The power supply is one of the most important and underrated components for users with little experience. As opposed to what the manufacturers want us to believe, nowadays, computers consume a lot less energy than in the past. Therefore, it's not needed to pick a power supply with exorbitant Watts.
Power supply calculators are often unreliable, suggesting power supplies with more watts than you would actually need. The fundamental factor is build quality: a good 500 watt power supply can be better than an economic 700 watt one. Unfortunately, the quality is very variable and also power supplies of the same brand can still differ a lot. The only advice is to read about it on specialized websites.
Picking the right wattage requires evaluating CPU and graphic card consumption first. Every computer with an APU o low-end graphic card will consume very little, so power supplies like the Corsair CX are a best-buy. A good cheap alternative is the EVGA. For a mid/high-end computer, a good quality 550W power supply is more than enough, 650W if you choose video cards that consume a lot, and you don't want to take risks. Models like the SeaSonic Gold and Corsair RM offer great quality and Gold efficiency certification. To save some money, Bronze certified power supplies are a good alternative.
Above 650W is a sensible choice only for those who have a top GPU configuration. An excellent model with 80 PLUS Platinum certification is the Corsair HX750i. For the highest build quality, there is the Corsair AX.
Remember: NEVER buy cheap power supplies, as they can cause irreversible damage to other components.