What is the average energy generation for each energy source considered?
SELECT ?energy_sources_labels (AVG(?elec_gen_value) AS ?average_elec_gen_value)
WHERE {
  orkgr:R153801 orkgp:compareContribution ?contrib.
  ?contrib orkgp:P43135 ?energy_sources.
  ?energy_sources rdfs:label ?energy_sources_labels;
                  orkgp:P43134 ?elec_gen.
  ?elec_gen orkgp:HAS_VALUE ?value.
  BIND(xsd:float(?value) AS ?elec_gen_value)
}