This is something you can look up online, either generic averages or for specific modular components on their respective datasheets. High end or overclocked motherboards can draw a few watts more than others, around 30W, larger or faster spinning fans a few more than others, around 3W or less if throttle controlled.
HDDs are closer to 15W spinning, SSDs closer to 3W in use, but both negligible power at idle. Same is true for an optical drive except roughly half dozen watts more than HDD when active.
Notice I wrote active vs idle. It is rare to have everything in a system consuming max power simultaneously. More often the high load comes from either CPU or video card, or both but then one bottlenecks the other so one isn't at max TDP in real life uses.
You can usually guesstimate an average system should have 75W for those components and adjust from there based on where the system deviates from average, but will get longer life from the PSU if you overshoot required wattage so it is running at a lower % of max capacity, all else equal.
Then there's amps per power (voltage) rail. You need to know that too, but mostly if you have enough 12V current.