What Is Geothermal Energy? A Plain-English Guide

Break the word in half and it explains itself: geo (earth) plus thermal (heat). Geothermal energy is the heat stored inside the planet you're standing on — and there is a staggering amount of it. The Earth's inner core runs somewhere around 10,800°F, roughly as hot as the surface of the sun, and that heat radiates outward through the mantle and crust all the way up to the ground under your feet.

Most people meet the word "geothermal" in one of two places: a news story about a power plant that makes electricity from steam, or a contractor's quote for a heating system that pulls warmth from their backyard. Those are related ideas, but they are not the same thing, and confusing them is one of the most common mistakes made on this topic. We'll keep them straight throughout.

Here's the short version, and then we'll earn it: geothermal energy is heat from the Earth, it is renewable, it is not a fossil fuel, and while it powers less than one percent of America's electricity today, the version most homeowners actually buy — a ground-source heat pump — works almost anywhere.

What Geothermal Energy Actually Is

Two things make the Earth hot on the inside. The first is leftover heat from the planet's violent formation about 4.5 billion years ago — primordial heat that has been slowly bleeding outward ever since. The second, and a key reason this energy counts as "renewable," is the steady decay of naturally occurring radioactive elements — isotopes of potassium, thorium, and uranium — scattered through the crust and mantle. As those atoms break down, they release heat, continuously, and they will keep doing it for billions of years.

That heat gets more intense the deeper you go. Near the surface, temperature rises at an average of about 25°C for every kilometer of depth — a figure known as the geothermal gradient (National Geographic Education). Go down a few kilometers in the right geology and you reach rock and water hot enough to spin a turbine. The U.S. Energy Information Administration notes that conventional geothermal power plants tap resources in the 300°F to 700°F range, often drilling wells a mile or two deep to reach them (next-generation "enhanced" systems go deeper still).

Is Geothermal Energy Renewable? (Yes — Here's Why)

Yes. Geothermal energy is renewable, and the reason is refreshingly concrete.

The U.S. Department of Energy puts it plainly: the Earth's heat is "continually replenished by the decay of naturally occurring radioactive elements and will remain available for billions of years." The EIA describes the same source as heat from "the slow breakdown of radioactive particles" inside the Earth. In other words, the planet is generating this heat right now, faster than any realistic amount of human use could draw it down. That's the definition of renewable: a resource that replenishes on a human timescale rather than one we deplete and never get back.

There's one honest nuance worth stating, because a careful reader will ask about it. While the resource is renewable, an individual geothermal reservoir can be locally over-drawn if a power plant pulls heat and fluid out faster than that specific site recharges. Well-run plants manage this by re-injecting the water they use and sizing the operation to the field. So the correct framing is: geothermal energy as a resource is renewable and effectively inexhaustible; a poorly managed single field can be temporarily depleted.

Is Geothermal a Fossil Fuel? (No)

No. Geothermal energy is not a fossil fuel, and the distinction is clean.

Fossil fuels — coal, oil, natural gas — are buried carbon that we dig up and burn. The energy comes from combustion, and combustion is where the carbon dioxide comes from. Geothermal is the opposite category. There is no fuel to mine, store, transport, or set on fire; you are simply moving heat that the Earth already made. As the DOE notes, geothermal involves "no fuel storage or combustion." If you've ever seen "geothermal" grouped with solar and wind on a list of renewables and wondered whether that was right, it is — for exactly this reason.

Three Different Things People Call "Geothermal"

Here is the section that clears up most of the confusion on the internet. The word "geothermal" gets attached to three genuinely different applications. They share a root — heat from the Earth — but they operate at wildly different depths, temperatures, and scales.

Geothermal power (electricity). This is the "power plant" version. It needs a genuinely hot resource — 300°F to 700°F — which exists only in specific geology, usually near tectonic plate boundaries or volcanic regions. There are three plant designs: dry-steam (the oldest), flash-steam (the most common today), and binary-cycle (which works at lower temperatures). The world's largest known dry-steam field is The Geysers in Northern California, which has been generating electricity since 1960 and remains a significant contributor to the state's renewable generation. The very first geothermal power plant went online in Italy in 1904 (EIA).

Direct use. Sometimes hot water sits close enough to the surface to be piped up and used as heat, no electricity required. Boise, Idaho has heated buildings this way since 1892 (DOE). District heating systems, greenhouses, aquaculture, and the famous hot-spring bathing culture of places like Iceland all fall under direct use.

Geothermal heat pumps. This is the one homeowners actually buy — and it is the odd one out, because it does not use the deep, hot resource at all. A geothermal (or "ground-source") heat pump exploits the fact that a few feet down, the ground holds a nearly constant mild temperature of about 40–70°F year-round (DOE), regardless of the weather above. It uses that stable ground as a place to pull heat from in winter and dump heat into in summer. Because it relies on shallow, mild ground rather than volcanic heat, it works almost anywhere in the country. When a homeowner says they "went geothermal," this is almost always what they mean.

If you want the deep dive on the home version, we cover exactly how that works in how geothermal heat pumps work and, for the whole-home heating-and-cooling picture, geothermal heating and cooling.

How Geothermal Energy Is Used, and Where

Geothermal energy shows up in three broad forms of use, matching the three applications above: making electricity, heating things directly, and running heat pumps.

On the electricity side, geothermal is a global business, even if a modest one. Roughly three dozen countries generate geothermal power, and depending on the dataset the world had somewhere around 15,000–17,000 MW of installed geothermal power capacity as of 2024 (industry tracker ThinkGeoEnergy puts it near 16,900 MW; the intergovernmental agency IRENA's figure is lower). The United States leads that list, with Indonesia and the Philippines next, followed by a cluster that includes Türkiye, New Zealand, Kenya, Mexico, Italy, Iceland, and Japan — the exact order shifts between sources. Within the U.S., California (led by The Geysers), Nevada, and Hawaii are the standouts, along with several other western states where the geology cooperates.

Iceland is the country most people picture, and for good reason: it heats the large majority of its buildings with geothermal energy and generates a meaningful share of its electricity from it, thanks to sitting squarely on a volcanic rift. (We're describing that qualitatively on purpose — the precise percentages get quoted inconsistently, and we'd rather say "the large majority" accurately than cite a specific figure we can't stand behind.)

For direct use and heat pumps, geography matters far less. Direct-use district heating runs in places like Boise. And ground-source heat pumps — the home version — are installed across every U.S. climate zone, from Minnesota to Florida, precisely because they only need the mild shallow ground that exists everywhere.

How Much of Our Power Comes From It

Time for perspective. Despite all of the above, geothermal is a small slice of the U.S. electricity mix today. As of 2025, the country had roughly 2.7 GW of geothermal power capacity, and seven states generated utility-scale geothermal electricity, producing on the order of 0.4% of total U.S. utility-scale electricity — around 16 billion kWh a year (EIA). (Counts vary with the metric: industry market reports that tally operating plants rather than generation put the state count a bit higher.)

So why does anyone get excited about a source that's under one percent? Two reasons.

First, geothermal punches above its weight on reliability. Unlike solar and wind, a geothermal plant runs around the clock, in any weather, because the heat source never sets or stops blowing. That gives it a high capacity factor — it produces close to its maximum rating a large fraction of the time. (Published figures vary a lot with the year and method: the DOE describes geothermal as typically running at 90% or higher, while the EIA's recent fleet-wide capacity-factor figures have come in nearer 66%, so treat it as a range rather than one tidy number.) Either way, geothermal delivers something solar and wind can't on their own: firm, baseload power.

Second, the ceiling is high. The DOE's analysis projected that next-generation techniques could eventually unlock up to 90 GW of geothermal electricity capacity in the U.S. by 2050 — dozens of times today's figure — alongside DOE GeoVision estimates of more than 17,000 geothermal district-heating installations and the equivalent of tens of millions of geothermal heat pumps. Whether that arrives on schedule is a separate question, but the resource is not the constraint. The engineering and the economics are.

The Honest Environmental Picture

Geothermal is genuinely clean, but "clean" is not the same as "zero impact."

The upsides are real. Geothermal power is baseload and runs nearly 24/7, its land footprint is tiny compared with the sprawl of a wind or solar farm producing the same energy, and because there's no combustion, its lifecycle emissions are very low (DOE, National Geographic).

The honest downsides are worth naming too. National Geographic points to several: injecting fluid deep underground can trigger small induced earthquakes; geothermal fluids can carry dissolved gases and minerals — including arsenic and other toxins — that must be managed rather than released; heavy extraction can cause land subsidence; and some plants vent carbon dioxide and hydrogen sulfide, at levels well below a fossil-fuel plant but not always zero. None of these is a dealbreaker, and modern plants engineer around them, but a homeowner comparing options deserves the full ledger, not just the brochure.

What This Means for Your House

If you found this page because you're weighing geothermal for your home, here's how the big-picture story lands on your specific decision.

You are almost certainly looking at a ground-source heat pump — the shallow-ground version, not a backyard power plant. That's good news, because it means the exotic requirements of geothermal electricity (volcanic geology, 300°F-plus resources, two-mile wells) don't apply to you. Your system needs only the mild, stable temperature a few feet down, which your yard already has.

What it does ask of you is a significant upfront investment to install the ground loop, in exchange for very low operating costs afterward — the classic geothermal trade-off. That math — high upfront cost, long payback, and big savings that mostly pay off if you stay in the home long enough — is the heart of the home decision. A common rule of thumb among owners is that if you're likely to move within a handful of years, the numbers rarely work in geothermal's favor. We work through the full calculation in our geothermal payback period and installation cost guides.

Weigh geothermal against your alternatives — a clear-eyed pros and cons is the right next stop, and our complete guide to geothermal ties the whole subject together.

Frequently Asked Questions

Is geothermal energy renewable or nonrenewable?

Renewable. The Earth's internal heat is continuously replenished by the decay of radioactive elements and will remain available for billions of years, far longer and faster than any human use could deplete it (DOE, EIA).

Is geothermal energy a fossil fuel?

No. Fossil fuels release energy by burning buried carbon; geothermal simply moves heat the Earth already produces, with no combustion and no fuel to mine or burn. That's why it's classified as a renewable, alongside solar and wind.

Is geothermal energy clean?

Largely yes — no combustion means very low lifecycle emissions and a tiny land footprint. But it isn't zero-impact: some plants vent small amounts of CO2 and hydrogen sulfide, injection can trigger minor seismic activity, and geothermal fluids can carry minerals like arsenic that must be managed. Modern plants engineer around these issues.

What's the difference between geothermal energy and a geothermal heat pump?

Geothermal energy is the broad resource — the Earth's heat, most dramatically used to make electricity from 300–700°F underground steam. A geothermal heat pump is a home appliance that uses only the mild, stable ~40–70°F ground a few feet down to heat and cool a building. Different depth, different temperature, different scale — but the same underlying idea.

How is geothermal energy used?

Three ways: generating electricity at power plants, heating buildings and greenhouses directly with naturally hot water (direct use), and running ground-source heat pumps that heat and cool individual homes.

Can I get geothermal energy anywhere?

For electricity, no — geothermal power needs specific hot geology found in places like California, Nevada, Iceland, and Indonesia. But geothermal heat pumps work almost anywhere, because they rely on the mild shallow ground temperature that exists under nearly every property.