You have a parts list, a budget, and one nagging question: what power supply actually fits? Guess too low and the system crashes or refuses to post under load — the single most common cause of "my new PC won't turn on" threads. Guess too high and you pay for watts you never use, often ending up with a louder, less efficient unit. Sizing the PSU is a planning decision, not a vibe.
The PSU Wattage Calculator turns that guesswork into a number. Enter your CPU, GPU, motherboard, memory, storage, and fans, and it totals the real board-power of each part — CPU and GPU dominate — then adds headroom for GPU transient spikes, ageing capacitors, and a future upgrade. You get a recommended wattage instead of a round-number guess, and a clear reason behind it.
Why the headroom matters: PSUs run most efficiently around 50–70% load, so a unit rated well above your peak draw stays cooler, quieter, and leaves room before you hit its limit. For a 450 W+ GPU, a quality 80 PLUS Gold unit on a strong +12V rail is the safe call. The calculator bakes that cushion in so you don't have to estimate it yourself.
Once you know the size, the PC Power Draw Estimator shows the other half of the story: idle versus load wattage and the annual energy cost you'll actually pay. That's useful when a bigger PSU also means a bigger always-on draw. Open both calculators, size the supply, and move on to the fun parts of the build.