What size battery do I need?: A quick guide

Battery storage system sizing is significantly more complicated than sizing a solar-only system. While solar panels generate energy, batteries only store it, so their usability (as well as their value) is based first and foremost on the energy available to fill them up (which usually comes from your solar panels). The size of the solar system installed (or to be installed) will usually be the primary dictator of the size range of the batteries which can be paired with it, followed by the home’s energy consumption levels and usage patterns; if a home uses a lot of energy during the day, there will be less solar available to go into the batteries.

In fact, the most important thing to look at is the relationship between the amount of energy that the household consumes on a daily basis and the amount of energy that the solar panel system produces day-to-day.

Further compounding the matter is the fact that people have different goals when it comes to home solar batteries: The primary goal for most people is to save money, but energy independence and backup power are also held in fairly high regard. Battery storage system sizing is therefore a very particular and highly individualised matter. If a battery is too large, it will be underutilised, and if it is too small it may not achieve the energy independence goals of the household where it is installed.

To account for this in the table, where the solar system size is large enough we’ve included two figures: The first being the maximum recommended battery size for financial purposes (trying to optimise for payback period and return on investment), and the second being the recommended maximum for energy independence (the number of days the home can go without drawing from the grid). Please note that systems whose purpose is energy independence will be significantly more expensive and usually have much poorer payback times than those that don’t because of the need for redundant, excess capacity which will sit idle and unused most of the time.

Bearing all this in mind, the table below aims to provide some ballpark figures, which should at least work as a starting point for solar & system size selection. The limitations of the below table include:

• Figures are based on a scenario where the solar panels are north-facing and pitched at a 30 degree tilt angle – other tilt angles and orientations will bear different (and often less favourable) results
• The figures should be appropriate for most of Australia, but may be a bit optimistic for Victorian and Tasmanian homes, where larger solar systems may be necessary
• The model scenario also assumes that the household uses a significant amount of energy during daylight hours (‘high days, evening peak’ usage pattern) – a home that uses less energy during the day could potentially benefit from a larger battery bank size (depending firstly on the size of the solar system)
• The battery system sizes are given in kilowatt-hours (kWh); in our modelling, this number refers to the full, usable capacity of the battery in question, after ‘depth of discharge‘ is factored out

• ‘Maximising returns’ – refers to the battery largest battery bank size (in kilowatt-hours, kWh) that can be installed which the solar system can charge up to full capacity at least 60% of the days of the year. The figures in this table are for the largest recommended size; smaller battery banks will usually offer better returns.
• ‘# days of energy autonomy’ denotes the maximum estimated number of consecutive days that a home can go without drawing energy from the grid in the event of several days of bad weather; you can use this as an approximation of how ‘off-grid ready’ your system will be (as long as your energy consumption levels don’t increase). Best practice is to have at least 3-5 days of energy autonomy built into your system if you plan on going off-grid (which is an option you should pursue only after careful consideration).
• ‘# days blackout protection’ refers the the amount of time that you’d be able to continue consuming energy as per usual in the event of a grid outage (you may be able to extend this by rationing your energy usage). Having blackout protection requires some excess battery capacity, as well as equipment that enables backup power functionality (read more here). We’ve only included a figure for battery backup in instances where the battery can be charged to full capacity at least 60% of the days of the year.