Units

In GCAM the "coefficient" of any technology and input simply represents the ratio between units of input and units of output.* Conversely, "efficiency" represents output/input; which tag is used for any technology is a choice (i.e., either could be used). In general, if there are multiple inputs, coefficients are used (so that the inputs share a common denominator), and similarly, coefficients tend to be used if there is a fundamental change taking place (e.g., from an energy good to a material good such as natural gas -> fertilizer).

The units of the "output" of any technology are indicated in the tag output-unit specified in the corresponding A*.sector.csv file. The units of the input are also indicated in the input-unit tag, but note that the input-unit is not specified at the level of each individual input. The failsafe way of determining the model's units of any input is to find the supplysector or resource of the same name and look up its output-unit. For example, in the N fertilizer sector, the output-unit is Mt and the input-unit is EJ, so the (input-output) coefficient of all technologies therein is EJ/Mt, which we simplify** to GJ/kg.

* The one exception to this is the infamous "tranTechnology", where the coefficient is in btu/vehicle-km and the ratio between inputs (EJ) and outputs (million passenger or tonne kilometers) includes several unit conversions and the load factor of the vehicle.

** By a factor of 9. GCAM's standard of simplifying indicators by powers of 9 comes from the order of magnitude of the global population.