I just ran a quick calculation in PV Watts to double check our figures and using the default figures it returned 38,918kWh per year. I would really question the 51,000kWh per year figure for a 32kW system in Perth unless you are using the best panels (e.g. Sunpower with very low combined losses and superior generation guarantee warranties), the best inverters, oversized wiring and scheduling cleaning of the panels at regular intervals (e.g. fortnightly). Remember this is the generation averaged over the 25 year life of the system (not the first year’s generation).