You don't need to convert 24 volts to 12 volts, you just use an equalizer
http://www.vanner.com/manuals/EQUALIZER-70M-SERIES.pdf- it splits the battery banks in to 12 volt banks, and draws from the equally-
The Vanner VANN-Guard Power Management System is an efficient and highly reliable method of
obtaining a 12 volt DC power source from a 24 volt DC electrical system. The VANN-Guard makes the
batteries look like they are in series and parallel at the same time. In addition to providing regulated 12
volt power, the system ensures that battery voltages remain equal which significantly extends battery life.
Ideally suited for vehicle and alternate energy applications, the VANN-Guard is designed to save your
batteries and the money you would spend replacing them. Users of the Vanner VANN-Guard know that it
is the most cost effective and dependable solution for dual voltage systems.
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Should be much more efficient than converting 24 to 12.