The wattage printed on a power supply (PSU) is the most it can deliver at once. It is not what your PC draws all the time. A 750 W unit in a PC that needs 300 W simply delivers 300 W. The number only matters in the worst moment: a demanding game where your graphics card and processor both work hard together.
So the question is not "how big can I go?" but "what is my PC's worst moment, and how much room do I want above it?" The tool below lets you try it.
Take the maximum power of your graphics card, add the processor, then add roughly 80 W for everything else. Graphics card makers list the card's board power on the product page, and processors list a power figure too. Both are honest starting points. The total is your worst case, and almost no game pushes every part to its limit at once, so it is a safe ceiling.
Graphics card makers also publish a recommended PSU size for each card. Use that as a floor: if the box says 650 W, do not buy 550 W because your processor is small.
Three reasons to leave space above the worst case, instead of buying exactly what you need.
Modern graphics cards can briefly pull much more power than their rating, for a few milliseconds. Newer supplies built to the ATX 3.x standard are designed to tolerate these short excursions. A supply running at 95% load has no room for them.
A PSU turns wall power into the power your PC uses and wastes the rest as heat. Efficiency is usually best around the middle of its range, roughly 40% to 60% load. Run it near its limit and the fan spins faster, so the PC is louder.
Graphics cards are the part people swap most often, and each generation can draw more power. A supply with some spare capacity usually survives your next card. A PSU often lasts through several PC builds, which is why it is a sensible place to spend a little extra.
The 80 Plus rating (Bronze, Silver, Gold, Platinum, Titanium) is about efficiency, not power. Bronze supplies are around 85% efficient at typical loads and Gold around 90%. A Gold unit wastes less as heat and usually uses better parts. Over a few years of gaming the savings on your bill are small, so choose Gold for build quality and quieter running, not for savings alone.
The label does not say how many watts you need, and a high rating on a poorly made unit is not a guarantee. Stick to known brands with a long warranty, since a bad PSU can damage other parts.
At a tight budget, get a well-known brand at 80 Plus Bronze with enough watts for your card, and accept that the cables are fixed. In the middle, aim for 80 Plus Gold, fully modular cables (you plug in only the ones you need, so the case stays tidy), and the right connector for your card. At the top end, pick extra watts for the next card, an ATX 3.1 unit with the 12V-2x6 connector many newer graphics cards use, and a long warranty, often ten years.
Across all budgets, check that the supply has the right graphics card power connectors, and that its size fits your case. Read the picks below as examples at different prices, not reviews.