Without having read it myself, perhaps they mean 5% of total usage. So the gas generation is built to be able to handle 2/3rds of the power demand, in case of outage as a backup, but in normal operation will only contribute 5% of the energy demand. That way, in the event of a failure of the renewable energy source for whatever reason, or a failure in the batteries, the gas can kick in and keep the servers online while cutting disposal operations that represent 1/3 of the total.