EROI Definition

It takes energy to get energy. Mining coal, refining oil, manufacturing solar panels, and enriching uranium all consume energy before a single useful kilowatt-hour is produced. EROI measures whether the payoff justifies the investment.

EROI (Energy Return on Investment) is the ratio of energy delivered to society by a source divided by the energy consumed to produce it. An EROI of 20:1 means 20 units of energy out for every 1 unit invested. Conventional oil historically delivered 30-40:1. Modern oil sands deliver 3-5:1. Solar PV delivers roughly 10-20:1. Nuclear delivers 40-75:1 (estimates vary widely by methodology). The threshold where a source ceases to power a complex industrial society is debated, but most estimates place it around 5-10:1. Below that, too much of the economy's energy output is consumed just producing more energy.

Compare fossil fuels over time. U.S. oil EROI dropped from roughly 100:1 in the 1930s (easy shallow wells) to about 10-15:1 today (deepwater, shale). The physical effort to extract each barrel has risen as the easiest reserves were depleted first.

Apply to solar PV. Manufacturing a solar panel takes about 1.5-2.5 years of its own output (the energy payback period). Over a 30-year lifetime, EROI is roughly 10-20:1.

Does a lower EROI necessarily mean a worse energy source?

EROI is not LCOE. A source with lower EROI can still be cheaper if the energy inputs are inexpensive. Solar's EROI is lower than nuclear's, but solar electricity is cheaper in most markets. EROI measures physical efficiency; LCOE measures economic cost. They answer different questions.

EROI matters for understanding the physical productivity of the energy system as a whole, even when it does not directly determine which technology wins in markets.

---