The real issue was that I didn't get that you were talking about Solar Orbiter, I thought you were saying that the irradiance of the sun was 20kW/m2, which seemed low to me, but I didn't even know the word "irradiance" so I didn't know what to type on Google to check it. Thanks for your detailed calculus :)
It's basic algebra. Calculus would involve derivatives or integrals.
https://www.merriam-webster.com/dictionary/calculus
The usual sense involves integration and derivation but look at senses 2 & 4. It also means any calculation.