How Much Power Can You Harvest?
The human body contains an average of 137,000 kcal of energy which be converted to 57,320,800 joules. Multiplied by the ~7.6 billion people on Earth, that is ~1.041e15 kcal which if all released at once would result in a one gigaton explosion. (20 times bigger the the biggest nuke ever tested.)
What About Non-human energy?
Humans are only about 0.001% of the world's biomass (give or take an order of magnitude since estimated figures on this vary wildly). This means if your weapon were to consume all biomass on an Earth like planet it would have a potential energy of about 10 Teratons.
What if it can also consume fossil fuels?
Fossil fuels are also an organic(ish) source of energy; so, maybe your weapon can consume this as well. Current estimates of the world's remaining fossil fuels are as follows:
- 1.730e12 barrels of crude oil @ 6.118e9 joules per barrel totaling about 1.058e22 joules.
- 1.1e12 tonnes of coal @ 2.931e10 joules per tonne totaling about 3.224e22 joules.
- 6.923e15 cu.ft. of Natural gas @ 1.055e6 joules per cu.ft. totaling about 7.304e21 joules.
This totals out to about 5.012e22 joules or about 12 additional Teratons of potential energy yielding a total energy of 22 Teratons or about 1/4 of the energy releasesed by the Chicxulub impactor.
Storing that much power
The most efficient way to store power in any sort of science based way is antimatter. One gram of antimatter reacting with one gram of ordinary matter results in 43 kilotons-equivalent of energy; so, storing the entire bio-chemical energy of an Earth like planet would require just over 1/2 a million kilograms of antimatter.
While that is a lot of mass, since your monster has superhuman strength you can work around this by compressing your antimatter into a super dense ball. Matter and antimatter are both mostly just empty space. If you were to collapse this fuel down to the density of a neutron star, it could be contained in a space just over one cubic millimeter.
Consuming that much power
A single person only contains about as much power as a 1/2 gallon of gasoline. Weighing in as much as a WWII destroyer, your monster would have to spend more calories walking from one victim to the next than it would get from each meal. To offset this, I would couple your monster with some sort of grey goo feeding system where it sends out a swarm of self replicating servitors that scower the planet of all life bringing the spoils back to the super dense monster that stores all the antimatter to power and control things.