From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Battle power (戦闘 力, Sentō Ryoku; FUNimation "Power level", Viz "Battle strength"; Literally meaning "Combat power") is a concept first introduced in the Dragon Ball manga when Son Goku learns to sense Ki after drinking the Ultra Divine Water. Later in the series, the Z Warriors are able to detect battle powers via the Ki Sense ability.
The highest battle power ever officially read aloud from a Scouter is Ginyu's reading of Goku's battle power, which after powering up is 180,000. The highest battle power ever officially stated is from the Dragon Ball: Daizenshū, which is Super Saiyan Goku with a battle power of 150,000,000 while battling Freeza on Planet Namek.
The use of battle powers was to get an estimation of how a battle would fare between two opponents of known battle powers. In general, if one had a higher battle power than one's opponent, then they could defeat the one with the weaker battle power. For example, "Person A" has a battle power of 10,000 and "Person B" has a battle power of 5,000. "Person A" will most certainly defeat "Person B". The factors of speed, strength, and technique, however, also come into factor when deciding the outcome of a fight.
It should be noted that when factoring in the edge a fighter has over an opponent, the difference in battle powers as a flat number is apparently not important, but rather the relative or percentage difference between the battle powers of the two combatants.
In general, the battle power of a normal person in the Dragon Ball universe seemed to remain pretty constant throughout their life. Battle powers could be increased, usually very slowly, through intense training or other special means. Some people were able to increase their battle power faster than others; the most clear example of this was Goku and Son Gohan.
The battle power of most people was usually the same independent of the fact that they were resting or powered up. However, there were several important exceptions. The most common way to increase one's battle power temporarily during a fight was by using Transformation, usually altering their external appearance. Examples include Saiyans (who transform into Great Apes and Super Saiyans), Zarbon, and Freeza. There were also some people that learned the skill to change their apparent battle power at will using a Power Up technique. This allowed them to "supress" their battle power when they wished to appear weaker, be undetected, or conserve energy, and then increase their battle power to the maximum during a fight. Namekians and highly-trained Earthlings (such as the Z Warriors) seem to be examples of races that were able to do this with the most ease, though Saiyans were also shown to be able to learn this ability. Other characters known to possess this ability were Ginyu and Freeza, whose races are unknown.
Saiyans possessed a unique ability that allowed them to easily increase their permanent battle power. If they fought a strong opponent and were left near death, but were able to recover, they would gain a great amount of battle power and increase their maximum battle power. The first character to display this in the series was Vegeta.
Characters that had a mixed Saiyan and Human heritage seemed to display special abilities regarding battle powers. They were able to reach high battle powers much faster than other races. Their battle power at a given time also fluctuated a lot depending on their emotional state. Intense rage, for example, could make them increase their battle power significantly for a limited amount of time.
 Influence and similarities on other series
Akira Toriyama and his work in the Dragon Ball series has influenced several other manga work. Similar works use an identical kind of "inner power", and that "inner power" made the characters grow stronger.
- Ring ni Kakero (Shonen Jump,77) authored by Masami Kurumada. Many concepts from this series are similar to Dragon Ball, such as tournament fighting, exciting fighting styles, and chi power.
- Masashi Kishimoto states in his comic Naruto that he was influenced by Toriyama. In Naruto, the concept similar to Ki is called chakra level; the trademark technique being the Rasengan(Spiraling Sphere), which requires a high level of chakra control.
- In the manga Saint Seiya, authored by Masami Kurumada, the protagonist Seiya is the titular character of the namesake show. He is one of the 88 saints of Athena, and serves loyally at her side. Seiya dons the bronze cloth (an armor that denotes sainthood) of Pegasus and draws power ("cosmos") from the constellation that connects to it. The saints can grow stronger if they raise higher levels of "cosmos".
- Bleach is a manga and anime series by Tite Kubo. All Bleach characters are "souls". Living humans contain souls within their bodies. Disembodied souls, or spirits, have a form composed of particles of spiritual energy called "reiatsu" (霊子, ectoplasm), Certain unique humans naturally have both the power to sense and the strength to fight with spirits. Ordinary humans can gain the ability to interact with spirits by spending time around a large source of spirit energy. Ichigo Kurosaki is the exact example, and considered to be one of the strongest characters.
- YuYu Hakusho is a manga and anime series by Yoshihiro Togashi. Yu Yu Hakusho characters are either human or yokai (demon), and have spiritual powers called reiki (for humans) and yoki (for demons). There is also a system of yoki class or equivalent-to-yoki class that ranges from E-S+, E being the weakest and S class being the strongest level of yokai. Yokai that are soldiers of one of the three of demon kings, Yomi have devices similar to scouters that let them read an enemy's power level.
- Shaman King is a manga and anime series by Hiroyuki Takei. The Shaman King characters are shamans or mediums that uses a spiritual energy called Furyoki to materialize their spirit guardians. This process is known as Over Soul, and the inner power is measured by an arm device whose function is similar to a Scouter.
- One Piece is a manga and anime series by Eiichiro Oda. Douriki (approximately meaning "power level") is a measurement of the power of one's Rokushiki abilities among the CP9 (Cipher Pol Number 9). It is measured in a similar way to Dragon Ball's power level but in the form of hitting a member of CP9 (Fukurou) who will then after being hit follow on to tell one his Douriki. However, this measurement of power was short-lived and was only ever shown to measure the power of the CP9 members and a single marine.
- Ghost Sweeper Mikami is a manga and anime series by Takashi Shiina. A type of spiritual energy labelled as reiki is common among all the characters in the series. It is measured in a fashion similar to the scouters in Dragon Ball, but in more relation to this manga it is gauged with a ring-shaped measuring device which enlarges from its original ring size, surrounds the target and temporarily drains them of their spiritual energy. The end result is a rating defined as "might" which can alternatively be seen as a power level.