South Australians are heading to the polls in an election widely regarded as the most unpredictable in the state's history, with a record number of early votes meaning a result may not be clear until next week.

The South Australian Electoral Commission says it is expecting more than 300,000 declaration votes, where ballots are cast at pre-poll centres, as absent votes or by post.

This represents about a quarter of the total votes in the state, where just over 1.2 million people are registered for Saturday's election.

Labor Premier Jay Weatherill is bidding to win a record fifth term in a row for the ALP while Opposition Leader Steven Marshall is hoping to seize power after falling just short in 2014.

The man in the middle is SA-BEST leader Nick Xenophon, who is seeking to win a seat in parliament's lower house after quitting the Senate.

He also hopes to have enough of his colleagues elected to hold the balance of power in the SA parliament.

Power prices, energy security and jobs have been the key issues while Labor is battling the 'it's time" factor after 16 years in office.

The government has also been hit by fallout from the Oakden nursing home scandal and problems over the protection of vulnerable children in state care.

The election may ultimately be decided by what happens with Mr Xenophon's challenge, and with several independents who could strike deals with the major parties to form a minority administration.