Solar Battery Size Calculator: What size battery do I need?

Concept of a home energy storage system based on a lithium ion battery pack situated in a modern garage with view on a vast landscape with solar power plant and wind turbine farm. 3d rendering.

What size solar panel array do you need for your home? And if you’re considering battery storage, what solar battery size would be most appropriate? This article includes tables that provide an at-a-glance guide, as well as links to more comprehensive calculators.

Solar system sizing table (no batteries)

If you’re considering solar panels for your home, you’ll get the most value from them by directly ‘self-consuming‘ the energy that they produce during daylight hours. The simple reason for this is that using the solar yourself allows you to avoid purchasing expensive energy from the grid instead of earning only a small credit for sending it into the grid. This is a crucial fact about going solar in Australia today.

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Your rate of self-consumption will depend on the amount of energy that you use daily, the pattern of your consumption throughout the day, and the size of the system that you install. You can increase your solar self-consumption ratio by running more of your devices during daylight hours – possibly manually, on timers or using a home energy management system.

The table below contains very rough solar self-consumption ratio estimates for a range of popular solar system sizes and energy consumption levels. Generally, we recommend keeping to a system size that means your self-consumption ratio remains above 30%.

 Your daily energy consumptionSelf-Consumption Ratio for Different Solar System Sizes
2kW Solar3kW Solar5kW Solar7kW Solar10kW Solar
5-10kWh30%25%17%13%9%
11-15kWh48%38%26%20%15%
16-20kWh57%46%34%26%20%
21-25kWh66%53%40%32%24%
26-30kWh73%59%44%37%28%
31-40kWh82%67%50%42%34%

Remember: The table above is a highly generalised, indicative guide; it does not take into account your location or the tilt & orientation of your roof – not to consider system prices or financial details like payback period. If you’d like to take a more detailed look, use our Solar PV System Payback Estimator or our Simple Solar System Sizing Estimator.

The below video gives a run-through on how to select the right solar system size

Picking the Correct Solar and Battery System Size

Using Sunwiz’s PVSell software, we’ve put together the below table to help shoppers choose the right system size for their needs. PVSell uses 365 days of weather data Please read the paragraphs below and remember that the table is a guide and a starting point only – we encourage you to do more research (and get more opinions) before deciding on a system size.

 About solar & battery system sizing

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.

About solar batteries & energy goals

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.

 A few caveats…

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

The solar battery size calculator

The solar battery size table

Your daily energy consumption
Largest recommended battery size for…
Solar system size
2kW 3kW 5kW 7kW 10kW
5-10kWh
Maximising returns: 5kWh 4kWh 4kWh 4kWh 4kWh
Maximising energy independence: 22kWh (3 days of energy autonomy) 35kWh (5 days of energy autonomy) 35kWh (5 days of energy autonomy) 35kWh (5 days of energy autonomy)
11-15kWh
Maximising returns: 4kWh 7kWh 9kWh 8kWh 8kWh
Maximising energy independence: 40kWh (3 days of energy autonomy) 65kWh (5 days of energy autonomy) 65kWh (5 days of energy autonomy)
16-20kWh
Maximising returns: 3kWh 6kWh 13kWh 12kWh 11kWh
Maximising energy independence: 55kWh (3 days of energy autonomy) 90kWh (5 days of energy autonomy)
21-25kWh
Maximising returns: 2kWh 5kWh 12kWh 16kWh 15kWh
Maximising energy independence: 23kWh (0.77 days of blackout protection) 70kWh (3 days of energy autonomy)
26-30kWh
Maximising returns: 1kWh 4kWh 11kWh 18kWh 18kWh
Maximising energy independence: 40kWh (1.5 days of blackout protection)
31-40kWh
Maximising returns: 3kWh 10kWh 17kWh 23kWh
Maximising energy independence: 27kWh (0.7 days of blackout protection)

 Glossary for this table

  • ‘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.
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Since 2008 our knowledge and sophisticated software have allowed over 300,000 Australian households and businesses to make a well-informed choice on their solar & battery installer.
Jeff Sykes

Comments

  1. Can you tell me when new back up batteries ( new technology) will be available and when the government will subside there batteries .
    I have 5 ks system Thank You

    1. Hi Sinikk. Backup power is already possible with the right battery setup. As for a government subsidy, a few small jurisdictions (ACT and City of Adelaide) offer battery incentives, but there is nothing on the federal level.

      Hope this helps!

  2. These tables are all well and good but when determining the economic criteria it falls apart because of the current huge disparity in costs between different battery storage makes. A Powerwall 2 with a stated capacity of 13.5 kWh and a cost of say $12000 installed compared to a Sonnen 6kwh with a installed cost of $12000 it’s a no brainer whatever else is variable i.e. Size of PV system home consumption or usage patterns. Even if you don’t use the full capacity of the PW2 battery it still represents better value and you don’t need a complicated chart to tell you that. Or am I missing something?

    1. Hi Patrick,

      Thanks always for your thoughtful input. You are well on point with this observation. We’ve written lots of articles on the financial case for solar & batteries and regularly point out (e.g. here) that – for now – solar makes more sense on its own. We’ve also pointed out that smaller battery banks tend to have faster payback periods than larger ones for the majority of homes. Our calculators yield the same results as well.

      The focus of this article is sizing, with the aim of educating people about how large a battery bank they can actually make use of – giving them a starting point for shopping around. This will help people to avoid being oversold on system size, especially in instances when the bigger battery bank does indeed cost significantly more than the smaller one. If the larger battery bank is more cost-effective and can operate at partial states of charge (so that it’s not a problem if it doesn’t reach ‘full’ most of the time – if ever), then by all means the larger one would be the smarter choice – and leaves room for expansion.

      As you’ve pointed out there are some significant price disparities at present, but we expect these to even out as time goes on and the market matures. At that point in time, these tables will become even more relevant.

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