Geothermal Energy Facts: The Numbers That Actually Check Out

Search "geothermal energy facts" and you get listicles. Read a few and you'll notice the same handful of numbers moving between them, often with no traceable source attached — and some of them are a decade stale.

So this list is built differently. Every number below is traced to a primary source: the Energy Information Administration, the Department of Energy, NREL, USGS, or a national statistics office. Each one carries the year it describes, because energy statistics age fast. And where a commonly-repeated "fact" couldn't be pinned to a source we could reach, it's in the last section rather than quietly included.

The Scale: How Much Geothermal Is There

1. US geothermal electric capacity is 3,969 megawatts. That's the installed nameplate figure as of 2024, from the DOE-funded market report. It's up from 3,673 MWe in 2020 — about 8% growth in four years, after a stretch from 2015 to 2019 when capacity grew roughly 1%.

2. Geothermal generates about 0.4% of US utility-scale electricity. Around 16 billion kWh in 2025, per EIA. This is the number that surprises people who've read a lot of enthusiastic geothermal coverage: it is a small slice of the American grid. The interesting part isn't the current share — it's the reliability profile behind it, which is the next section.

3. California produces roughly 69% of US geothermal electricity. EIA's 2025 figure is 68.6% of the national total from one state. Geothermal power generation in the US is not geographically distributed the way solar or wind is; it clusters where the resource surfaces.

4. The Geysers in California is the world's largest geothermal power complex. Calpine operates 13 plants there with about 725 MW of capacity — that figure is one operator's fleet within the field, not every plant in the complex. It sits about 70 miles north of San Francisco, and it is the same field that has been producing since the 1960s.

The Reliability Fact Nobody Quotes

5. US geothermal ran at a 64.6% capacity factor in 2024. For comparison, in the same finalized EIA data: wind managed 34.3%, solar photovoltaic 23.2%, and solar thermal 25.0%.

This is the most useful geothermal number we found, and it rarely shows up in the fact lists — it takes a paragraph to explain, which is probably why. Geothermal produces roughly twice as much actual electricity per megawatt of installed capacity as wind, and nearly three times as much as solar PV. The reason is unglamorous: the heat underground doesn't care whether it's night, cloudy, or calm.

That is why an industry with 0.4% of US utility-scale generation gets a disproportionate amount of attention from utilities and data-center operators. They aren't buying megawatts. They're buying megawatts that show up at 3 a.m.

Where It Started

6. The first geothermal electricity was generated in 1904 at Larderello, Italy. Prince Piero Ginori Conti used geothermal steam to drive a turbine and light five bulbs. Five bulbs is a genuinely tiny demonstration — and it was the entire beginning of the industry.

7. The first US commercial geothermal power plant opened at The Geysers in 1960. Pacific Gas and Electric built it with 11 MW of capacity. The same field now anchors the world's largest complex, which is an unusual amount of continuity for an energy asset.

The Ground Under Your House

This is where geothermal stops being about power plants and starts being about houses — and where two genuinely different technologies get conflated. If that distinction is fuzzy, our guide to what geothermal energy actually is separates them properly.

8. About 30 feet down, ground temperature holds steady between roughly 50°F and 59°F year-round. DOE gives that as 10–15°C at about 9 meters. It doesn't swing with the seasons, which is the entire physical basis for a residential ground-source heat pump: the ground is warmer than winter air and cooler than summer air, so you're always moving heat across a smaller gap than an air-source system is.

9. The Earth's crust warms roughly 24°C per kilometer of depth. DOE states the gradient as about 1°F per 75 feet, which converts to approximately 24°C/km. Actual gradients vary a lot by location — that variation is precisely why some regions have power-plant-grade resources and others have to drill much deeper for the same heat.

Worth being clear about the scale difference here: a residential ground loop goes down a few hundred feet and is harvesting a stable temperature, not heat from the Earth's core. A geothermal power plant is a different proposition entirely. Our drilling and wells guide covers what actually happens in a residential yard.

The Environmental Numbers

10. We pulled a lifecycle-emissions fact and could not stand it up. We had NREL median figures for geothermal electricity — single-digit-to-forties grams of CO₂e per kWh depending on plant type — and the NREL page they came from is gone. Numbers we can't check against a live source don't go in a fact list, so they're not here.

What we can say without a source dispute is in fact 11: the land-use comparison, where the underlying report loads and the figures are checkable.

11. In one DOE-hosted facility comparison, geothermal used far less land per unit of energy than solar. That comparison gives 160–170 m²/GWh for geothermal flash or binary plants excluding wells, and 900 m²/GWh for a flash facility counting wells and pipelines. The same comparison puts solar thermal at 3,200 m²/GWh and utility-scale solar PV at 7,500 m²/GWh.

Even using the geothermal figure that includes all the wells and surface pipe — the least flattering way to count it — the flash plant in that comparison used roughly one-eighth the land per gigawatt-hour of the utility-scale PV case. That's one report's selected facilities, not a universal law about every geothermal and solar plant ever built.

The Land-Use Comparison, Done Carefully

Fact 11 deserves unpacking, because land-use comparisons between energy sources are unusually easy to do dishonestly.

The DOE-hosted figures give geothermal two numbers, not one: 160–170 m²/GWh excluding wells, and 900 m²/GWh for a flash facility counting wells and pipelines. Those describe different things. The first is the plant footprint. The second is the plant plus everything drilled and piped to feed it.

Which number you quote changes the story by roughly 5.3–5.6× (900 ÷ 170 = 5.29; 900 ÷ 160 = 5.63), and geothermal advocates predictably quote the first while critics quote the second. The useful comparison takes the bigger number — 900 m²/GWh, wells and all — against solar thermal at 3,200 m²/GWh and utility-scale solar PV at 7,500 m²/GWh.

Geothermal still wins by roughly 8× against solar PV on that basis. It doesn't need the flattering number.

Part of why the comparison lands where it does: land use per GWh has generation in the denominator, so a technology running 64.6% of the time gets more GWh out of the same footprint than one running 23.2% of the time. Capacity factor contributes to the land figure — it doesn't determine it on its own, since footprint, wells, pipelines, plant lifetime, and methodology all feed in too.

Iceland: About 90% of Domestic Heating

12. About 90% of Iceland's domestic heating energy came from geothermal in 2020, per Iceland's national energy authority. Not 90% of its electricity — 90% of the energy used to heat homes. Icelandic cities are heated by district systems piping naturally hot water from geothermal fields directly into buildings.

This one gets garbled a lot, with "90% of Iceland runs on geothermal" applied to electricity, or to the whole energy system. The 90% figure is specifically about domestic heating. Iceland's geothermal electricity share is a different number entirely — it's in the last section, not here.

The Heat Pump Count Nobody Has

13. Nobody publishes a clean count of US ground-source heat pump installations. The 2025 market report is explicit that existing GHP installations have to be estimated from EIA's Residential and Commercial Buildings Energy Consumption Surveys, and that both surveys have limited representativeness for this purpose. The one hard series that exists is EPA's annual ENERGY STAR shipment data, which covers only certified residential units — a slice of the market, not the whole of it.

We had a widely-cited "about 2 million units in service" figure from a DOE assessment. The DOE PDF behind it now returns a 404, and no working copy survives to check it against, so we won't assert it.

What's clear regardless: the power-plant side of geothermal sits in a handful of Western states, while the heat-pump side is in ordinary houses across the country. That's the side most Americans would ever actually encounter.

14. Residential vertical loops typically go 100 to 400 feet deep, in boreholes about 4 inches in diameter — a detail worth knowing because it's the part of the job that dominates the installation cost.

15. A rough sizing rule is 150 to 200 feet of vertical bore per ton of capacity — the DOE building-science chapter gives that as a preliminary estimate specifically for heating-dominated climates. Final sizing depends on the building load and the ground's thermal conductivity, so treat it as a sketch rather than a design figure.

16. ENERGY STAR residential GHP shipments peaked in the early 2010s. EPA has collected annual shipment data for ENERGY STAR qualified geothermal heat pumps since 2010, broken out by closed-loop, open-loop, and direct-geoexchange; the market report notes the highest shipment years came early in that series. It only covers ENERGY STAR certified residential units, so it undercounts the total market — but it's the most consistent annual series anyone publishes.

17. The US geothermal power industry grew 8% in four years — after growing about 1% in the four years before that. Capacity went from 3,673 MWe in 2020 to 3,969 MWe in 2024, against roughly 1% growth across 2015–2019. The absolute numbers are small; the change in trajectory is the story.

18. Geothermal electricity is geographically concentrated. California generated 68.6% of US geothermal utility-scale electricity in 2025, and The Geysers is the largest complex in the world. Geothermal power, as currently built in the US, is a handful of places. Enhanced geothermal systems are the attempt to change that, by engineering a reservoir where nature didn't provide one.

What We Couldn't Verify

What we went looking for and are leaving out rather than guessing at:

• Iceland's geothermal share of electricity. The 90% domestic-heating figure is solid and sourced. The electricity share is a different number, and we couldn't pin it to a primary source. Most of Iceland's electricity is hydro, which is why the two figures differ — but we're not putting a percentage on it we can't cite.
• Lifecycle CO₂ emissions per kWh, for geothermal or anything else. We had NREL medians for geothermal and no reachable page to re-verify them against, and we never got matching coal and gas figures from the same methodology. Half a comparison is worse than none.
• A US ground-source heat pump unit count. See fact 13 — the commonly-quoted 2-million figure traces to a DOE PDF that now 404s.
• A residential-only drilling cost benchmark. The per-foot borehole costs that exist in the 2025 market report are market-wide benchmarks drawn from developer and driller interviews, not a residential price schedule. They're in the drilling guide with that caveat attached.

If you came here for a round number to drop in a school report, that last section is probably annoying. Those round numbers are all still circulating in other fact lists — now you know what to make of them.