Sometimes it is much cheaper to build a new computer buying the parts separately, but the problem arises when we do not know which Power Supply to choose, and although being guided by Watts is the most common, it is not really that easy to estimate.
A simple way is to search on the Internet for the maximum consumption of each part, and thus give us an idea of the minimum Watts that we would need from a source. But since it is something tedious or many times due to lack of time, it is something that not everyone could do; and we would end up buying a power supply that does not satisfy the consumption of our computer. The consequences of this are slowness, frequent errors and freeze screen for a few moments.
Although the maximum consumption is not the only thing to consider, there are also the characteristics such as how much amperage the source supports in each voltage output, but this is another issue.
To avoid all this and do things right, ASUS, a company that manufactures computers parts like motherboards, etc; made an online tool that allows us to know what is the minimum that we have to use of Watts in a Power supply, we only have to select the parts that our computer has to determine it. Previously I put an old tool but the link is already updated.