What is EROI (energy return on investment)?

EROI, energy return on investment, measures how many units of energy a resource delivers for every unit of energy spent extracting or building it. Early easy oil returned as much as 100:1. Today, as extraction shifts to shale, deepwater and oil sands, the ratio typically falls to between 10:1 and 20:1, while modern solar PV systems return 10 to 30.

The historical trajectory of EROI points downward. At Spindletop in 1901 you drove a pipe into the ground and a fountain of wealth came up: easy oil of that kind returned as much as 100:1 (U.S. Energy Information Administration). Today, as extraction moves toward deeper and more complex unconventional oil — shale, deepwater, oil sands — EROI has fallen markedly, typically to somewhere between 10:1 and 20:1 (BP Statistical Review of World Energy, now published by the Energy Institute).

A long-running myth holds that manufacturing a solar panel consumes more energy than the panel will ever produce. That claim may have held in a 1970s laboratory, but it has since been disproved. Modern PV systems have an EROI of 10 to 30, meaning that for every unit of energy invested in mining, refining silicon and shipping modules, the system returns 10 to 30 units over a 30-year life. The companion metric, energy payback time, currently stands at 1 to 3 years depending on the site's solar resource — a three-year payback on a thirty-year asset is a considerable return in energy terms.

EROI matters because it sets the thickness of the energy surplus. Fossil fuels became the foundation of civilisation precisely because they once delivered an enormous surplus, which is what allowed humanity to build cities, schools and hospitals. A gallon of petrol holds roughly 130 million joules, about 33.7 kWh; performing the same work with human muscle would take some 400 to 500 hours of uninterrupted heavy labour. When EROI contracts, it is not just that fuel gets more expensive — the lever that let civilisation accelerate quietly gets shorter.

Put the two trend lines together and you get what this book calls the thermodynamic wall: fossil fuel EROI is falling while the complexity of renewable systems is rising, and the two are converging on the same band. This contraction of the energy surplus is a rarely discussed driver of today's shifting economic landscape, and it is why the monetary cost per megawatt-hour alone is not enough to judge what an energy source is really worth to a society.

As fossil fuel EROI declines and renewable systems grow more complex, we are running into a thermodynamic wall.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第二章 2.2.4 + 第七章 7.6