Illegal connections, meter-tampering and billing errors have cost Eskom more than half a billion rand in the past 12 months.
Minister of Public Enterprises Barbara Hogan gave the figure in a written reply to a parliamentary question from Manie van Wyk (DA).
As she did so, reports emerged of Eskom executives receiving an average salary increase of 83 percent.
Executive salary and bonus hikes were raised at negotiations between Eskom and employees as the electricity generator yielded to a union refusal of 8.5 percent, tabling a 9 percent pay offer and upping a housing allowance from R1 000 to R1 500.
Another Eskom move that raised eyebrows was the purchase of World Cup tickets worth R12.6-million.
On losses to Eskom, Hogan said a range of electricity losses not arising through Eskom faults were "typically" because of electricity theft.
In the 2009/10 financial year, between 3 210 GWh and 5 136GWh - or up to R625-million in electricity - had been lost to illegal connections, meter-tampering and billing errors. This was enough electricity to supply a large number of communities.
Hogan said that although Eskom had lost 5.8 percent of the electricity generated in 2009/10, it had performed well because of the way it managed energy losses.
"The international benchmark exercise carried out in 2007 put Eskom in the first quartile of the top-performing distribution utilities in terms of total energy losses," she said.
"The Eskom result is within the benchmark parameters of 5.6 percent to 12 percent."
Hogan said Eskom's policy was to remove illegal connections immediately they were discovered and to fine the culprits.
Eskom did not supply electricity to informal settlements on land that was not zoned for residential use.
It had launched Operation Khanyisa in an attempt to change people's attitudes to legal electricity use and to encourage them to report illegal connections.
It had also proposed amending the Electricity Regulation Act to classify electricity theft as a crime, said Hogan.
She said fines were increased for customers found to have tampered with their meter more than once.
Eskom was to audit 33 percent of all large power consumers for illegal connections, Hogan said. Unlike household consumers, electricity was cut off to large power consumers found to have illegal connections because they "not only pose a revenue loss risk, they are also a safety hazard".