Re which see my below about more useful concept of
EG/EI (Energy Gain(loss)/Energy Invested).

Are we talking about thermodynamics or economics? I see ER/EI as thermodynamic.

Sheesh, you guys throw out all these high falutin terms and concepts like thermodynamics and stuff... I mean, next thing you know ya'll will be expectin us common folk to be able to understand the Krebs Cycle or sumthin.

Seriously though I wonder how many people of even college level literacy have any understanding of thermodynamics and are able to grasp why classical economic theory is pure bunk.

It seems obvious that there are two quantities here, and making sure they are obvious and separate is a sensible idea.

First there is EROEI talking in simplistic terms about energy and efficiency of extraction. Second there is ROI talking in simplistic terms about money. Now EROEI informs ROI, but so does location, market prices, shade of sky colour in your part of the world, etc.

EROI, energy returned on investment, is a bit of a muddle in the middle, conflating energy with finance. It doesn't really tell you anything and confuses the issue by being too close to ROI in the minds of economists. Don't confuse the poor dears.

Load up your spellchecker with EROI => EROEI and get rid of it from articles unless the author is really incorporating energy with finance. If they are, don't let them get away with the woolly thinking that so exemplifies economists and accounting - they need to define inclusions, exclusions and assumptions.

PS Plastics and fertilisers is a red herring on EROEI. What is output from the resource recovery process is potential energy. What you use it for is another question. Plastics, fertilisers, etc. have high 'energy' inputs because they can use feedstocks that could be energy.

can EROI be negative? ROI can be, ER/EI can't be

ER/EI never negative? On a strict thermodynamic basis this must be true, for energy can neither be created nor destroyed, thus each term must always be positive. (Unless they manage to harness Dark Energy, that is.)

But, to be practical, we are talking about useful energy here, so there is a subjective judgement in what precisely constitutes energy.

For instance, say I use a motorised winch to hoist a heavy weight. Neglecting friction, I have an EROEI of one because the potential energy gained is equal to the work done.

If the rope breaks and the weight falls and smashes my winch, I've ended up with net negative energy, because I've gained zero useful energy from burning my fuel, plus I've lost the energy used to make the winch.

Thus I can get negative EROEI in a catastrophe.

(I realise I have cheated in re-drawing the boundaries of the problem between start and finish, but I am prepared to argue that this is acceptable in a practical situation.)