A direct answer to the most common power supply question, and the reasoning behind it so you can adjust for your own build.
The short version
Integrated graphics or an entry card: 450 to 550 W.
Mid-range card, one 8-pin connector: 550 to 650 W.
Upper mid-range, two 8-pin connectors: 700 to 750 W.
High-end card: 850 W.
Flagship card with a high-core-count processor: 1000 W and above.
Use our calculator for a figure based on your actual components rather than these bands.
Where the numbers come from
Take the processor sustained figure, add the graphics card total board power, add roughly 60 to 80 W for everything else, then multiply by about 1.35 and round up.
A 120 W processor with a 250 W card comes to roughly 430 W sustained, which becomes about 580 W with headroom, so a 650 W supply is a comfortable choice.
Is a bigger supply wasteful?
Not in the way people assume. A power supply draws what the system needs, not its rated capacity. A 1000 W unit in a system using 300 W draws about 300 W plus conversion losses.
There is a mild efficiency cost to running far below the optimal load point, and there is a real cost in money for capacity you will not use. But an oversized supply does not waste power in proportion to its rating.
The genuine argument for buying somewhat above your current requirement is future upgrades, since graphics cards have trended upwards in power draw.
Signs a supply is undersized
An undersized supply rarely announces itself clearly. The usual symptom is the machine restarting or switching off abruptly under load, typically while gaming, with no error and no warning.
That happens because a protection circuit saw a momentary overload and shut down rather than delivering unstable power. It is doing its job. A machine that is stable in every other situation but restarts during demanding games is showing the classic pattern.
Less obvious symptoms include coil whine that rises sharply with load, the fan running at full speed constantly, or the machine failing to start after a graphics card upgrade that otherwise looks compatible.
If you suspect this, the test is to remove load: lower the graphics settings substantially, or temporarily remove an overclock. If stability returns, capacity is the likely cause.
Do not count peripherals as free
Devices drawing power from the motherboard add to the total. USB devices, RGB controllers, fan hubs, capture cards and additional drives each take a share.
Individually these are small, typically a few watts each. A machine with a dozen USB devices, a fan hub and several drives can add 50 W or more that no component calculator will show, because none of it appears on a specification sheet.
This is part of why the headroom multiplier exists rather than being an arbitrary safety margin.
Where a graphics card manufacturer publishes a recommended capacity, treat it as a floor rather than a suggestion. They have measured the transient behaviour of that design on instrumented equipment. A calculation that adds sustained figures cannot see that behaviour.