Investment income can be used to demonstrate the required financial income for the South African retired visas.
For temporary residency, the capital of the investment can be used to prove the prescribed amount on a monthly basis (for a maximum period of 48 months). If two (a couple), were to apply for temporary residency, then the prescribed amount would need to be proven for each applicant per month (for a maximum period of 48 months).
For permanent residency, the investment income must show that this is a life-long guaranteed income that is received monthly by the applicant.