The quantity, quality and type of biomass available for use in the biogas plant constitutes the basic factor of biogas generation. The biogas incidence can and should also be calculated according to different methods applied in parallel.
It should be kept in mind that the various methods of calculation can yield quite disparate results that not only require averaging by the planner, but which are also subject to seasonal variation.
The biomass supply should be divided into two categories:
This is a time-consuming, cumbersome approach, but it is also a necessary means of adapting values from pertinent literature to unknown regions. The method is rather inaccurate if no total-solids measuring is included. Direct measurement can only provide indication of seasonal or fodder-related variance if sufficiently long series of measurements are conducted.
According to this method, the biomass supply can be determined at once on the basis of the livestock inventory. Data concerning how much manure is produced by different species and per liveweight of the livestock unit are preferable.
Dung yield = liveweight × number of animals × specific quantity of excrements [ kg/d ]
Often, specific quantities of excrement are given in % of liveweight per day, in the form of moist mass, total solids content or volatile solids content
This approach leads to relatively accurate information, as long as other biogas plants are already in operation within the area in question.
This approach is necessary if green matter is to be included as substrate.
The energy demand of any given farm is equal to the sum of all present and future consumption situations, i.e. cooking, lighting, cooling, power generation etc. The following table helps to collect all data concerning the energy demand. To get more information about the specific energy demand, for example, see "Biogas appliances".
Table: Outline for determining biogas demand |
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| Energy consumers | Data | Biogas demand [l/d] |
| 1. Gas for cooking Number of persons Present energy consumption Gas demand per person and meal Specific consumption rate of burner Total anticipated cooking-gas demand |
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| 2. Lighting Specific gas consumption per lamp |
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| 3. Cooling Specific gas consumption * 24 hours |
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| 4. Engines Specific gas consumption per kWh |
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| 5. Miscellaneous consumers Gas demand |
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| Anticipated increase in consumption (%) | ||
| Total biogas demand 1st-priority consumers 2nd-priority consumers 3rd-priority consumers |
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The following alternative modes of calculation are useful:
Determining biogas demand on the basis of present energy consumption, e.g. for ascertaining the cooking-energy demand. This involves either measuring or inquiring the present rate of energy consumption in the form of wood, charcoal, kerosene and bottled gas.
Calculating biogas demand via comparable-use data: Such data may consist of
Estimating biogas demand by way of appliance consumption data and assumed periods of use: This approach can only work to the extent that the appliances to be used are known in advance, e.g. a biogas lamp with a specific gas consumption of 120 l/h and a planned operating period of 3 h/d, resulting in a gas demand of 360 l/d.
Then, the interested party's energy demand should be tabulated in the form of a requirements list. In that connection, it is important to attach relative priority values to the various consumers, e.g.: