To assess sustainability, it is essential to consider all the variables (economic, social, technical, legal, even geopolitical). Know the limitations of the fossil energy system in the country of reference and assess their renewable energy capacity is essential.
Measuring biomass partly depends on what form the biomass takes. Microorganisms can often be filtered from suspension, dried, and weighed as a mass, to give a dry weight. This is probably the simplest method. Usually this would be connected to another parameter, such as volume of suspension, to give g biomass / ml., suspension, or g biomass/quantity of substrate utilized in growth, etc.