Tuesday, September 30, 2008

Power Supply Energy Efficiency - Active PFC vs no PFC

Well, I've heard the chatter that the new PFC Power Supplies can save power, and I thought I'd give it a go.

I took a digital watt meter, and booted up a bench system with a standard ThermalTake TR2 430W non-PFC power supply. It was consuming ~120 watts under load (boot or stress test) and ~98 watts during idle.

I switched to a Seasonic S12-II 330W power supply with Active-PFC, and ran the same tests again.

With the new power supply, it was burning ~84 watts under load, and ~50 watts on idle.

I tried the same test on a few different motherboards, with some Enermax power supplies (also non PFC) and I've seen a significant drop in all. Another system that was 150w under load dropped to 109W with the Seasonic S12-II power supply.

A few items to consider:

1) There may be a slight drop in energy consumption because I'm using a 330W power supply vs a 430W power supply - but I expect it to be very slight, maybe a few Watts.

2) It could just be brand differences - I don't have a Seasonic Passive PFC or non-PFC power supply to compare. Running this test with more controls would make for a more accurate reading.

3) I've heard Passive PFC is more energy efficient than Active PFC, but I don't have the power supplies to compare.

4) The more power you burn, the more of a difference an Active PFC equiped power suuply makes. This makes good sense, as efficiency is a % rating of total power consumed - As you consume more power, your % lost to heat is greater.

5) I tested a cheaper Seasonic 330W power supply (generic OEM replacement, not the S12) and it had nearly identical stats for energy consumption as the S12-II, so you don't need to buy the more expenisve power supply to reap the savings.


I'm starting to phase out my non-PFC power supplies in my lab, because this does add up to a savings. I have ~20 machines in the lab, and about 10 of them are on 24/7.

I'm able to get power supplies at cost, $35 for a generic Seasonic, and $45 for the S12-II 330W, so this doesn't cost me very much. I'm also able to sell off my old power supplies as used for nearly the new cost of the Seasonics, so this will save me money if we don't count the time I have spent.

Otherwise, you'll have to take a serious look at if it's going to make $$ sense to switch your PS. In the best case scenario from my tests (a converted system running wide-open) is saving ~36 Watts. We pay ~.11 cents per killowatt hour here, so I'm saving ~.10 cents per day, or ~$2.85 per month.

That means it's over 15 months to pay back a $45 cash investment in this power supply, so your ROI isn't amazing. If you're not getting any money for your old supply, you are also throwing away the $$ you spent for that, unless it was time for a new PS.

On the other hand, when you consider that you'll probably have this solid Seasonic power supply for 5 years, and electricity will only go up in price, then it may make good sense.

Lastly, don't forget to account for the savings in A/C if you use it - You're making less heat now, which means less electricity to cool your office or home. In my lab, I think this will make a big difference, as on hot days we can just barely keep the temp from rising. When I unlock the lab doors in the morning before the A/C has turned on, I can always feel the heat from the room from these running systems.

With a lower heat generation load in the building, we should be able to keep our existing A/C system, which is a large cost savings.

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