Power supplies are rated for their maximum output. So if you've calculated all the components and what they use at their maximum, then as long as your power supply can feed that needed power then it'll work. It's important to make sure you don't go over the rating of the PS though, as it'll most likely result in a voltage drop and it certainly isn't good for many of the components.
But to answer your other question, no if the PS only needs to put out 50W and it's a 1000W power supply (to use extremes), it won't suck 1000W out of your outlet. It'll suck near 50W. More efficient power supplies are more efficient at delivering power and use less themselves. Often bigger ones may "eat a little more themselves" for the fan and circuitry, but it shouldn't be huge.
Adding 20% is a safe option since you definitely don't want to hit the limit. But also keep in mind that you may add components to the system in the future, which is admittedly hard to predict.
Generally, one of the most expensive times is actually at boot time. That's when all the components fire up and are eating power near their maximum. Typically after that the power drawn is actually quite a bit less.