N900 charges from an USB so 2.5 watts (5V, 500 mA). Unless it's way over the limit of good measure, it's under that. Leaving it to bake or running designed benchmarks can put it over that so it relies on battery to provide the rest, but I very much doubt you can push it over that during normal use.

The wall wart goes up to 1000 mA, so that's upwards of 5W, however, I don't think you need that unless you plan on cooking the device.

Non-negotiated USBs deliver 100 mA instead of 500, and, from experience, the phone charges if left locked or simply displaying an image. Navigating the net or playing is more than that so it actually goes down. I'm guessing you need some 200 mA for net, and maybe 400-500 for games?

To suit your needs, charge the battery (1200 mAh) and do what you do normally, see how long it takes to die and plan accordingly. If you kill it full-to-off in 1 hour, that's 4V 1.2 Ah, 4.8 Wh in an hour, that's 950 mA roughly.

Don't think you can kill it in an hour, though.