I tried solar charging with a 12V 1.5W (that has degraded to less than 1W) panel with DC-DC converter to 5V/USB. This setup is really weak, it won't charge if the N900 is being used at all, including if the backlight is on.

However my N900's display stays off until unlocked - it stays off when I plug in/disconnect a charger... In Settings/Display there's a "Display stays lit when charging" option, is that on?