You'll never find the exact numbers on a sales website, because it depends entirely on how you configure your server.
What you can find however is the wattage of the installed power supplies. For normal rack servers, the power supplies are 1+1, which means that the power draw will never exceed the total of one power supply (so, 2x 850 watts is still just 850 watts). Blade servers are a different story (as they have 6 power supplies).
For the ubiquitous Dell R710, it has the option of 570w or 870w power supplies (in my region at 240v). 0.5kw is only 500w, so you could really only get one server at peak load before going over your power limit.
We have some R710's though and I can tell you that at 240v with 48Gb of RAM, 2x 6-core Xeons and 4x 15,000 RPM SAS drives they draw no more than about 300w under peak load. That's measured by the UPS and confirmed by the iDrac logs. So you might be able to get two of them under normal load into 500w (this is based on the assumption that your rack provider will simply bill you exorbitant amounts based on excess usage. If you're going to trip a breaker by exceeding your allocated power usage, don't take any risks).