Do not take this (too) seriously, after all, Factorio is a game and not a physics simulator, so steam doesn't cool down and inserters know exactly what to grab and where on the belt it is and so on.
\begin{Tongue-in-cheek}
20 kW for 5 minutes is 20,000 W * 5 min * 60 s/min = 6,000,000 W min s/min == 6,000,000 W s = 6MJ
6MJ is the same energy that is in 3 wood or 1.5 coal or half a solid fuel.
It is more energy than one completely charged accumulator (albeit the accumulator can dispense it 15 times faster).
A small scale study[1] drawing athletes from a swimming club[2], did measure the performance of endurance and sprint swimmers from <0.1s time frames to long term (hours) endurance.
For the peak power from a squat jump, <0.1s : Mean[3]: 3396 W, max 5226 W.
For the same jump, but averaged the whole jump until the feet leave the pressure plate: Mean: 1480 W, Max: 22478 W
Bicycle ergometer, a few seconds, at 4, 8, 10, 12 and 14% body weight, until the increasing of the load did decrease the resulting peak power: Mean: 844 W, Max: 1515 W
Bicycle ergometer, 30s, load 10% body weight: Mean: 613 W, Max: 1054 W
Bicycle ergometer, stepping up the power required slowly, measuring lactate concentration, pulse, blood pressure, breathing. The idea is to find where the point of "more lactate being produced in the straining muscle than the body can get rid of", indicating the point where "can endure over a very long time" becomes "you are on borrowed time with the lactate clock ticking". And they got ... Mean: 190W, Max: 227W
Generating 6MJ in 15s means 6,000,000 J / 15 s = 400,000 J/s or 400 kW. That is 4/9th of a steam engine, slightly under half.
600 W versus 400 kW is 3 orders of magnitude. When someone has more (actually a lot more) than 100 times the strength of an ordinary human, they clearly are superhuman :-)
[1] https://online.medunigraz.at/mug_online/wbabs.getDocument?pThesisNr=17980&pAutorNr=63942&pOrgNR=1)
[2] required: 4+ years training at least 2h hours on 3 days per week, 2+ years of regular participation in national (or international) competitions and healthy.
[3] Average is the addition of all values, then dividing by the number of values.
Mean is "sort the values by their size, then take the value[4] in the middle."
Take a list like this one: 1, 1, 2, 2,100. Average is 106/5 = 21.5. Mean is 2.
This is why you ask for the mean when someone only wants to give you an average. A company with one CEO making $5,000,000 a year and 100 workers making $20,000 per year has an average yearly salary $69,307 but a mean of, you guessed it, $20,000.
[4] If the list has an even number of items, (e.g. "a, b, c, d, e, f") then the middle is between 2 values ("c" and "d" in our example). In this case, take the average of them (("c" + "d") / 2).
\end{Tongue-in-cheek}
Note again, I have no problem at all with the mod (it's a game, not a highly realistic simulator of colonising other planets, and even then ... ), I actually had fun looking up how possible or impossible such a generator may be (and learned some things from the study, too).
Now, you could capture some biters and use a capstan (naval) powering a generator, dangle a 'carrot' in front of them so they run and run and run ... and ... PROFIT!
