One of - if not the most - important system on a ship, is undoubtedly power. For without power, you cannot move, fire weapons, use labs, keep your crew alive through life support, and do many other functions on a ship.
Power comes in three forms - Warp, APR and Battery.
Warp represents the main drive of the ship, and power coming from the warp engines can be used for any function on the ship requiring power. In addition, movement power can only be supplied from the warp drive, making it the most important and most numerous power system available.
Auxiliary Power Reserves represents the many generators and auxiliary power reactors on a ship. Power from these sources may be used for any function, with the exception of movement. Thus having an excess amount of APR will allow a ship to devote its warp drive for movement, and its APR for weapons and other functions.
Batteries are sources of additional reserve power. If charged, they can be used as additional APR, but once used, they must be recharged before being used again - taking valuable power from other sources. They do not have as much of a use in this BSG conversion than they do in standard SFB, but they are still useful to have around. When your engines have been hit, it is always useful to have an additional source of power available.
Power ships have an additional requirement - BEAMED APR or B-APR. This is the power that can be beamed to other ships. This is in addition to normal APR.
Power may be allocated on a ship as the designer sees fit, provided other SSD box requirements are fulfilled (such as control and hull).