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Why does the Sun emit much more radiation per square metre than Earth’s surface?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

The Sun emits significantly more radiation per square metre than Earth’s surface primarily due to the vast difference in their absolute temperatures.1

According to the Stefan-Boltzmann law, the total radiant energy emitted by a surface per unit area is proportional to the fourth power of its absolute temperature ( $W = σ T^{4}$ ). Because the Sun’s surface (the photosphere) is approximately 6,000 K, while Earth’s surface is significantly cooler at roughly 300 K, the Sun radiates vastly more energy per unit area. Specifically, because the Sun is about 20 times hotter than Earth, it emits $(6, 000 / 300)^{4} = 20^{4} = 160, 000$ times more electromagnetic energy per square metre than Earth’s surface.213

This temperature difference also dictates the type of radiation emitted: the Sun’s higher temperature causes its peak emission to occur in the visible portion of the electromagnetic spectrum, whereas Earth’s lower temperature results in peak emissions in the infrared range.43