Ocean thermal gradient: Abundance

Terawatts in principle, kilowatts in practice

What this tab covers How much of this energy there is: the natural flow or stock, the share that could be harnessed, and how much is used today, compared with world primary energy use (592 EJ in 2024).

A very large flow in principle (several terawatts), but almost untapped: the largest operating plants are around 100 kW.

A flow: renewed continuously

How much there is

Measure	Value	Compared with world energy use	Note
Maximum net OTEC power (3-D ocean model)	about 30 TW²	about 1.6 times world primary energy use (945 EJ vs 592 EJ)	Would require flows large enough to strengthen the global overturning circulation.
Net power with little effect on ocean temperatures	about 7 TW²	about 37% of world primary energy use (220 EJ)	Considered the more practical upper bound.
Net power limit, 1-D model, by discharge design	2.7-4.3 TW¹⁰	about 14-23% of world primary energy use	Mixed discharge 2.7 TW; separate warm and cold discharges 4.3 TW.
IRENA estimate of global OTEC potential	300 EJ per year¹	about half of world primary energy use	IRENA calls this 90% of the global ocean energy potential.
United States resource estimate (DOE assessment)	4,642 TWh per year³	about 2.8% of world primary energy use (16.7 EJ)	Judged 'unrealistically high' by the National Academies review.
Largest plant operating in Hawaii (grid-connected 2015)	0.1 MW⁵	negligible
Kumejima demonstration plant, Okinawa, Japan (built 2013)	about 0.1 MW⁶	negligible	Deep water is reused for cooling, prawn farming and agriculture.

Estimates span an order of magnitude because the resource is self-limiting: pumping deep cold water up and discharging it warms the deep ocean and cools the surface, shrinking the very gap the plant relies on. One-dimensional models give a few terawatts depending on how effluent is discharged; a three-dimensional circulation model gives about 30 TW at most, but only about 7 TW without noticeably altering ocean temperatures.

World primary energy use was 592 EJ in 2024 (Energy Institute Statistical Review 2025), so even the cautious 7 TW figure is more than a third of it. Actual use is a rounding error: Makai's 100 kW plant in Hawaii (grid-connected in 2015) and a demonstration plant of about 100 kW on Kumejima, Japan, built in 2013.

Sources

Ocean Thermal Energy Conversion: Technology Brief IRENA, 2014
Estimates of global Ocean Thermal Energy Conversion (OTEC) resources using an ocean general circulation model (Rajagopalan and Nihous, Renewable Energy 50) Elsevier, 2013
An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments, chapter on ocean thermal energy National Academies Press, 2013
Makai Ocean Engineering connects world's largest OTEC plant to U.S. grid Power Engineering, 2015
The Century-Old Renewable You've Never Heard Of (Kumejima OTEC plant) Eos (AGU), 2022
A Preliminary Investigation of the Effect of OTEC Effluent Discharge Options on Global OTEC Resources (Nihous, JMSE 6) MDPI, 2018