The number appears to me as ~20 x 10^22 J

I tried to replicate your maths using 1Bn W for a power plant and I get ~6.3M years. Did you use a different W?

I chose 1Bn from reading https://www.energy.gov/ne/articles/infographic-how-much-powe...

I'm glad your comment triggered me to do this exercise!

Boggling numbers all the same, and this is only the top half-mile of ocean