The original story here: http://www.instructables.com/id/Super-Size-Your-Battery-Run-Time-D-Cell-Battery-Pa/
Based on information from Panasonic I have discovered the following in regard to battery run time:
Using the same load for each size alkaline battery….
1 D cell = 3 C cells
1 D cell = 7 AA cells
1 D cell = 18 AAA cells
6 D cells = 200 9 volt batteries.
Think of the cost savings replacing 72 AAA batteries with 4 D cells.
To greatly increase my run time I have made some D cell battery backs as well as modifying an existing product to do double duty as a battery pack.