Average modem power consumption in watts?

Honestly, it will depend...  With you being off-grid you shouldn't depend on the "average" power usage, but the possible highest usage as well so that you don't overload your personal grid, unless you can predict when the signal will drop out and the modem ramps up transmission power and processing to try and keep a signal lock.

The label on the power brick states (at least on mine) Input at 100-130V @ 2.0A (which at maximum theoretical value is 260w), and the output is listed as 46w.

The only time these modems will use a TON of power is during heavy cloud cover, rain, and when it's acquiring a signal after being powered on, so you may want to plan for possible high wattage surges during those scenarios.

Power usage also depends upon how well your satellite is aimed, as well as general location in your beam assignment.