One of the most common questions from homeowners considering solar battery storage is "what size battery do I need?" It's a crucial question because choosing the wrong size can significantly impact your return on investment. A battery that's too small won't provide the coverage you need, while an oversized battery means paying for capacity you'll never use. This guide walks you through a systematic approach to calculating the optimal battery size for your Australian home.
Step 1: Understand Your Daily Energy Consumption
The foundation of battery sizing is understanding how much energy your household uses. Start by gathering at least 12 months of electricity bills to capture seasonal variations. Look for your total kilowatt-hours (kWh) used over the year, then divide by 365 to get your average daily consumption.
The average Australian home uses between 15 and 25 kWh per day, but this varies enormously based on household size, the number and type of appliances, whether you have a pool or electric hot water, heating and cooling requirements, and whether anyone works from home. Your bills will give you a personalised baseline far more accurate than any average.
Pro Tip
Many electricity retailers now provide detailed usage data through online portals or apps. This data often breaks down your consumption by time of day, which is even more useful for battery sizing than monthly totals.
Step 2: Analyse Your Usage Patterns
Not all of your daily energy consumption needs to come from your battery. You only need to store enough to cover the periods when your solar panels aren't generating sufficient power. This means understanding when you use electricity throughout the day.
Identify Your Evening and Overnight Usage
For most households, the battery needs to cover consumption from around sunset until the following morning when solar generation resumes. This typically means covering about 12 to 16 hours of consumption, depending on the season. In summer, your battery might only need to cover from 8pm to 6am, while in winter, this window could extend from 5pm to 8am.
Estimate what percentage of your daily usage occurs during these non-solar hours. For many households, this is between 40% and 60% of total daily consumption. If you're home during the day and run major appliances while the sun is shining, your evening percentage might be lower. If everyone is at work or school and the house is empty during peak solar hours, your evening percentage could be higher.
Calculate Your Evening Energy Needs
Multiply your average daily consumption by your estimated evening usage percentage. For example, if you use an average of 20 kWh per day and estimate that 50% of that is consumed during non-solar hours, your evening energy need is 10 kWh. This figure represents the minimum battery capacity that would allow you to cover your typical evening usage entirely from stored solar energy.
Key Takeaway
You don't need a battery that matches your total daily consumption—you only need enough to cover your evening and overnight usage when solar panels aren't generating power.
Step 3: Consider Your Solar System Size
Your battery size should be proportionate to your solar system's generation capacity. There's limited benefit in having a 20kWh battery if your solar system only generates 10kWh of excess energy per day—you'll never fully charge it. Conversely, a tiny battery with a large solar system means you'll still export lots of cheap energy to the grid.
Calculate Your Excess Solar Generation
If you already have solar, check your monitoring app or inverter display to see how much energy you export to the grid on a typical sunny day. This exported energy represents what could potentially charge your battery instead. If you're installing solar and a battery together, your installer can model expected generation based on your system size, roof orientation, and shading.
A useful rule of thumb is that your battery capacity in kWh should roughly equal your solar system size in kW multiplied by 1 to 1.5. So a 6.6kW solar system pairs well with a battery between 6.6kWh and 10kWh. This ratio ensures your solar system can typically charge the battery on a sunny day while leaving capacity to power your home during daylight hours.
Step 4: Factor in Round-Trip Efficiency
Batteries aren't 100% efficient—some energy is lost in the charging and discharging process. Most lithium-ion batteries have a round-trip efficiency of 85% to 95%, meaning that for every 10kWh you put in, you get 8.5 to 9.5 kWh out. Factor this into your calculations by adding a buffer of 10% to 15% to your required capacity.
For example, if your calculations suggest you need 10kWh of usable storage, you might actually want a battery rated at 11 to 12kWh to ensure you can actually access that full 10kWh. Always look at usable capacity specifications rather than nominal capacity, as some manufacturers advertise higher nominal ratings that don't reflect real-world usable storage.
Step 5: Account for Seasonal Variations
Your energy usage likely varies significantly between seasons, particularly if you have electric heating or cooling. A battery that perfectly covers your summer evening usage might fall short in winter when days are shorter and heating demands are higher.
Winter Considerations
In winter, solar generation is reduced due to shorter days and lower sun angles, your battery needs to cover a longer evening period, and heating loads may increase your overall consumption. Consider whether your battery needs to cover your absolute peak winter demand or whether you're comfortable drawing some grid power during the coldest months. Most homeowners opt for a battery sized to cover typical conditions rather than worst-case scenarios, accepting some grid usage during winter extremes.
Summer Considerations
Summer brings its own considerations. Air conditioning can dramatically increase energy consumption and solar generation is typically at its highest, potentially providing excess charging capacity. Evening energy needs may be higher due to air conditioning running into the night. If summer evening cooling is a priority, ensure your battery has sufficient power output (kW) to run your air conditioning unit, not just sufficient capacity (kWh).
Step 6: Define Your Backup Power Requirements
If backup power during grid outages is important to you, this may influence your battery sizing. Not all batteries support backup operation, and those that do may have different capabilities.
Consider which circuits or appliances you want to power during outages. Essential loads might include refrigeration, some lighting, phone charging, and internet equipment. If you want whole-house backup including air conditioning, you'll need significantly more battery capacity and power output.
Backup Power Note
Backup power capability adds complexity and cost to battery installations. If outages are rare in your area, you might decide the extra expense isn't justified. If outages are common or you have medical equipment, prioritising backup capability makes more sense.
Step 7: Balance Size with Budget
Larger batteries provide more coverage and faster payback on a per-kWh basis, but they also cost more upfront. Finding the right balance between capability and cost is essential for a good return on investment.
Consider the price difference between battery sizes. Sometimes there's a "sweet spot" where a slightly larger battery offers much better value per kWh than a smaller option. Also consider your timeline—if you might expand your solar system in the future, buying a slightly larger battery now could make sense. Think about whether any rebates you're eligible for have capacity limits that make certain sizes more attractive. Finally, calculate the payback period for different battery sizes based on your electricity rates and usage patterns.
Putting It All Together: A Worked Example
Let's walk through a practical example. The Jones family uses 22 kWh per day on average and has a 6.6kW solar system that currently exports about 8 kWh to the grid daily. They estimate 50% of their usage (11 kWh) occurs during evening and overnight hours. Adding 15% for efficiency losses gives them a target of approximately 12.6 kWh.
Their solar system exports about 8 kWh daily, which is less than their evening needs. This suggests their solar system might be slightly undersized for complete self-sufficiency, or they could adjust usage habits to shift more consumption to daylight hours. Based on these calculations, a battery in the 10 to 13 kWh range would be appropriate. This would cover most of their evening needs while not dramatically overshooting what their solar system can charge.
Final Recommendations
After completing these calculations, discuss your findings with a qualified installer who can verify your assumptions and recommend specific products. Be prepared to share your electricity bills, existing solar system details if applicable, and any specific requirements like backup power needs.
Remember that some uncertainty in sizing is normal. Real-world conditions vary, and even the best calculations are estimates. A well-chosen battery in the right size range will serve you well, even if it's not perfectly optimised to the last kilowatt-hour.