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n00b question - power consumption

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DoubleZedRed

Gawd
Joined
Jan 12, 2009
Messages
849
does a power supply constantly draw what it's rated at? or is it based on connected hardware?

ex: 850 watt psu plugged in

will it draw 850 watts regardless? or would it only draw what it needs up to the capacity?
 
Mavvok said:
does a power supply constantly draw what it's rated at? or is it based on connected hardware?

ex: 850 watt psu plugged in

will it draw 850 watts regardless? or would it only draw what it needs up to the capacity?
Click to expand...

The components will only draw as much power from the PSU as they need - up to the maximum rated (or practical) capacity.

As for the power draw of the PSU itself from the wall, it also depends on the connected components (but will use a bit more than the components actually draw due to efficiency and PFC factors).
 
Inefficient PSUs waste power so try to get an 80+ Silver or Gold rated unit. If you have to make a choice its better to cheap out on something else and get a good PSU.
 
E4g1e said:
The components will only draw as much power from the PSU as they need - up to the maximum rated (or practical) capacity.

As for the power draw of the PSU itself from the wall, it also depends on the connected components (but will use a bit more than the components actually draw due to efficiency and PFC factors).
Click to expand...

are active PFC psu's better than passive or non PFC?
 
Mavvok said:
are active PFC psu's better than passive or non PFC?
Click to expand...

Usually. The lack of a PFC is almost always a giveaway that the PSU in question is an older and/or lower-efficiency design that may not be suitalbe for a modern build. The inclusion of a manual AC voltage selector switch at the rear of the PSU is a giveaway that the PSU in question lacks an active PFC. All active PFC PSUs have no manual AC voltage selector whatsoever.
 
Theoretically, if internally the PSU's are the same, the passive PFC is better for a regular home user because it uses no current to correct the sine wave output. However, the amount used is negligible (approximately 2-3 watts or less, I believe, don't trust me on this number). Active PFC is better for industrial/commercial users because it uses less VA (volt-amperes) compared to a passive/no PFC. Households usually get charged per watt, whereas industrial/commercial users (normally large ones) get charged per VA. VA is always higher than wattage, and PFC lowers VA closer to that of wattage, usually within 1% for active PFC. But this is just high-level tech talk.

In reality, it is what E4g1e said. Active PFC is an indicator of a newer design which is generally better than older designs. It's not always the case, but holds true at least 90% of the time. PFC, however, is not really a good indicator of a power supply's quality. Quality power supplies today almost exclusively use active PFC.

To answer your original question, the PSU will draw from the wall approximately 1.2 times the amount of power your components are drawing. Thus, if your components are drawing 100 watts, it'll draw approximately 120 watts. The energy difference is due to energy loss converting AC to DC. How large the difference is is based on the efficiency of the PSU. More efficient PSUs have smaller differences between wattage being pulled from the wall and wattage supplied to the components.
 
so if i was going to build a machine that i was starting with some low end parts (low end cpu and gpu) and wanted to upgrade later i'd be better off getting a PSU that provides the estimated amount that i would eventually need instead of just getting one for what i currently need as there won't be a difference in power draw until i change cpu/gpu?
 
Just get the power supply that you believe will last you through all planned component changes. Power supplies last a very long time, especially quality ones. There is the fact that lower wattage power supplies are more efficient at lower power levels, whereas higher wattage power supplies are less efficient. However, the difference is usually about 5-10 watts, and that is relatively miniscule. It'll cost much less to buy one large power supply now to handle all upgrades than to buy multiple power supplies eventually ending up with that one large power supply anyways.
 
Mavvok said:
so if i was going to build a machine that i was starting with some low end parts (low end cpu and gpu) and wanted to upgrade later i'd be better off getting a PSU that provides the estimated amount that i would eventually need instead of just getting one for what i currently need as there won't be a difference in power draw until i change cpu/gpu?
Click to expand...

Correct. There's some variance with efficiency as load varies. Efficiency is highest around 50% load, and lowest near 0% or 100% load, but it only varies slightly, on the order of 2-3 percent, and that would be at the extremes. For instance, 80 Plus Gold requires 90% efficiency at 50% load, and 87% efficiency at 20% and 100% load. So with that 850w PSU, if you get one that's 80 Plus Gold, you will always be 87%-90% efficient for any load from 175w to 850w.
 
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