By the numbers: Energy from the Sun

It starts with the sun… Photosynthetic organisms use sunlight as a power source. Therefore, the rest of us that have to eat plants (or the other delicious organisms that have to eat photosynthetic organisms) are also indebted to the sun for energy. How much are we talking about? Let’s talk numbers in the first installment of ‘By the numbers’:

The average amount of solar energy hitting the earth’s surface is 4.2 kilowatt-hours of energy per square meter for a 24 hour day. Of course this will vary significantly depending on the day of the year and whether you are in a place like Forks, WA or Tuscon, AZ. Not impressed? Let’s think of it another way…

One acre is ~4046 square meters. This gives you 16,993 kWh of incident energy per day per acre. Ok, that’s bigger, but probably still out of context for you.

Think about this: a gallon of gasoline has an energy content of 36.6 kWh. So the average acre receives the amount of energy from the sun equivalent to ~464 gallons of gasoline. Each. Day. Are you surprised yet?

How about this: The total U.S. Energy consumption for the year 2011 for all sources (coal, oil, electricity, renewable whatever)* was 97.301 Quadrillion BTU = 2.85 x1013 kWh. From the numbers above, the average acre on earth will give you 6.2 x106 kWh per year. If we could harness all of that energy, then we would need 4.6 x106 acres or 7187.5 square miles to supply all of our current domestic energy needs. If these numbers are still making your head spin, it corresponds to a land area equivalent to about 13.8% of the state of Louisiana. It’s only 6.3% of the land area of Arizona, the state with the highest values of incident solar energy, according to estimates made by the National Renewable Energy Lab (NREL) in Golden, Colorado. See map below.


Of course, in these calculations we are just talking about potential- in this case, a nearly upper limit for possible energy available with 100% conversion efficiency. At every step from incident photon to electricity useful for human consumption, there are losses. If you make some assumptions, like 15% conversion efficiency, the amount of land area required to replace the annual U.S. energy consumption is ~48000 square miles, just less than half the size of Arizona. This number may be less impressive, but it is still in the realm of ‘possible.’

With such an abundance of energy, it’s no wonder scientists are trying to tap solar for our energy needs. That’s why it’s important to study how photosynthetic organisms capture and convert this energy- not solely for the purpose of making ‘super plants’ but also using engineering principles from biology to create economical and self-sustaining photovoltaic cells.

Interactive bonus: NREL has an on-line tool called In My Backyard that allows you to estimate how much energy you can generate by installing various-sized solar panels at your home (or business) address. It also estimates installation cost and how long it will take to recover your costs.


*Since we are speaking of numbers today, you may be amazed at the sorts of things that scientists somewhere are counting. If you ever wanted to know numbers related to energy, go to the U.S. Energy Information Administration.

1 thought on “By the numbers: Energy from the Sun

  1. Pingback: Artificial Photosynthesis | New Under The Sun Blog

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