Because few conversion technologies convert resources directly into useful energy, MESSAGE works on 5 energy levels:
Technologies can take in from one level and put out at another level or on the same level. The energy forms defined in each level can be envisioned as a transfer hub, that the various technologies feed into or pump away from. The useful energy demand is given as a time series. Technologies can also vary per time period.
The mathematical formulation of MESSAGE (if properly set up) ensures that the flows are consistent: demand is met, inflows equal outflows and constraints are not exceeded.
An example of an RES is given in the figure below:
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