PC Power Supply Calculator

Add up your CPU, graphics card, drives, fans and peripherals to estimate peak system draw, the PSU wattage you should buy with headroom for GPU transient spikes, and what the PC costs to run each year.

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Results will appear here once you enter your values.

How the PSU size is calculated

The calculator adds each component's peak draw times its count, applies any overclock margin, then adds headroom and rounds up to the next retail size (450, 550, 650, 750, 850, 1000, 1200 or 1600 W).

Peak = Σ(watts × qty) × (1 + overclock %)
Minimum PSU = peak × (1 + headroom %)

Worked example: a 142 W CPU (a 105 W TDP chip's package power limit), a 300 W graphics card, 50 W motherboard, two 5 W DIMMs, one 7 W SSD, four 3 W fans and 15 W of USB gear total 536 W. With 40% headroom that is 750.4 W, just over a 750 W unit, so the recommendation is 850 W.

Why 40% headroom: GPU transient spikes and ATX 3.x

Recent graphics cards can pull close to double their rated board power for a few milliseconds when a game scene changes. Older ATX 2.x supplies sized with no margin could trip over-current protection and shut the PC off even though average draw was fine.

The Intel ATX 3.x design guide adds power-excursion requirements so a certified supply rides through brief peaks well above its rated output instead of shutting down. A 40% margin also keeps the PSU close to 50% load while gaming, where 80 PLUS efficiency is highest.

  • Use the CPU's PPT or PL2, not the headline TDP. An AMD 170 W TDP part is allowed 230 W package power; Intel K-series desktop chips list 253 W PL2.
  • Seat 12V-2x6 / 12VHPWR plugs fully and avoid a tight bend right at the connector.

Wall draw, 80 PLUS efficiency and running cost

A PSU converts wall AC to DC. The PC's DC load divided by efficiency is what your meter sees. The 80 PLUS program's 115 V internal requirements at 50% load are 80% (80 PLUS), 85% (Bronze), 88% (Silver), 90% (Gold), 92% (Platinum) and 94% (Titanium).

Example: the 536 W build averages about 70% of peak in games, 375.2 W DC. On a Gold unit that is 375.2 / 0.90 = 417 W from the wall. Four hours a day for a year is 417 × 4 × 365 / 1,000 = 609 kWh, or $103.47 at $0.17/kWh. Moving from Bronze to Platinum on the same PC saves about 7% of wall energy, roughly $8 a year here, so buy higher tiers for quiet fans and build quality rather than payback.

Common sizing mistakes

  • Adding the TDP of every part. TDP is a cooling spec, not a power ceiling; peak draw can be 35% higher.
  • Buying the biggest unit available. A 1,600 W supply running a 300 W idle desktop sits at under 20% load, where efficiency drops.
  • Ignoring the circuit. A 15 A, 120 V household circuit is limited to 1,440 W of continuous load (80% rule, NFPA 70), shared with monitors and anything else on the outlet.
  • Counting the monitor. Monitors and speakers plug into the wall, not the PSU; leave them out of this total.

Frequently Asked Questions

Common questions about the PC Power Supply Calculator

Add the card's total board power to your CPU's peak package power plus about 100 W for the rest of the system, then add 40%. A 350 W card with a 230 W CPU lands near 950 W, so a 1,000 W ATX 3.x unit is the usual pick. Always check the card maker's recommended PSU as well.

Sources & References

80 PLUS Program Certification Requirements

Efficiency requirements at 10/20/50/100% load for 115 V internal non-redundant power supplies by tier.
→Source: CLEAResult 80 PLUS

ATX Version 3 Multi Rail Desktop Platform Power Supply Design Guide

Power excursion requirements and the 12V-2x6 connector specification for ATX 3.x supplies.
→Source: Intel

Desktop Computers and Energy Use

ENERGY STAR computer specification, including internal power supply efficiency requirements.
→Source: ENERGY STAR

Average Price of Electricity to Ultimate Customers

Residential electricity prices by state used for the default $0.17/kWh rate.
→Source: U.S. Energy Information Administration

NFPA 70: National Electrical Code

Continuous-load 125% sizing basis behind the 80% branch-circuit rule of thumb.
→Source: NFPA

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