Overview
There are two kinds of power packs: symmetric power packs and assymetric power packs. Symmetric power packs split power equally between multiple flash heads. Assymetric power packs allow the user to vary the power supply to different flash heads.
Monolights, on the other hand, are compact substitutes for a power pack/flash head combinations. Their size limits their power, but they are often desired for their portability.
Having all the controls on the power pack makes it easier to change the settings of multiple flash heads in one place quickly. Monolights come with the controls built directly into the light itself. So if you have multiple monolights, you'll need to control and adjust the settings of each one separately. Head/power pack kits also offer faster flash durations than monolights, which can really come in handy when you want to freeze action during fast motion. They also have faster recycle times.
If you're shooting outdoors or with complicated modifier setups, you are going to need a lot of power to overcome the sun or the light loss caused by the modifiers. Head/pack systems have a big advantage in the power department. Whereas, if you're shooting indoors or inside your studio, monolights will probably be more than enough for regular work.
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