi-meter.

My little cheapie m-meter in my general toolbox will handle MUCH less power than my large and much more expensive Fluke m-meter. (and I would not trust the little cheapie with it's full rating, either). :P

I agree with the recommendation of using Kill-A-Watt. It's easy, has no wire cutting involved, and much less hazard to non-electricians.

Adak

[four4875]

yeah, i didnt consider the $10 mms. ive grown accustomed to an expensive radioshack one and a fluke, fluke much more lately than the RS.

[Kamel]

well, with some applications you can calculate load by using a series of resisters inside the circuit then running it at full load. the only problem is, usually the device wont function like this, but you can figure out usually how much load there is.

i don't think this would work with something like a PC though, it's a bit too complicated. at the electronics goldmine (www.goldmine-elec.com) there used to be an electricity meter that told you a ton of things about your current and over time energy usage, but it doesn't seem to be there anymore. you might do some searching around for a "GE smarthome electricity cost meter" or something similar to that. the GE and smarthome parts are right, so that should be the most important thing. i purchased one and am very glad i did. the accuracy of the current wattage draw is very nice, and has a 1 watt fidelity. i can find fluxuations in power with my system just by burning cd's or playing a game.

[Skeith]

Well in theory the power the system uses by the low voltage busses should sum to the wattage drawn from the mains socket. This is the law of conservation of energy. The wattage acctualy consumed by the pc components will be between 80 and 90 percent of the wattage drawn from the mains outlet because of the poor efficiency of converting the high voltage to low voltage. Simply put some energy is lost to heat in the PSU.
Some power supplied will have an efficiency rating on them or on the box or manual. Some cheap PSUs will have an efficiency as low as 62%. Most of those were AT style PSUs though.

Now if you put an ampmeter between your live wire on your AC input and the PSU you will get a ampere rating. This rating is multiplied by your mains voltage to give you the wattage. Here in Canada our voltage is 120vac on the mains line.

So if my PSU is drawing 4amps off of the 120v line then the wattage its consuming is 4*120 = 480watts. This is the total wattage. To find the wattage that the components inside the PC draw you would multiply by the efficiency percentage. So if my PSU is only 80% efficient then the components in my PC would draw 480*0.80 = 384 watts. The rest of the wattage that isnt used by the computers components is disipated in the PSU as the current is switching elelctronics need and mostly heat.

You can probably get the efficiency rating of your PSU from your PSUs manufactuer if you e-mail them and ask them.

That is the easiest way unless you want to mesure each low voltage bus seperately or just estimate the current draw with an online calculator or known current ratings.

[klingens]

Those ratings on the PSU are useless: it only works if The PSU actually used at the rated wattage. Something you will never manage in the real world.
The less percentage of the rated wattage you use, the less efficient it is. E.g it's a bad idea to run a 600W PSU on your old athlonxp.