Laptop vs. Desktop Power Consumption

Devonavar · Post by **Devonavar** » Sun Apr 30, 2006 11:19 am

Linus: We have two AC/DC bricks to test with the PicoPSU, a 70W and a 120W IIRC.

Hifriday: Actually most high powered AC/DC bricks are switching power supplies in black plastic. Lower capacity models (<15W) may use transformers, but switching power supplies are the only practical design that I know of for high powered AC/DC conversion. This means that they ARE affected by PF.

And, PF does affect current draw. Without PFC, you will always see a higher current draw than you would with it. The formula looks like this: Watts = Voltage Ã— Current Ã— PF. When PF is low, current must increase to compensate to maintain the same power (wattage) output. With electric (vs. electronic) devices, PF = 1 because the load is purely resistive, so you can factor PF out of the equation to get the more familiar Watts = Voltage Ã— Current.

At the same time, PF does not affect efficiency, because efficiency is used to measure power, not current. If you are billed in kWh (power over time), you will not be billed for consuming more power if you don't use PFC, even though your current draw will be higher. If you are billed in VAh (Volts Ã— Amps Ã— hours), you WILL see a higher power bill, since they are billing you based on your current draw, not power.

Linus · Post by **Linus** » Sun Apr 30, 2006 11:33 am

Hifriday - I think you're measuring VA with the clamp meter, not necessarily watts consumed. When I plug my LCD power brick up to my Kill-A-Watt, it reads 32W, 74VA, and 0.43 PF. That fits with Devon's VA * PF = W.

Interestingly, that means the wattage of your systems is, if anything, lower than you reported

. Part of it may be the higher efficiency at 220V.

Also, if the original AOpen PSU had APFC but the brick doesn't (like my LCD power brick obviously doesn't), then the difference between the two in straight-out efficiency is even bigger than your numbers suggest.

Devonavar - I'm glad you'll have more than one brick to test. Would you test a 180W/220W brick if I lent you one? This thread has me considering a move to a brick & DC-DC PSU combination, and I think a lot of people would be interested in the results from bricks big enough to power "normal" systems.

Devonavar · Post by **Devonavar** » Sun Apr 30, 2006 12:04 pm

Assuming the PicoPSU can handle it I'd be interested, but ultimately it's Mike's call. It may also depend on where you are located and how long shipping takes. We want to get this done in the next week or two (we're trying to get an EPIA system running with it). Why not e-mail him ([email protected]) and see what he says?

Linus · Post by **Linus** » Sun Apr 30, 2006 1:04 pm

Nevermind...forgot the PicoPSU maxes out around 120W. I'll just wait patiently for the results of your review.

I want to thank all of you for what I think has become a very informative thread. All of this info will help a lot when I build my next PC.

Eunos · Post by **Eunos** » Sun Apr 30, 2006 7:51 pm

I'm excitedly looking forward to this PicoPSU review - sounds like an incredibly compact and efficient solution. I'd be able to buy a much smaller case like an NSK2400.

My only concern that high-end regular power supplies have a host of important safety features, eg over-temperature, over-voltage, over-current, over-power, short-circuit and under-voltage protection. I have had a cheap power supply kick the bucket before and it killed almost every component in my PC just like that.

jaganath · Post by **jaganath** » Mon May 01, 2006 3:18 am

First of all, thanks to Devon for his concise explanation of PF and how it affects current draw. Highly educational.

(we're trying to get an EPIA system running with it)

I fear that this might limit the scope or usefulness of the article; I think it's pretty much a given that the PicoPSU can power a VIA EPIA mini-ITX system (that is, after all, what it was designed for) but the really interesting possibility with the PicoPSU and the PW-200M more generally is their ability to power a low-wattage micro- or ATX system. It will be good to know the efficiency of the Pico anyway, but perhaps a test involving a minimalist Sempron/Athlon S754 system might yield more generally applicable results.

Devonavar · Post by **Devonavar** » Mon May 01, 2006 7:58 am

That's not all we're doing. It will see some time on the test bench, as well as in an actual system where thermals can influence it.

Hifriday · Post by **Hifriday** » Mon May 01, 2006 8:22 am

Ok thanks Linus and Devon for straightening me out. For the longest time I thought measuring VA was the same as the wattage, but now I realize for AC circuits with reactive loads, this is only "Apparent Power" (measured in VA) and not "True Power" or "Real Power" (measured in Watts). Apparent Power is a combination of True Power and Reactive Power. Some interesting reading clarifying PF that I should have read sooner here and also Mike's explanation here.

Linus wrote:Interestingly, that means the wattage of your systems is, if anything, lower than you reported . Part of it may be the higher efficiency at 220V.

Also, if the original AOpen PSU had APFC but the brick doesn't (like my LCD power brick obviously doesn't), then the difference between the two in straight-out efficiency is even bigger than your numbers suggest.

So it is as Linus pointed out, the Real Power consumption (Watts) should be even less then my reported VA figures. However most of my systems are using swithcing power supplies and as Devon pointed out should have active PFC (the fact they are all universal input voltage 110-220v should have rang some bells in my head). This probably means PF is close to 1 (maybe 0.95 or higher?) so my Real Power watts figure won't be too much lower than the Apparent Power VA figures I posted. I really should try to get hold of a 220v watt meter.

As for the AOpen stock PSU, I doubt it has active PFC, probably only passive PFC (0.6-0.

and as you pointed out that is why I measured such a large VA difference compared to the Pico. When I replaced the the Antec Aria stock PSU (which has Active PFC) with the PW200M, the Apparent power reduction was only 9-12VA.

Devon thanks also for the clarification of minimum draw/efficiency.

Linus · Post by **Linus** » Wed May 10, 2006 11:14 am

SPCR's review of the picoPSU is here: Tiny, Silent and Efficient: The picoPSU.

Nothing all that surprising to me, really. Decent efficiency improvements versus the Seasonic SS-300SFD 80 Plus, but nothing earth-shattering.

Linus · Post by **Linus** » Sun May 28, 2006 5:13 pm

New winner of the low-power crown: SPCR built and tested an EPIA-based system that idled at 17W with a PicoPSU/brick combination for power (an efficient ATX PSU brought that up to 28W). See VIA EPIA EN12000E: Today's most efficient CPU & board.

Since the board is built from the start with low power in mind, I wonder if it utilizes some laptop-style power conservation measures, like turning off motherboard features when they're not in use.

mb2 · Post by **mb2** » Tue May 30, 2006 5:14 am

Devonavar wrote:it would require incrementing the load in very small increments, something that our current test setup cannot do easily.

could it not just be loaded with fan(s) of various ratings?

Devonavar · Post by **Devonavar** » Tue May 30, 2006 11:21 am

Yes, I suppose it could, but to what benefit? The loading resistor should still drop out of the circuit entirely once the motherboard finishes booting. The amount of time that the resistor would be in use is so small that I don't see it being an important factor in long-term efficiency. It seems like a lot of work for very little knowledge that can be put to practical use.