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GCAM Output Price and Quantity Units

Key Concept

GCAM sector output uses two independent unit fields:

• output-unit: unit of physical-output (e.g. Mt, EJ, Mt NH3)
• price-unit: unit of sector/cost (e.g. 1975$/kg, 1975$/GJ, 1975$/kgNH3)

These two units are NOT always compatible. The product sector/cost * physical-output only represents a monetary proxy when the units are closed (e.g. Mt * 1975$/kg can be converted because Mt -> kg is a known factor).

Decision Rule

Before interpreting cost * output as revenue, check unit compatibility:

1. If both units refer to the same physical commodity (e.g. Mt and 1975$/kg for cement), the product can be converted to 1975$ by applying the correct scale factor.

2. If output-unit is energy-based (EJ) and price-unit is energy-based (1975$/GJ), the product represents a service expenditure proxy, not a physical commodity revenue. This is common for chemical and other industry.

3. If the units are mismatched (e.g. Mt output, 1975$/GJ price, as seen in some GCAM-China configurations for iron and steel), the product is not directly interpretable as revenue. The sector’s price-unit should be confirmed in the model input configuration.

Example from GCAM-China

sector	output_unit	price_unit	compatible for revenue proxy?
ammonia	Mt NH3	1975$/kgNH3	yes (Mt -> kg conversion)
cement	Mt	1975$/kg	yes (Mt -> kg conversion)
chemical	EJ	1975$/GJ	yes, but as service proxy
other industry	EJ	1975$/GJ	yes, but as service proxy
iron and steel	Mt	1975$/GJ	NOT compatible; check config

Where Units Come From

Both price-unit and output-unit are defined in the GCAM input XML for each sector. They are set via input CSV files and written by R processing scripts into the model XML. BaseX output faithfully reflects whatever was loaded into the model.

In GCAM-China, these are typically set in files like:

• A323.sector.csv or A323.sector_China.csv for detailed industry
• A321.sector.csv for cement
• Generated XML like detailed_industry_CHINA.xml, cement_CHINA.xml

A mismatch in the input configuration (e.g. iron and steel with price-unit=1975$/GJ but output-unit=Mt) flows through to BaseX output. This should be addressed at the input configuration level.

Reference

GCAM official documentation:

• Price Outputs: https://jgcri.github.io/gcam-doc/outputs_prices.html
• Quantity Outputs: https://jgcri.github.io/gcam-doc/outputs_quantity.html
• Energy Demand: https://jgcri.github.io/gcam-doc/demand_energy.html

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