According to Rangeland Gateway, there are two methods to determine biomass:

Direct methods involve techniques that weigh or estimate the actual biomass of plants in quadrats. Indirect methods are based on developing a relationship between plant weight and an easier-to-measure attribute such as plant height, rainfall, or cover.

The most suitable approach to determine biomass in an inventory or monitoring program depends on the type of vegetation, skills of observers, sample size requirements, and time and budgetary constraints.

Direct Methods to Determine Biomass. Biomass sampling is usually conducted using a sample unit with defined boundaries, for example, some quadrat so that biomass can be expressed relative to a known area. This type of sampling is best suited to areas dominated by herbaceous or shorter shrub species that can be accommodated in relatively small quadrats. With these techniques, the quadrats are directly evaluated to assess biomass. With these techniques, the quadrats are directly evaluated to assess biomass.

The most common direct methods of determining biomass are:

• Harvesting to Determine Biomass
• Estimation Approaches to Determine Biomass
• Sample Units for Direct Methods to Determine Biomass

Indirect Methods Determine Biomass. Indirect methods to determine biomass are based on developing a relationship between plant weight and an easier-to-measure attribute such as plant height, rainfall, or cover. This approach is usually performed in three stages. First, preliminary sampling must be conducted to establish the relationship using regression analysis, with biomass as the dependent variable. The preliminary work is then followed by rapid measurement of the indirect attribute in the field. The final stage occurs after collecting and converting the data to biomass values using the previously established regression equation.

Indirect methods may be selected over direct methods to determine biomass in inventory or monitoring programs because they are non-destructive and usually less time-consuming. These approaches are often used for taller trees and shrubs and when large sample sizes are required to sample extensive or highly variable areas. The disadvantage of these methods is that the relationships between the indirect attribute and biomass are generally restricted in their applicability to the time and place that the preliminary data was collected.

• Climatic Records
• Plant Dimensions
• Reference Unit Method

Jorge Morales Pedraza thank you so much sir

If you are talking on the energy content then you need to do calorimetric analysis. A device named Bomb Calorimeter is handy for this.