PSU Calculator
Free PSU calculator. Get the right power supply wattage for your CPU, GPU and drives, with headroom, 80+ rating and the connectors you need.
How to size a power supply
A power supply should be chosen for the peak your system can draw, not its average. We start from the rated TDP of your CPU and graphics card, add the fixed overheads every build carries — roughly 50 W for the motherboard, 5 W per memory module, 5 W per SSD, 10 W per hard drive, 3 W per fan and 15 W for an AIO pump — then apply a transient allowance and your chosen safety margin.
Why headroom matters more than the number on the box
Modern graphics cards draw power in extremely short bursts. An RTX 40-series or 50-series card rated at 320 W can spike to well over 500 W for a fraction of a millisecond. Those transients are far too brief to show up on a wall meter, but they are long enough to trip a power supply's over-current protection, which shows up as a sudden reboot under load with no error message. Headroom is what absorbs them.
Efficiency ratings
The 80 PLUS badge describes how much wall power reaches your components. Bronze is roughly 85% efficient at half load, Gold about 90%, Platinum about 92%. Higher efficiency means less waste heat and quieter fans, though the electricity saving alone rarely repays the price difference — you can check that for your own tariff with the Electricity Cost calculator.
Running at about half load is the sweet spot
Power supplies are most efficient near 50% load, and most units keep their fan stopped there. Sizing so your typical draw lands around half the rated wattage gives you a quiet, efficient, long-lived unit with room for a future GPU upgrade.
Connectors matter as much as watts
A 1000 W unit is useless if it lacks the connector your card needs. High-end cards use the 12V-2x6 (formerly 12VHPWR) connector; check that your PSU provides one natively rather than via an adapter, and that any adapter is fully seated. The PSU Compatibility Checker covers this in detail.
Frequently asked questions
Is a 750W PSU enough for an RTX 4070 build?
Comfortably. An RTX 4070 (200W) with a mid-range CPU lands around 400W peak, so a quality 650W unit is sufficient and 750W leaves room for a future upgrade.
Does a bigger PSU use more electricity?
No. A power supply draws only what the components ask for. A 1000W unit running a 300W system draws about 300W plus conversion losses, the same as a 600W unit would. The only cost of oversizing is the purchase price and slightly lower efficiency if you run far below half load.
What does the wattage on the graphics card box mean?
Manufacturers print a conservative "recommended system PSU" figure that assumes a power-hungry CPU and a mediocre power supply. This calculator works from the card's actual board power instead, which is why its number is often lower.
Should I buy 80+ Gold or is Bronze fine?
Below about 650W a good Bronze unit is fine. Above that, Gold is worth it for the reduced heat and quieter operation. Build quality matters far more than the badge - a well-made Bronze unit beats a poorly made Gold one every time.
Why does my PC shut down under load with a PSU that should be big enough?
Almost always GPU transient spikes tripping over-current protection, or an ageing unit whose capacitors have degraded. Adding headroom fixes the first; the second needs a replacement. Power supplies typically lose capacity after five to seven years.