MicroSD flash block size
By aligning your block writes with the flash card blocks you might increase the write speed since you'll only be writing one block rather than two or more, and thus decrease the amount of time the card has to be powered.  Further, one flash block write will consume less power than the two that would be required if you wrote across a boundary.

However your greatest power savings will be in communicating with the card as fast as possible so you can power it for as short a period of time as possible.

Even though a flash block write consumes quite a bit of power, communicating slowly with the card and leaving it powered during communications will eat quite a bit more if you don't speed up communications.