So what if we go off grid entirely?
According to our power bill, we use 816 kWh in 95 days, or 8.6 kWh a day. This is actually crazily low, and AGL actually states that the average 1 person household uses 1216 kWh in 95 days or 12.8 kWh a day. Chalk that up to efficient LED lighting, off-peak hot water and no air-conditioning.
At 8.6 kWh a day, we need at least two Powerwalls to cope with cloudy days. Even so, we would need to be extra frugal through long periods of cloudy weather.
Fully installed, we are unlikely to end up with much change out of $30,000.
On the plus side, we can now save our entire power bill, which includes the supply charge. In this case, $2.61 a day. Unfortunately, this actually means our payback time is around 31 years, even worse than being tied to the grid (at current electricity costs).
http://www.businessinsider.com.au/this-is-what-teslas-home-battery-powerwall-will-cost-to-run-in-australia-2015-6