A mid-range mobo by itself can use 40w easily. Some of the high end mobo's use 50-60w. A hdd runs @10w, each fan @3-4w, each stick of ram 2-3w. By themselves pretty insignificant wattages, but added up that's @ 65-70w. But thats max. You are also talking about an i3 in a gaming pc. It's not going to have an easy time, users generally want the best settings possible, so that i3, especially on games like gta5, is going to see hard usage.
My general rule of thumb is add up all that's possible, gaming will be @70-75% of that and the psu should be double, edited for quality. So with an absolute maximum possible of 225w, doubled, the psu is a 450w, which is perfect for a 1050ti system.
Each system is different, some more efficient, some less so, some only have 2x fans or 1x stick of ram or 2x SSDs etc so I figure a mid range system at 70w, high end at 100w, +cpu, +gpu. It's not ever going to be accurate, that's impossible, but a range is acceptable.