You'd need a "wattage meter" to know specifically. However in the first post of this thread, there is a link to a video card power use article which shows the average power use at idle is 108-to-140ish Watts. So unless you're running a high-performance video card, any 400-600W PSU will be enough for a trading rig.
No one has mentioned the efficiency ratings. Generally this is where the higher prices are. Be sure to get at least 80 PLUS. I recommend spending the money and get an 80 PLUS Silver or 80 PLUS Gold. http://en.wikipedia.org/wiki/80_PLUS cT
There are in-line wattage-meters you can get. Looks kinda like a power-strip/surge-protector. Another way is a clamp-meter with a ring you clamp around the power-cable: http://uk.rs-online.com/web/search/...ponents.com/largeimages/R6973963-01.jpg[/img] http://uk.rs-online.com/web/search/..._sp=ProductPage-_-Calibrated-_-6990952#header The thing is, you should size your PS for the maximum load of your system, not the average. So you want all your fans spinning a full-speed, all the drives spinning and the CPU at full-load. Good way to load it up is to run a benchmark program like Prime95 and play some full-screen videos. One of my current systems uses around 120-130w at average workload, but when it's crunching charts and running simulations, the power-consumption goes up to 350w. Then you want sufficient overhead so that the PS isn't overloaded; a 100% safety buffer maybe. So I use a 700w PS. Also what kills a PS most of the time isn't power-overload, but overheating. The fan typically fails first, then the PS overheats and cooks some components on the circuit boards (typically a capacitor). So I use server-type systems with on-board diagnostics that can tell me when a PS-fan goes out, or a hard-drive fails or some memory starts going bad. These systems also have redundant PS and drives, so I can swap out parts without turning off the systems.
Your use is atypical. Most traders do not "crunch" charts, whatever that means.... nor do they "run simulations". Running several/many tic charts and/or "custom" indicators will use a lot more CPU, however. Most trading rigs run near idle.
I have one of these. Kill-A-Watt is another brand. http://www.amazon.com/Energy-Meter-...VK52/ref=sr_1_1?ie=UTF8&qid=1299420401&sr=8-1