Reality has been that few of these power supplies had sufficient output to run a full compliment of phones at the specified ratings. (The different PoE specs have a variety of maximum outputs.) Fortunately, few have run into problems because few organizations fill up these chassis with phones on every port, and phones don't necessarily pull the maximum power all the time, so this issue, while real, was seldom discussed.
Recently, the situation has improved, as power supplies with obscenely high output are now available so that it may now be possible to get all the power you need (Important safety tip: Do not touch your tongue to a 4200W power supply).
To do a power budget without access to a handy calculator, you would of course use the formula "Watts = Volts * Amps." Aggregate the power consumption for all the devices you intend to plug into the switch, and then compare that to the ratings on your power supplies. Don't forget to include the power required by the switch itself. But in any case, you should check with the manufacturer, because there can be a lot of little gotchas.
Since these new supplies can be expensive, you'll want determine which one you need before you buy. If you're rolling out VoIP on existing boxes, you should calculate your power requirements to determine if you need to upgrade your supply before you do your deployment.