That is just a general minimum rating needed for a maximum setup (all hard rives and peripherals installed) but it excludes the power ration for graphics cards, s o...
One thing to consider is not just the motherboard, but if you want to get a good graphics card-- that one alone needs 400Watt + 400Watt Motherboard .. So many gaming PC's use 850Watt power supplies. (you will notice when you buy a high end GFX card they suggest using 600/800/1000Watt and this is the max for everything used)
It does not meaning uses the 850Watts constantly, it is a switched power supply and is controlled via the motherboard and/or software of your operating system. That is why it is important to enable power saving features and install the correct drivers for your OS.
So when you are not doing anything it can use as little as 50Watts.
But when gaming it jumps up to 600-800watts.
One thing to keep in mind- do not buy the cheapest pwoer supplies- because they tend to blow out when you start using the near maximum rating. I have seen this happen dozens of times, and people come back later and buy a proper PSU(you can tell by comparing the weight of the same rated PSU) The better PSU's sue high quality capicitors and coil transformers and a bit more clever technology to decrease the chance of a blow out, they also usaully come with 3 to 5 years warranty, where as chinese el-cheepo only 1 year.
Usually when i am in doubt i use one of these wall power meters and monitor the usage.