Battery storage and electricity bills in a post-FiT world

Our solar system was installed in October, 2009, and due to the premium feed in tariff (FIT) that was available in Western Australia at the time we have not had a bill since then, only cheques.
Our latest bi-monthly invoice from Synergy (it’s a credit note really) arrived covers July and August, and it got me thinking about what the end of the premium FIT in 2019 will mean.
The extract below makes it pretty simple to determine that once 10 years is up our 40 cent premium FIT will end and with it the loss of that $202.80 credit. The overall credit for the period was $144.05 after subtracting our consumption from the grid.
This effectively means that I will have to pay for power for the first time in 10 years. Time to start planning how to avoid this.

As we have kept all our energy records over the past 7 years we determined that our energy consumption has largely remained constant at 11.8kWh/day, despite now have two energy monsters that were only babies when the system was installed.
We put this down to a couple of things. Firstly, all lighting was incandescent at the start and these were initially changed over to CFLs and then to LEDs about 5 years ago, after an extensive trialling period with various brands and styles. The second reason is that as electrical equipment has failed and needed to be replaced we have selected far more efficient appliances.
Our solar system has generated, since installation, a total of 38.4MWh. Our self-consumption is at 40%. This is actually pretty good as we tried to minimise the use of power during the day to get the maximum benefit from system. Once the premium FIT ends we will need to adjust our power usage to align usage with solar production. This will be easier with some items such as washing clothes, dish washer and to some extent cooking, but harder with others.
Our 3.5kW system has an orientation of 30 degrees West of North, so it suffers from being a bit slow to wake up in the morning. Being on the (now not available) Synergy SM1 Smart Power Tariff means that the morning peak in winter time (April to September) is from 7am to 11am.
Given our solar orientation, this means that the morning usage is almost all at peak rates for the winter months. At the other end of the day the winter night peak (5pm to 9pm) is also at peak rates with little or no solar contribution.
Based on the system production to date there is sufficient solar production even during the winter months to be energy self-sufficient. The issue is how to align production with usage.
If I look at the obvious choice and install a battery to cover the Peak, High and Low Shoulder energy usage then we would move from 40% to 73% self-usage. This would require a daily battery usage capacity of 3.7kWh. Well within the capability of a single Tesla Powerwall. To extend this to a theoretical 100%, the battery would need to supply 6.7kWh.
That is beyond the capacity of a single Powerwall. In reality, a single Tesla Powerwall would likely get us to somewhere in the high 90% range given peak loads would at times be beyond the capacity of the Powerwall and have to be taken from the grid. The existing solar system would easily handle the production required to charge the battery with exports back to the grid in each case. A 10% efficiency penalty has been assumed for energy sourced from the battery.
For each case the data has been modelled to determine what the impact would have been on the Synergy bill over the same period assuming the tariffs of today. In case 1, for offsetting only peak and low and high shoulder then the cost over the 6.83 years would have been $801.58 while for case 2, offsetting all imported energy then the costs drops to $459.80.
So, supplying 100% of our own energy while still remaining connected to the grid for times when the solar / battery system could not keep up with demand would cost around $0.18 per day. The case 3 option is to do nothing. For the privilege of giving away 27.4 MWh I would be billed $1.13 per day. Clearly the do nothing option is not for us.

The cost of the Powerwall is currently around $9,500 (fully installed) as an add on to the existing system, but this is today’s price and we don’t need it for 3 years.
David Martin in an “early adopter” and has had an interest in solar power since university, some 25 years ago, and has been in power systems all my working life.  “The reason I installed the solar system originally was to help provide a better future for my kids.  The cost was secondary.  It has however turned into a great investment.  I guess I just want to put the message out that there is a better option than burning fossil fuels.  I see the change as a gradual one that will ultimately be won of price.”