Energy quality refers to differences in the ability of a unit of energy to produce goods and services for people. The usefulness of an energy system is determined by a complex combination of physical, technical, economic, and social attributes. These include gravimetric and volumetric energy density, power density, emissions, cost and efficiency of conversion, financial risk, amenability to storage, risk to human health, and ease of transport. No single metric of an energy system captures all such attributes. It stands to reason, therefore, that a comprehensive and balanced comparison of energy technologies should employ a range of metrics, with their strengths and weaknesses duly noted. The most common way to measure energy is by heat content because all forms of energy can be completely converted to heat (Btus, joules, calories, kilowatt-hours). The aggregation of different energy types is accomplished by multiplying their mass or volume used times their heat content per unit mass or volume