I noticed that you didn't mention Event Zero's 'Greentrac' as a PC power management solution, perhaps you've never heard of it.
Greentrac goes above and beyond 1E's nightwatchmen and all the other PC power saving software available. While those other programs basically just shut down the computers at night, Greentrac's focus is on saving power during the day (though it also has shutdown capabilities).
Greentrac installs across an enterprise's network (no size limits) and tracks each individual user's power wastage (something you mentioned you were having trouble doing). It provides a user interface that users log in to and can see how much power they waste during the day by leaving their computer idle. The users can see how they're doing in relation to other employees, and are given a score on how much energy they waste.
The idea behind Greentrac is that users have the power to save far more energy than automated software – they know when they’re going out to lunch, they know when they’ll be away for a meeting etc. Greentrac simply gives users the motivation to want to save power. The user interface provides tips on saving power, and encourages users to either turn their computer off or put it to sleep whenever they’re not using it.
Even if you're already using automatic PC shutdown software like Nightwatchman etc, reducing power wastage during the day by 80% on 10,000 PCs (which Greentrac makes possible) can save you a further $200 000 on your electricity bill and cut CO2 emissions by 1.4 million kilograms annually.
The Greentrac software has a huge number of other features as well, it really is the best power saving software available to date, there's plenty of more information on the Greentrac website (http://www.greentrac.com).