There are 2 version of the Speedlights Power Index, depending on the primary usage scenario of a flash. Both show the same guide numbers but use different subsets of flashes.
Digital TTL Power Index
The first index lists speedlites to use in the camera hots-shoe, such as the Canon 580EX II, a Nikon SB-700, or the Nissin Di866.
The number next to the blue bar stands for the guide number obtained in the tests: the Nissin Di866, for example, has a tested guide number of 41 (meters) at the 35mm reflector position and ISO 100, while the official specifications list a GN of 40 (light blue bar).
In advertising, manufacturers often use the maximum guide number at the tele zoom reflector position: to see these values, go to the respective specs & test results page, e.g. here for the Nissin Di866 test results.
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Manual-Flash Power Index
The second version of the index lists flashes that are usable for wireless flash with radio triggers, wireless TTL or optical slave mode. That’s why you find here pure-manual speedlites such as the Vivitar 285HV or Yongnuo YN-560 next to a Nikon SB-800 or speedlites from Canon.
On the other side, a Nikon SB-400 is excluded since it can’t be triggered through the hot shoe with normal radio receivers such as Cactus V4 or Yonguo RF-602. Other incompatible flashes include the Nissin Di622 (Mark I) for example or the Sigma EF-610 DG ST.
The latest evolution of the Speedlights.net Power Index shows the guide number from our tests as “RealGN” with metric values for the 35mm reflector position, and compares with the official manufacturer specs in the light blue bars. The Index is sorted by the test guide numbers in descending order so that the most powerful flash is found at the top of the list.
To get the feet-values for the flash guide number, simply multiply the metric guide number with 3.28 (example: GN 30 (meters) equals 30 * 3.28 = 98.4 (feet).
For comparison, the built-in pop-up flashes on DSLR camera bodies typically have a guide number of 12 – 14 at ISO 100.