Blame TV Tropes, they sent me here:

Humans can be resized by SCP-1056 as low as 0.50 and as high as 1.75 with minimal change in function.
In a minority (roughly 8%) of cases, an object altered by the device experiences a material instability and begins to undergo atomic decay within 36 hours of alteration.
This decay produces significant heat and energy – approximately 150 gigajoules per kilogram - around six orders of magnitude less than typical matter-antimatter decay but sufficiently high to render frequent or high-mass transformations inadvisable.

If it takes the whole 36 hours, that's 129,600 seconds. So 150,000,000,000 / 129,600 = 1,157,407.407~ joules per second using only 1 kg. However, going by this, we're dealing with an American high schooler.

The device was recovered by SCP personnel following reports of unusual behavior among students at ████ High School in ████, PA. School officials began an investigation when teachers reported that a number of students were behaving unusually. Specifically, the students displayed significantly impaired language skills, abnormally poor attention span, long-term memory, and impulse control. Medical examination of the students revealed the presence of numerous vascular and nervous system abnormalities. Foundation personnel recovered the device at the home of one of the students, who had presumably been using it for recreational purposes.

Ages 14 to 18. 16 years old average weights 53. 14 years old average weights 47.6 (Girls) + 50.8 (Boys), and for age 16, 53.5 kg (F) + 60.8 (M). Taking the average of 14 y/o weights across genders, we get 49.2 kg, & for 16 y/o, it's 57.15. If we're taking the average of those ages, it should be for a 16 year old.

Anyway, 150 gigajoules per kg, so 150,000,000,000 * 57.15 = 8,572,500,000,000 joules, and now dividing that by 129,600 seconds, 66,145,833.33333~ joules per second. So assuming a 16 y/o of unknown gender (& thus averaging the weight across gender.), & taking the maximum amount of time, you get that.

Now, it could also be higher because people who use it change size. For humans, it's done up to 1.75x bigger.

When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier.

The average human supposedly has a body density of 985 kilograms per cubic meter.

Since our hypothetical 16 y/o is assumed to be 57.15 kg before size changing, & the density is supposedly 985 kg/m³

Volume 0.05802 cubic meters; square of our multiplier (1.75) is 3.0625, because squaring a number is making it n^2. Likewise, Cube of 1.75 is 1.75^3, so 5.359375.

I THINK mass is multiplied by the cube: so our sized up teen weighs 306.28828125 kg. 306.28828125 * 150,000,000,000 = 45,943,242,187,500. Which, divided by 129,600 = 354,500,325.52083333~ joules

So yeah, 354,500,325.52083333~ joules per second (at slowest, assuming the human gets the highest multiplier for a human, then undergoes atomic decay taking 36 hours) which is slightly over a third of maximum Small Building level.

Credits to Imaginym for doing the calc.

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