PSU question

Scruff

Are PSUs rated at their maximum wattage that they can provide or how much they provide when are running?
E.g. if i have a 300W psu powering my rig but the components are only using 200W is the psu sucking down 300w from the mains or just the 200W + conversion inefficiency?

uberpixie

Scruff wrote: »

Are PSUs rated at their maximum wattage that they can provide or how much they provide when are running?
E.g. if i have a 300W psu powering my rig but the components are only using 200W is the psu sucking down 300w from the mains or just the 200W + conversion inefficiency?

There are rated at their "maximum" level of stable power they can provide at 100% load. Most cheap PSUs generally lie about their ratings so a pinch of salt needs to be taken in general with specs.

There are a lot of 200W PSUs dressed up as "700W" PSUs. The amount of amps that the 12.0V lines on the PSU can take is also very important especially in high end gaming rigs. Cheap PSUs are epic fail in this regard also.

Basically your PSU provides your 200W + "conversion ineffciency" as you neatly put it.

"conversion ineffciency" varies depending on the load placed on the psu. e.g. a PSU is generally very efficent running at 50% and 75% load and the "conversion ineffciency" would be lower on a modest PSU VS lets say a 1200W psu that is only loaded at 10% of it's possible output. So matching the PSU to the load it will run is very important if you do not want to waste money on electricity.


http://en.wikipedia.org/wiki/80_PLUS (eplained better...)

"
The efficiency of a computer power supply is the amount of energy it supplies, divided by how much energy is input. The remaining energy is converted into heat.



For instance, a power supply operating at 60% efficiency, supplying 300 watts to a computer would draw 500 watts, and give off 200 watts of heat. An 80% efficient supply would draw 375 watts to produce the same 300 watts, giving off 75 watts of heat.


For a given power supply, efficiency varies depending on how much power is being delivered. Supplies are typically most efficient at between half and three quarters load, much less efficient at low load, and somewhat less efficient at maximum load.



Older ATX power supplies were typically 60% to 75% efficient. To qualify for 80 PLUS, a power supply must achieve at least 80% efficiency at three specified loads (20%, 50% and 100% of maximum rated power).



However, 80 PLUS supplies may still be less than 80% efficient at lower loads. For instance, an 80 PLUS, 520 watt supply could still be 70% or less efficient at 60 watts (a typical idle power for a desktop computer).[2] Thus it is still important to select a supply with capacity appropriate to the device being powered."