The current browser has the accelerometer/portrait mode disabled by default. However, most people have enabled it by pressing ctrl-shift-o. I have yet to see anyone report additional power usage based on having it enabled (assuming ctrl-shift-o actually does turn on the accelerometer and it is off by default).

An easy way to test power consumption will be to turn on the setting where the phone app opens when rotated.

I don't think it would be that great of a power drain. I would much prefer to have auto rotation in most apps.