Tuvar

If you use a UPS with some basic metrics you can also pull metrics. You'll likely need to only have the server on it, unless it can log the wattage per socket. That's getting into enterprise level UPS's though. Kill-a-Watt is the most basic option as someone commented though. Since most people have their equipment on a UPS it can give you logs over time. Depends on how much you want to invest, but some sort of UPS is highly recommended.