Geothermal energy and nuclear energy are the two major energy sources that do not originate from the sun. This surprises many people because nearly every other energy source we use, including fossil fuels, wind, and hydropower, traces back to solar radiation in one way or another. Tidal energy also largely falls outside the sun’s influence, driven mostly by the moon’s gravitational pull.
Why Most Energy Sources Are Really Solar Energy
The sun’s dominance over Earth’s energy picture is hard to overstate. Fossil fuels like coal, oil, and natural gas are all derived from ancient plants and animals. The energy stored within them is chemical energy that originally came from sunlight through photosynthesis, as plants used solar radiation to convert carbon dioxide into sugars and other high-energy carbohydrates millions of years ago. When you burn gasoline or natural gas, you’re releasing stored sunlight that’s hundreds of millions of years old.
Wind energy is also solar in origin. The sun heats the atmosphere unevenly, warming equatorial regions more than the poles, and this temperature difference creates pressure gradients that drive wind patterns. Hydroelectric power follows the same logic: the sun evaporates water from oceans and lakes, lifts it into the atmosphere, and deposits it as rain or snow at higher elevations. That water flows downhill through turbines, but the energy that got it uphill in the first place was solar. Even biomass energy, from wood to ethanol, depends on photosynthesis happening right now rather than in the distant past.
Geothermal: Heat From Earth’s Interior
Geothermal energy taps heat generated deep inside the Earth, completely independent of the sun. The planet’s internal heat budget is roughly 45 trillion watts (45 TW), and it comes from two main sources: the primordial energy left over from when the Earth formed 4.5 billion years ago, and the ongoing radioactive decay of elements in the Earth’s crust and mantle.
The radioactive component is significant. Four isotopes do most of the work: two forms of uranium, one of thorium, and one of potassium. One widely used model estimates that radioactive decay alone contributes about 20 TW of heat, with uranium and thorium each responsible for roughly 8 TW and potassium adding another 4 TW. The remaining heat comes from the original gravitational energy of planetary formation and friction from the Earth’s core solidifying over time. Scientists still debate the exact split between these sources, but none of them involve the sun.
For commercial geothermal power, wells tap underground reservoirs where fluid temperatures reach at least 300°F (149°C), though some plants can operate with temperatures as low as 210°F (99°C). Countries like Iceland, which sits on a volcanic hotspot, generate a substantial share of their electricity and heating this way.
Nuclear Energy: Forged in Ancient Stars
Nuclear power plants run on uranium, and uranium was not made by our sun. It was forged in cataclysmic events that predate our solar system entirely. The universe’s first stars, often more than ten times the mass of our sun, burned through their fuel and produced elements up to iron. When those massive stars died in explosions called core-collapse supernovae, they ejected those elements into space.
The heaviest naturally occurring elements, including uranium, gold, and platinum, required something even more extreme. Scientists believe they formed in violent, neutron-rich environments like supernovae or the collision of two neutron stars. Under those conditions, atomic nuclei capture neutrons so rapidly that they build up into much heavier elements before they have time to decay. The uranium sitting in fuel rods at a nuclear plant was created billions of years ago in one of these events, long before the Earth or our sun existed. So while nuclear energy involves atoms, the energy source itself has nothing to do with solar radiation.
Tidal Energy: Mostly the Moon
Tidal energy occupies an interesting middle ground. Ocean tides are driven primarily by the moon’s gravitational pull on Earth’s water. The sun does contribute, but solar tides are about half as large as lunar tides and show up as variations in the lunar tidal pattern rather than as a separate set of tides. During a new or full moon, the sun and moon align, and their combined pull creates stronger “spring” tides. When they’re at right angles, the result is weaker “neap” tides.
Because the moon is the dominant force, tidal energy is generally classified as non-solar. But it’s not a perfectly clean category the way geothermal and nuclear energy are.
A Non-Solar Energy Source You Might Not Expect
Deep on the ocean floor, hydrothermal vents release superheated, mineral-rich water from beneath the Earth’s crust. These vents support thriving ecosystems with enormous biomass compared to the surrounding deep ocean, and none of it depends on sunlight. Instead, microbes at these vents use chemical energy from reactions between seawater and volcanic minerals. One key process involves the oxidation of hydrogen sulfide released from the vents, which provides the energy to drive biochemical reactions in a process called chemosynthesis.
Another reaction called serpentinization, where seawater reacts with certain rocks in the ocean crust, also releases chemical energy that microbial communities can exploit. Scientists have observed microbial blooms appearing at the start of new volcanic cycles, suggesting that communities of microbes sit dormant in the upper oceanic crust, ready to capitalize on fresh hydrothermal fluids. These ecosystems are a living reminder that not all energy on Earth traces back to the sun.
Quick Comparison of Common Energy Sources
- Solar origin: fossil fuels (coal, oil, natural gas), solar panels, wind, hydroelectric, biomass
- Non-solar origin: geothermal, nuclear (uranium fission)
- Mostly non-solar: tidal (roughly two-thirds moon, one-third sun)
Of these, nuclear energy currently plays a larger role in global electricity generation. In 2024, nuclear accounted for about 8% of the growth in global energy supply. Geothermal remains a smaller contributor worldwide, limited by geography, though it is a major energy source in volcanic regions. Both represent genuinely independent alternatives to the sun’s energy that has powered nearly everything else on this planet for billions of years.