Compasses cost virtually nothing in terms of power, the chips are cheap their signal is easy to read. Usually they're not even read constantly, an accelerometer is read to constantly adjust the compass which avoids even asking the fractionally slower chip. The accelerometer is essentially free to call. The compass is only queried periodically to ensure that the accelerometer hasn't caused any drift in value.
All-in-all... compasses cost nothing.
Having the screen powered... costs almost everything.
Basically to preserve battery, it's best to use the compass all the time.
That'll help, I usually have the maps on display with the screen brightness turn off, as I can be a little paranoid about missing a turn on a route I don't really know.
Well I presume it's still going to use the GPS to know where you are so that it can point the arrow at the next place. Otherwise it's going to be hard to navigate anything that isn't a long straight line.
Compasses cost virtually nothing in terms of power, the chips are cheap their signal is easy to read. Usually they're not even read constantly, an accelerometer is read to constantly adjust the compass which avoids even asking the fractionally slower chip. The accelerometer is essentially free to call. The compass is only queried periodically to ensure that the accelerometer hasn't caused any drift in value.
All-in-all... compasses cost nothing.
Having the screen powered... costs almost everything.