Following the Energy Trail

Find out how the sun's energy is used on earth.

Dr Viau working with numbers


 Without the sun, there would be no life on earth.

The sun's energy warms the planet, powers the weather and the water cycle, and makes it possible for plants to grow.

The solar energy that energizes the plants goes on to become the fuel that allows animals to live and grow.

Let's see how the sun's energy flows in the Biosphere.

You will be surprised to see how little of the sun's energy it takes to build and power all the life forms on earth!


The Solar Constant


The Solar Constant is the amount of solar energy per square meter that reaches the earth's surface at the equator at noon.

Chart about the sun's energy use

The earth gets only 2 billionths of the sun's energy, but that is still a lot. However, you can see on the chart that life (through photosynthesis) uses only .023% of the energy that reaches the surface of the earth.  

34% of the sun's energy is reflected back into space by snow and clouds. This reflective quality of a planet is called its albedo.

42% of the energy goes to warm the land and water. The warmth of the earth is constantly being radiated into space, and the sun's energy replenishes this warmth.

The water cycle -- evaporation and precipitation -- uses 23% of the solar energy.

Winds and ocean currents use 1%.

A Math Page:

To get the details on how to calculate energy available for plants, click here.

The table below tells how many KiloCalories (food calories) are available for animals to eat in different biomes.

Primary Productivity Table

 Ecosystem Type

 Net Primary Productivity
(Kilocalories per square meter per year)

 Approximate Kilocalories per square meter per day

 Rainfall per year in inches

 Growing Season

 Percentage of earth's surface covered by this biome
Tropical Rain Forest



 More than 60

 365 days


Estuary (the place where a river meets the sea -may have many channels and be a delta.)



water environment



Swamps and Marshes



 water environment

Deciduous Temperate Forest




 More than 120 days


 Boreal Forest (Evergreen Coniferous Forest)




 Less than 120 days

 Savanna (grass, scattered trees,
little or no winter snow)






 Temperate Grassland (cold winters)




 Polar Tundra



 Less than 10


includes sand and ice


  < 200

 Less than 1


 Open Ocean


Less than 1

 Less than 10


Cultivated land covers about 9% of earth's land surface, and has replaced tracts of forest and some of the more generously watered grasslands. Humans require land with a minimum of 20 inches of rain per year in order to grow their crops and to survive. In some places, the natural rainfall is suppliment with irrigation.

This table is from an online Geology course created by Dr, Michael Pidwirny at Okanagan College, British Columbia, Canada. I have added additional material from The numbers are not in perfect agreement, but they are close. (Dr Viau's additons are in red)

The Caloric Content of Foods

Some Animal Weights and Caloric Requirements on Earth

 Return to

Tundra KCalorie Pyramid Coniferous Forest KCalorie Pyramid
Decidious Forest KCalorie Pyramid Grasslands KCalorie Pyramid
Return to Lesson 7 Return to Lesson 10


Return to Science Notes

Elizabeth Anne Viau, 1999. This material may be used freely for instructional purposes but not sold for a price beyond the cost of reproduction. Please inform the author if you use it at