Electronic flash evolved from giant units that plugged into wall outlets to heavy shoulder-borne battery compartments to in-camera units powered by AA size batteries.
When I was a cub, I carried a “Little Giant” strobe loaded with “B” batteries. In those days portable radios used a twin voltage system. The “A” batteries were 1 ½ volt and the “B” batteries were high voltage packages. They were heavy, and there was danger when opening to change out the batteries. Anyway, the flash tube of even a modern unit requires high-voltage. The vintage versions didn’t step-up the voltage efficiently. Modern units thrive on solid state electronics.
In the early days, cameras had to be set manually. We used a “Guide Number” system. While still used today, this method has fallen by the wayside because cameras and electronic flash now use “chip logic” to set the exposure. The guide numbers were published. Say your unit had a guide number of 100 for 100 ISO film. We guestimated the subject distance, say 12 feet, divided 12 into 100, and set the aperture at f/8.
Finding a guide number that worked was the key. We used the published values or ran tests ourselves. The American Standards Association (ASA), forerunner to ISO, published the needed formulas.
Watt seconds = Capacitance in microfarads X voltage squared divided by 2,000,000. From this value, Effective Candle Power Seconds can be calculated.
From this answer, the Guide Number was calculated
Guide number = square root of 0.63 X ECPS X ASA
ECPS = Effective Candle power seconds
ASA = Forerunner of ISO
These formulas, ASA PH 2.4-1953 became the standard adopted by all electronic flash manufacturers and film makers who published the data sheets enclosed with the film.