How much PSU wattage do you actually need?

Power supply sizing attracts more bad advice than any other part of a build. Calculators pad their numbers, retailers benefit from selling you more, and forum guidance often dates from an era when efficiency was worse and spikes were smaller.

Start from the two components that matter

For almost every desktop, the graphics card and processor account for the overwhelming majority of power draw. Everything else (drives, fans, memory, the board itself) typically adds somewhere in the region of 50 to 100 watts combined.

So a workable estimate is:

GPU board power + CPU TDP + 100W for everything else.

That gives you the realistic sustained load. What you buy should sit comfortably above it, for reasons below.

Why headroom is not just padding

Transient spikes are the real constraint. Modern graphics cards draw power in very short bursts far above their rated figure. A card rated at 300W can momentarily pull well beyond that for periods measured in microseconds. Those spikes are too brief to matter thermally but long enough to trip a power supply's over-current protection, which presents as the machine rebooting under load, with no error, on a PSU whose rated wattage looks more than adequate on paper.

Efficiency peaks around half load. Most units are at their most efficient somewhere near 50% utilisation. A PSU running at 90% of capacity generates more heat, spins its fan faster, and ages sooner.

A reasonable target is for your estimated sustained load to be roughly 50–65% of the PSU's rating.

Efficiency ratings, briefly

The 80 Plus tiers (Bronze, Gold, Platinum, Titanium) describe how much wall power reaches your components rather than becoming heat. The practical differences:

• Bronze is fine for modest builds and is the sensible floor.
• Gold is the value sweet spot for most people.
• Platinum and Titanium cost meaningfully more for a few percentage points. They make sense for machines under sustained heavy load, less so for gaming.

Efficiency is not a proxy for build quality, though it correlates loosely. A Gold unit from a reputable manufacturer is a better buy than a Platinum one from a brand you have not heard of.

Connectors matter as much as watts

Current high-end graphics cards use the 12VHPWR (or the revised 12V-2x6) connector. A PSU without one requires an adapter, and adapters have been the source of well-documented failures when not fully seated.

If you are buying a card that uses it, buy a PSU with the connector native rather than adapted. Check the ATX Version spec too: ATX 3.0 and 3.1 units are designed with transient spikes explicitly in mind.

Modularity

• Non-modular: all cables permanently attached. Cheapest, most cable clutter.
• Semi-modular: essential cables fixed, the rest detachable. Good compromise.
• Fully modular: everything detachable. Easiest to build with, easiest to keep tidy.

This is convenience, not performance. It does not affect stability.

Worked example

A build with a 300W graphics card and a 120W processor:

• Sustained estimate: 300 + 120 + 100 = 520W
• Target 50–65% load: 800W to 1000W
• Buy: an 850W unit, ATX 3.x, with a native 12VHPWR connector

Not the 1200W a calculator will often suggest, and not the 650W that looks adequate on the sustained figure alone.

Every PSU on VantaSpec lists Wattage, Efficiency, ATX Version, 12VHPWR and Modular as specs, so you can filter on the things that actually decide the purchase.