Hard to say exactly, anything would be a rough estimate. The best way(s) to know for sure are either a $20 watt meter from Amazon, Wal-Mart, home improvement stores, etc. The meter comes in handy for finding the exact power usage of any appliance, not just the PC.
Alternatively and a bit more money, but many mid-range and up UPS backups will also show exact power usage. I'm happy with my CyberPower one and it also helps with avoid problems with brief power drops that are common at least in my area. Nothing worse than being in the middle of something and a split-second power drop would just be enough to cause a PC reset. Now I never have to worry about those, and I get a graceful shutdown on longer power outages.