PSU Watts And 12V Calculator

This PSU calculator can calculate both the wattage and 12V amps required to run your system. Data is based off measured values from any accurate manafacturer supplied data and the following sources:
Digit-Life, LostCircuits, SilentPCReview, StorageReview, TechReport, XbitLabs

Motherboard Chipset

Cpu

If your particular model isn't listed, pick a model with the same core as yours and use the change speed input to correct the speed to what it is. Use CPU-Z to find out what model cpu you have, and what core it is. It is perfectly normal for the default wattage to be a lot different from the the quoted TDP value of the CPU, due to it typically being applied to an entire range rather than to each individual model, and because AMD measure TDP differently to Intel. The 'overclocking' feature can also be used for undervolting / underclocking your cpu if required. NOTE: uncore power consumption (Intel Core i7) is not displayed here since its power usage is not determined by core speed / voltage, but is included in overall power calculations.

Override default wattage:

Number of CPUs:

Default voltage:	Default speed (GHZ):	Default wattage:
Change voltage:	Change speed (GHZ):	Overclocked wattage:

Graphics Card

Number of graphics cards (SLI/Crossfire)

Default core speed (GHZ):	Default memory speed (GHZ):	Default wattage:
Change core speed (GHZ):	Change memory speed (GHZ):	Overclocked wattage:

Dram

Memory voltage is typically 1.5V for DDR3, 1.8V for DDR2, 2.6V for DDR. Memory speed is typically 1066 for DDR3, 533 for DDR2, 400 for DDR1, obviously both of these are different when overclocking and/or higher spec ram.

Number of sticks of ram:

DDR or DDR2/3?:

Memory Voltage:

Memory Speed:

Drives

For optical drives, choose 'None' option if not applicable. If your hard drive isn't listed the 'generic' options are a reasonably substitute.

Hard drive type 1:

Number of drives:

Hard drive type 2:

Number of drives:

Optical drive 1:

Optical drive 2:

MISC

The usb values represent 'typical' draws. If you know what your usb devices actually draw add them up in the 'known usb power draw' field in preference to the the select boxes, however most usb devices state '100mA' regardless of what they actually draw. A typical high power usb device would be an external hard drive that powers off the usb bus. Windows can tell you a usb device draws by going into device manager and opening up the usb hubs in 'universal serial bus controllers' (again dependent on device, sometimes '100mA' is stated rather than actual power use). Note that if you are using a powered usb hub (ie one with a seperate power adapter), don't include any devices connected to it here, and any devices running off an unpowered USB hub need to be included.

Number of PCI cards:

Number of low power (100mA) USB devices:

Number of high power (500mA) usb devices:

Known usb power draw total (in mA)

Sound Card Model:

Cooling

If your case fan model isn't listed, it can be manually entered from manafacturer data, for example: this fan has a draw of 0.12A, while this fan has a power draw of 2.46w (convert to amps by dividing by 12. eg. 2.46W / 12V = 0.205A).

Cpu fan 12V amps draw

Connection

Case fan (type 1) 12V amps draw
Number of fans:	Connection

Case fan (type 2) 12V amps draw
Number of fans:	Connection

For water pumps and other extreme cooling methods, this data can be manually entered from manafacturer data as well, for example this Switftech pump uses 2A.

Water pump / peltier 12V draw (amps):

UPS and motherboard related features

NOTE: If your confused by any of the terms here, leave them on default values.

The motherboard related values are set to typical defaults of how a motherboard powers parts, if you know your motherboard is set to differently then that can be set here. The rest of factors merely influence the UPS calculations, they did not in anyway influence the size of the psu required. This includes psu efficiency - psu's are rated by dc output (unchanged by efficiency), not what they draw from the mains (influenced by efficiency). If you have any other devices connected to the UPS on top of the PC and monitor (eg modem/router, speakers etc), add up their power draw (hopefully provided in the manual) and include it in the 'other devices conncted to UPS watt draw' field. If your monitor has a power draw listed in its specifications, use that instead of the suggested value. The UPS power factor is its rated wattage divided by its rated VA, so for example a 420W / 700VA UPS has a power factor of 0.6, as does a 600W / 1000VA UPS. If this is not specified, assume its 0.6. The psu pfc type refers to if the psu has any sought of power factor correction, usually 'none' for cheaper models while higher end models have active power factor correction.

PSU pfc type

PSU efficiency at load (%)

UPS power factor

Monitor type and No. of

Other devices powered off UPS total watt draw

Motherboard/scsi card setup for 'staggered spinup' for hard drives

Motherboard powers dram off the 3.3V or 5V rail

Motherboard set so usb devices power off the 5V or 5Vsb rail

Actual System Power Consumption While Gaming

This is how much power the computer will actually be using off the psu while gaming. Obviously both of these should be well under what the psu is rated for. See the recommended size PSU at the end to work out how big a PSU you need, after taking care of the 'multiple 12V rail psu checks'.

Max sustained power consumption (watts)	3.3V consumption	5V consumption	12V combined

This is full hard drive (all drives) and CPU activity, perhaps a 'worst case' load for fileservers. Given that its rather unrealistic test that most users wouldn't be doing, the final recommended psu calcs don't consider it at the moment.

File Server full backup watts	File Server backup 12V combined amps	File Server backup 5V amps

Recomended minimum UPS Ratings

Use these values to work out how big an Uninteruptable Power Supply (UPS) you need to ensure your pc stays powered during blackouts. It is importent to note that the specs of UPS needs to exceed both the watt and VA (volt amp) ratings given here.

	Computer Only	Total (inc monitor etc)
Actual AC Watts drawn from wall outlet
Actual VA draw
Recommended Watts UPS rating (Watts / VA)

Multiple 12V Rail PSU Checks

These calculations are based on the typical configuration of a ‘by the books’ dual 12V rail psu, that is cpu gets 12V2 to itself, and everything else is on 12V1. Multiple (3,4,5 etc) 12V rail psu's vary a lot in configuration, so have not been coded for, instead go off the guideliness below. Note that regardless of what the PSU label says, the capacity of each individual 12V rail is at least 18-20A, sometimes more but almost never less. Also some newer psu’s have no split rails at all (ie only has one big 12V rail) such as new Seasonic built psu’s (despite being labelled as having multi 12V rails). Additionally, if any of this numbers are quite high (say more than 30A) you'll need to either research carefully on the rail distribution, or get a single 12V rail psu in order to avoid shutdowns.

General guidelines to pick a good rail distribution on multi 12V rail psu's (don't apply as much to rails rated for >25A rather than the normal 18-20A):

No more than 1 6pin pcie on the same rail as the 24pin atx connector or none at all for 3/4 graphics cards
No more than 1 6pin pcie on the same rail as an EPS or ATX12V connector
No more than 2 6pin pcie on any other rail (and no 8pin pcie).
No more than 1 8pin pcie on any other rail (and no 6pin pcie).
No more than a couple of hard drives on a rail that already has 2 pcie / 1 8pin pcie / atx connector + 6pin on it (approx)
No more than roughly 7 hard drives on a rail that is otherwise free (again approx)

Typical Gaming 12V1 draw

If this exceeds 18A, a dual 12V rail psu probably won’t work - psu will shut down while gaming. The only solution is to buy a non dual 12V rail psu*.

Startup 12V1 draw

If this exceeds 18A, you may have issues with a dual 12V rail psu thats ‘by the books’. If your motherboard supports ‘staggered spinup’ (SATA only) that should also help solve this issue. Don't be overly concerned if the recommended 12V combined amps ends up being very close to this, as its a peak draw that only lasts a second or two (as apposed to gaming 12V draw which is continuous and therefore a lot more stressful on the PSU).

Peak CPU (12V2) draw

High end overclocked cpu's can draw over 18A, so if it does a PSU with EPS connector or one that is actually single 12V rail is required.

* A dual 12V rail psu can often work with more than 18A power draw if it puts the second graphics card on the same 12V rail as the cpu (12V2), like the Silverstone Element 500w, very few do this however. Alternatively if the current limiters are set to more than 20A (eg. Enermax liberty 500w), or there aren't any (eg. just about any Seasonic) then a dual 12V rail psu will work, but in the majority of cases it won't, regardless of 12V combined amps and wattage rating of the psu.

Recommended Minimum PSU Ratings

When choosing a psu make sure all the ratings especially both the wattage AND 12V combined amps (given as amps and watts here since some PSU models quote the figure as amps, and others as watts) are met. See the OCAU PSU Wiki for a guide on how to work out 12V combined amps. When sizing a recommended psu, its not a disaster if the calc says you need a 403w psu and you've got a 400w, it'll still work as long as the psu is decent brand/model as there is plenty of 'headroom' in these calculations. Also its perfectly normal for the recommended minimum 3.3V and 5V ratings to be very small (they will be except for older systems), this just indicates that the psu you use doesn't need to have high ratings on these rails.

See the OCAU PSU Wiki for a list of what brands are classed as 'generic' and what are decent brands. Even amongst brands, some models are better than others, so reading up reviews on candidate PSU's is a very good idea to ensure you the PSU you end up buying can run reliably at 100% load. For example the OCZ gamexstream 1010w has out of spec voltage ripple past around 50% load, so may not run systems reliabily even though this psu calculator may indicate it could as HardOCP found out, whereas the OCZ proxstream 1000w in the same review was excellent even at 100% load.

For decent brand/model PSU:

Note: 'typ' references are purely placeholders so the table layout is very similar to most PSU labels to allow for easy comparison. They don't mean the PSU has to have that rating or better, same deal with number of 12V rails listed.

	3.3V	5V	12V1	12V2	12V3 etc	-12V	5VSB
Individual rail rating			18A typ	18A typ	18A typ	0.8A Typ
Combined Rating:						10W Typ
Total Wattage:

Additional minimum rating requirements for Generic brand/model PSU:

Best practise would be to not use a generic psu at all under any circumstance. However for those can't afford a real PSU ensuring your generic psu meets these numbers will minimise the potential harm done. If the calculator says the power draw is too high for a generic, don't use one, even temporarily!

Min PSU wattage	Min 12V combined amps

PSU Connector Requirements

Here the total number connectors of each type are given, so make sure your candidate psu meets the count. Note that if the calculator recommends an 8pin EPS connector rather than 4pin ATX12V connector you should ensure your PSU and Motherboard have 8 pin EPS connectors. Its made the recommendation because your cpu power draw is too high. Currently the calculator assumes that no adapters are being used. While it is fine often to daisy chain up low power fans with adapters off one or two molex connectors if your psu doesn't have enough, the same wouldn't be recommended for graphics cards. Its best to get a psu with all the required PCIE connectors in the first place. Also ocassionally some manafacturers put an extra PCIE connector or 8pin instead of 6pin on their 'overclocked' edition of a videocard, in which case the PCIE count listed here will be incorrect.

4pin ATX12V

8pin EPS

6pin PCIE

8pin PCIE

SATA

4pin Molex

This is a work in progress, send any feedback / suggestions to

Legal bs: This is a guide only, I take no responsibility if you buy a psu that destroys your system based on any information here.