Power Supply Unit Wattage and Rails Calculator

This PSU Wattage and Rails Calculator can calculate both wattage and individual voltage rail amperage (3.3V, 5V, 12V and 5VSB) required to run a PC system. Data provided by calculation is based on accurate manufacturer supplied information (TDP or otherwise) and measured values from various professional sources, like following: Lost Circuits, Silent PC Review, Storage Review, Tech Report, Xbit Labs, iXBT Labs.

Motherboard Chipset

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If a particular model isn't listed, select a model with same core and use change speed input to correct core speed appropriately, select an entry of similar power, or a generic entry. Use CPU-Z to find out exact model of a CPU, and its core count. It's perfectly normal for default wattage to be a lot different from quoted TDP value of a CPU, due to it typically being applied to an entire range rather than to each individual model, and because manufacturers measure TDP differently. Overclocking feature can also be used for undervolting/underclocking your CPU, if required. NOTE: uncore/non-core power consumption is not displayed here since its power usage is not determined by core speed/voltage, but it is included in overall power calculations.

override default wattage

number of CPUs / APUs

default voltage

default speed (GHz)

default wattage

change voltage

change speed (GHz):

overclocked wattage

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Graphics Card (GPU)

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

Second Graphics Card (PhysX, Crossfire)

PhysX users using a different GPU, or Crossfire users that have mismatched graphics cards (eg. a Radeon HD 4850 and a Radeon HD 4870 in Crossfire) can put their second graphics card here. If the second graphics card isn't actually being used for gaming, dropping the load factor to around 50% will make calculations less excessive since that card won't be using full power. For the sake of calculations, PhysX loads graphics card fully.

number of secondary GPUs (PhysX/Crossfire)

load factor

default core speed (GHz)

default memory speed (GHz)

default wattage

change core speed (GHz)

change memory speed (GHz)

overclocked wattage

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Memory voltage is typically 1.5V for DDR3, 1.8V for DDR2 and 2.6V for DDR. Memory speed ranges from 2133 to 1066 for DDR3, 1066 to 553 for DDR2, and 400 to 266 for DDR; obviously these values are different when overclocking and/or when using higher spec RAM.

number of memory modules

DDR or DDR2/3?

memory voltage

memory speed

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If a particular hard drive model isn't listed, the 'generic' option is a reasonable substitute; same goes for solid state drives. Choose 'none' for optical drives, if not applicable.

primary storage drives

number of drives

additional storage drives

number of drives

primary optical drive

additional optical drive

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USB values represent 'typical' draws. If known, add actual USB draw in the 'known USB power draw' field, in preference to selecting 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 a USB bus. Windows specifies a USB device draw inside device manager entries; dependent on device, sometimes '100mA' is stated rather than actual power use. NOTE: if using a powered USB hub (ie. with a separate power adapter), don't include any devices connected to it here. Any devices running on 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 (mA)

sound card model

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If a fan model isn't listed, enter its stats manually; a usual 12V-powered fan with a power draw of 2.46W has amps draw of 0.205A (2.46W / 12V = 0.205A).

CPU fan 12V amps draw


primary case fans 12V amps draw

number of fans


additional case fans 12V amps draw

number of fans


Data for water pumps and other professional cooling methods can be manually entered from manufacturer's information; many pumps use 2A total amps draw.

water pump / peltier 12V draw (amps)

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UPS and motherboard related features

NOTE: If the terms here seem confusing, leave them at default values.

These parameters merely influence UPS calculations; they do not, in anyway, influence the size of the PSU required. This includes PSU efficiency - PSUs 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.), sum up their total power draw (hopefully provided in a manual) and include it in the 'wattage draw of other devices connected to UPS' field. If a monitor has a power draw listed in its specifications, use that value instead of the suggested value. A UPS power factor is its rated wattage divided by its rated VA; example: a 420W/700VA UPS has a power factor of 0.6, as does a 600W/1000VA UPS. If this is not specified, assume it's 0.6. The PSU PFC type refers to the type of PSU's power factor correction, if any; 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 number

wattage draw of other devices connected to UPS

Motherboard related values are set to typical defaults; if known that a particular motherboard works differently, it can be customized here.

motherboard/SCSI card setup for 'staggered spinup' for hard drives

motherboard powers dram from 3.3V or 5V rail

motherboard powers USB devices from 5V or 5VSB rail

Multiple 12V Rail PSU Checks

These calculations are based on typical configuration of a by-the-books dual 12V rail PSU: CPU is powered from 12V2 rail, and everything else is on 12V1. Multiple (3,4,5 etc.) 12V rail PSUs vary a lot in configuration, so have not been coded for, instead consider these guidelines: 1. regardless what the PSU label says, the capacity of each individual 12V rail is at least 18A, sometimes more, but almost never less; 2. some PSUs have no split rails at all (ie. only one big 12V rail), such as some Seasonic built PSUs (despite being labelled as having multiple 12V rails); 3. additionally, if any of these numbers are quite high (more than 30A), either research carefully on the rail distribution, or get a single 12V rail PSU in order to avoid shutdowns.

General guidelines for a good rail distribution on multi 12V rail PSUs (doesn't apply as much to rails rated for 25A+; rather the usual 18A/20A rails):

  • no more than 1 6pin PCIe on the same rail as 24pin ATX connector, or none at all for 3/4 graphic cards;
  • no more than 1 6pin PCIe on the same rail as EPS or ATX12V connector;
  • no more than 2 6pin PCIe on any rail (and no 8pin PCIe);
  • no more than 1 8pin PCIe on any rail (and no 6pin PCIe);
  • no more than a couple of hard drives on a rail that already has 2 6pin PCIe / 1 8pin PCIe / ATX connector (approximately);
  • no more than roughly 7 hard drives on a rail that is otherwise free (approximately).

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 by-the-books dual 12V rail PSU. If your motherboard supports 'staggered spinup' (SATA only), that could help solve the issue. If PSU's 12V combined amps end up being very close to this, don't worry; it's a peak draw that only lasts a second or two (as oppossed to gaming 12V draw which is continuous and therefore a lot more stressful on a PSU).
peak CPU (12V2) draw High end overclocked CPUs can draw over 18A; in such case a PSU with an EPS connector or a single 12V rail PSU is required.

* A dual 12V rail PSU can sometimes work with more than 18A power draw if a PCIe is on the same 12V rail as CPU (12V2); the Silverstone Element 500W works in such a way; however, very few others do. Alternatively, if 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 a PSU.

Recommended Minimum PSU Ratings

When choosing a PSU make sure all ratings are met! The most important rating is 12V combined amperage and wattage (some PSUs quote figures only as amps, others only as watts, and some mention both). See OCAU PSU Wiki for a guide on how to work out 12V combined amps. When sizing a recommended PSU, it's not a disaster if calculations say you need a 403W PSU and you've got a 400W; it will still work as long as the PSU is a decent brand/model; there is plenty of 'headroom' in these calculations. Also it's perfectly normal for recommended minimum 3.3V and 5V ratings to be very small (except for older systems, these will be small in most situations).

Also, check OCAU PSU Wiki for a list of brand reputations. Even if same brand, some PSU models are better than others, so reading reviews on a candidate PSU is a very good idea. For example OCZ GameXStream 1010W has a bad voltage ripple when used past 50% its nominal load, thus it may not run systems reliably, even though calculations indicate it should, as HardOCP review found out; however, OCZ ProXStream 1000W was excellent even at 100% load.

Nominal ratings for decent brand/model PSU

NOTE: 'typ' references are purely placeholders, to allow for an easy comparison with usual PSU labels. These are NOT requirements; the important thing is the 12V combined amperage, as stated before.

3.3V rail
5V rail
12V1 rail
12V2 rail
12V3 rail, etc
-12V rail
5VSB rail
INDIVIDUAL rail rating
18A typ
18A typ
18A typ
0.8A typ
10W typ
TOTAL power

Additional rating requirements for generic brand/model PSU

Best practice is to not use a generic PSU under any circumstances. However, these numbers are given for conveniency reasons, to minimise a potential harm if one is used. If calculations say that power draw is too high for a generic PSU, don't use one, even temporarily!

minimum PSU power
minimum 12V combined

PSU Connector Requirements

Preferable total number of connectors of each type are listed here. These are not absolute requirements. NOTE: 1. if calculations recommend an 8pin EPS connector, rather than 4pin ATX12V connector, you should ensure your PSU and motherboard have 8pin EPS connectors, cause your CPU/APU needs more power, 2. calculations assume that no adapters are being used; it is fine to daisy chain up few low power fans off a single connector, but the same doesn't apply for graphics cards, 3. these numbers are just recommendations, and situation will depend much on particular graphics cards used.

4pin ATX12V

8pin EPS

6pin PCIE

8pin PCIE


4pin molex

Actual System Power Consumption

These ratings tell how much power computer will actually be using while gaming. Obviously, these are well under what PSU is rated for, as gaming doesn't tax all the components to max. The actual recommended PSU ratings are above.

maximum sustained power consumption
3.3V rail consumption
5V rail consumption
12V combined consumption

Following ratings represent maximum activity on all drives and CPU combined, perhaps a 'worst case' load for fileservers. This is a rather unrealistic scenario, so final PSU calculations don't consider it, at the moment.

fileserver full usage
fileserver full usage 12V combined
fileserver full usage 5V rail

Recomended Minimum UPS Ratings

These values will help you decide on Uninteruptable Power Supply (UPS). NOTE: actual UPS ratings need to exceed both wattage (W) and VA (volt amperage) ratings given here.

computer only
TOTAL (including monitor, etc)
actual AC wattage used from outlet
actual VA draw
recommended UPS wattage rating (W / VA)

Closing Notes

This is a guide only, final choice of PSU is your own responsibility.

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