What size battery is best for me? This question is often asked, and many people have different answers. The following is the writer's opinion. It may resonate with you, or you may completely disagree. Whichever is fine. This is an opinion for a balance between storage and cost.
The power from your battery needs to last from when the sun goes down until the sun comes back up again so that you can be self-sufficient when there is no generation.
It takes time to charge your battery. On a typical day, will you be able to charge your battery before the sun goes down? If you can't, that's not necessarily bad, as you can import power from the grid at generally very cheap prices. It is not uncommon with LocalVolts to be paid 3c/kWh to take from the grid. It is less common with Amber Electric due to their higher prices that have resulted from their price hedging fund, but negative buy prices can still occur.
This directly relates to how long it takes to charge your battery. The power draw your house load uses during the day also impacts how long it will take to charge your battery, as only excess solar (the surplus after using it for your house load) charges the battery by default. The more panels you put on your roof, the higher the solar generation, and the quicker the battery will charge. Note that these days, it may be more cost-effective not to put up additional panels, but to buy from the grid when it is extremely cheap or there is a negative buy price. If there are no extremely cheap daytime prices, like on bad weather days, you would have to purchase from the grid at a higher rate. However, people with lots of solar panels would also face low generation, but having not purchased extra panels, you'd probably still be in a better financial position.
If you want to be self-sufficient, you will need to be able to survive the night without taking from the grid. You will need to have that capacity available in your battery.
Smaller batteries generally cannot discharge as quickly as larger batteries. So if you get a small battery, expect low discharge rates. Ideally, you want to be able to discharge and export to the grid as fast as possible since the price spike periods are generally very limited.
The kW that you can export to the grid is dictated by your local electricity distributor. Generally, this is 5kW per phase, but some dynamic connections allow more while also being able to restrict to less.
Unfortuntately batteries degrade over time. In five year's time your battery may only be able to hold 85-90% of its original capacity. You may want to take this into consideration when right-sizing your battery.
Once you have answers to all of the above questions, you can think about the right size for your battery to suit your needs and also to make you some money.
Principles:
So, in this example, I would recommend a 25kWh battery: 15kWh to get you through the night and 10kWh available to export during a two-hour price spike. There will be times when you have excess battery (when there are no price spikes), but you can take this as an opportunity to export at a cheaper rate if no price spikes are forecast. Optimally, you should try to time the early morning sun with a battery that is nearly at the minimum SoC, but not quite.