Value is subjective. A subject is a self-preservation machine (cells, organisms, collectives, etc.) capable of resisting the effects of entropy by expending energy externally. Only subjects can ascribe value to something.

Energy has value to all subjects because subjects need energy to continue existing in spite of entropy.

Subjects engage in activities that produce energy for themselves. By producing energy, subjects contribute to increasing entropy, which consequently requires more energy to resist in the future.

Subjects must maintain a surplus of energy. They must create more than they consume.

Storing energy is valuable.

Living cells store energy in lipids - fatty compounds that cells can "burn" later for energy.

Organisms store energy in food reserves - by keeping food reserves, organisms can later convert the food to energy for their cells.

Collectives store energy in money - by keeping records of energy creation vs consumption, collectives can efficiently produce and allocate energy to/from the comprising organisms.

Humans are the only known collectives that use things which aren't food as money. (Although monkeys have been successfully trained to use coins to trade for food and sex)

Collectives usually have huge long-term aspirations (like building a city or going to the moon). These require tons of energy and may take decades. Food storage is not sufficient at storing this much energy for years at a time.

Anything can be used as money.

Its up to the collective to decide what is money. The money does not store the energy like fats or food. Rather the energy is "stored" as an IOU by the collective.

By spending money, you are demanding that others in the collective burn energy for you. By earning money, you expend energy to gain more energy (in the form of money).

A good money will keep accurate records of energy expended. A good money will be equally "hard" to acquire no matter which organism is competing to acquire it.

A worse money will become 'elastic' causing the collective's record of energy expended to become inaccurate to reality. A worse money is easier to acquire for some and harder for others. Not because some organisms can expend more energy than others, but because the money rewards them unfairly (Cantillon Effect).

In theory, the best money will become the most adopted by collectives.

A money that is adopted by a larger collective is more valuable than a money adopted by a smaller collective. "Large" and "small" here refers to the energy production of the collective, not necessarily the number of organisms.

A popular money can be exchanged for more energy and thus the money itself is more valuable.