What is the average energy generation of all energy sources considered?
SELECT (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 ?electricity_generation.
  FILTER(REGEX(?energy_sources_labels, "all sources"))
  ?electricity_generation orkgp:HAS_VALUE ?value.
  BIND(xsd:float(?value) AS ?elec_gen_value)
}