"A tool that measures the pure power that's needed to resurrect Majin Buu."
The Energy Meters are able to measure power levels far too high to be measured even by Frieza's best scouters (which could read up to at least 180,000). However, instead of using the measurements in scouters and extending them, they use much larger measurements called kili. The device has a needle and some lines on it, each line representing 1000 kili of energy. The needle moves over to one of the lines to show how many thousands of kili that person has. The older model wielded by Spopovich had the units in a clockwise manner, in which when the power was beyond measurable, the needle kept spinning around repeatedly. The newer model used by Babidi has the units more like a gauge, and even has a dial on the bottom right corner of the device.
The meter was used twice, once by Spopovich and Yamu to measure Gohan's energy at the World Martial Arts Tournament and once by Babidi himself to measure Goku's energy. The energy meter used by Spopovich and Yamu was an older model which could only read up to 3,000 kili of energy, while the one used by Babidi was a newer model which could read much higher levels, apparently up to 9,000 kili as that model had nine lines on it.
Due to its limited use, it is unknown if Power Meters can measure Godly ki or the energy used by Androids (which are unreadable using Scouter-based technology).
Video Game Appearances
In Dragon Ball Xenoverse 2, the Energy Meter appears as a wearable accessory that can Future Warrior that can be worn on their back. However it is depicted as golden colored with a white and yellow portions on the gauge.