Its a bit of a rubbery maths equation and the answer will vary depending on your feed in rate, daily consumption, daily production, time difference between production and consumption, and your billing tariff.
In our case, we installed a 5kw system that cost $10,300 around 8 to 10 years ago (cant remember exactly when?). At the time our average bill was $800 to $1000 a quarter. We were luck and locked in the 55c feed in tariff, and our buy price is 23c. Our daily production is around 25kw, depending on the whether and time of year. We are hi consumption users as we have 5 fridges, 2 water pumps (house tank & shed tank), an enviro HSTP which runs an air pump 24/7 and sulage pump on float valve, a pond with 2 constant supply water pumps and UV filters, heated spa on a timer, and we're lazy and leave everything on standby.
For us, the system was paid for from consumption savings and feed in rebates after roughly 7 years according to the meter on the inverter. We still get a bill every quarter, but in summer its about $70 to $100, and in winter it goes up to around $200. We could most likely reduce the bill if we better managed when we used our energy so we used more at night and less during the day so we fed more back at the higher rate, and once the feed in tariff reverts to the standard 8c I'll probably put a concerted effort into it.
