As more Australian households install solar panels, the question of whether to add a battery—and how big it should be—has become central to distributed energy planning. Battery sizing is not a one-size-fits-all calculation; the optimal capacity depends heavily on whether your primary goal is backup power during outages or maximizing self-consumption of your solar generation. This article breaks down the two approaches, with concrete examples, costs, and trade-offs to help you decide.
Understanding the Two Use Cases
Before diving into sizing, it's essential to distinguish between backup and self-consumption. A battery sized for backup is chosen to power critical loads during a grid outage. A battery sized for self-consumption is chosen to store excess solar energy for use in the evening, reducing grid imports. These goals lead to different capacity, power, and chemistry requirements.
Backup Power Sizing
Backup sizing focuses on three variables: the loads you want to keep running, the duration of an average outage, and the depth of discharge (DoD) you're willing to accept. For example, a typical Australian home might designate a refrigerator (150W), lights (100W), internet router (20W), and a chest freezer (200W) as critical loads, totaling around 470W. If you want to run these for 12 hours, you need at least 5.6 kWh of usable capacity. Factoring in inverter efficiency (~90%) and DoD (e.g., 80% for lithium-ion), the gross battery capacity required would be about 7.8 kWh.
Common backup targets include:
- Short outages (2-4 hours): 2-5 kWh usable capacity, often covered by a small battery like the Tesla Powerwall 2 (13.5 kWh gross, 13.5 kWh usable at 100% DoD) but sized down for cost.
- Extended outages (8-24 hours): 10-20 kWh usable capacity, requiring larger batteries like the LG Chem RESU10H (9.8 kWh usable) or the BYD Battery-Box Premium HVS (10.2 kWh usable).
- Whole-home backup: 20+ kWh usable, often requiring multiple batteries or a generator.
In practice, most Australian homes can achieve critical-load backup with a single 10-13 kWh battery. The Tesla Powerwall 2, for instance, offers 13.5 kWh of usable capacity and can power essential circuits for a day or more, depending on load. However, if you need to run air conditioning or an electric oven, you'll need a larger capacity and a higher inverter rating.
Self-Consumption Sizing
Self-consumption aims to store excess solar generation for later use, reducing electricity bills. The optimal battery size depends on your daily solar surplus and evening consumption. A typical Sydney home with a 6.6 kW solar system might generate 25 kWh on a sunny day, with daytime consumption of 10 kWh, leaving 15 kWh of surplus. If evening consumption is 12 kWh, a battery with 12 kWh usable capacity would cover most of it. However, on cloudy days, surplus may be only 5 kWh, making a smaller battery more cost-effective.
Key factors for self-consumption sizing:
- Solar system size: Larger systems produce more surplus, requiring larger batteries.
- Consumption profile: Homes with high daytime usage need less battery; those with low daytime usage need more.
- Electricity tariffs: Time-of-use (TOU) tariffs with high peak rates increase the value of shifting solar energy to evening.
- Feed-in tariffs (FiTs): Low FiTs (e.g., 5-8 c/kWh in NSW) make self-consumption more valuable than exporting.
A common rule of thumb is to size the battery at 1-1.5 times the average daily solar surplus. For a 6.6 kW system in Melbourne, with average daily surplus of 10-12 kWh, a 10-13 kWh battery is typical. For a 10 kW system in Brisbane, surplus might be 20-25 kWh, suggesting a 20-25 kWh battery.
Financial Trade-Offs: Backup vs. Self-Consumption
The economics differ markedly between the two use cases. Backup power provides resilience but rarely pays for itself through energy savings. Self-consumption can reduce electricity bills by $500-$1,500 per year, depending on tariff and location.
Backup Economics
Batteries sized for backup typically have a longer payback period because they are not fully cycled daily. For example, a Tesla Powerwall 2 installed for $12,000 (including installation) might be cycled only 20 times per year during outages, yielding negligible bill savings. However, the value of uninterrupted power can be high for those in bushfire-prone areas or with medical needs. Some households pair backup with self-consumption to improve economics.
Self-Consumption Economics
Self-consumption batteries are cycled daily, generating savings by avoiding grid imports. In Victoria, with a flat tariff of 25 c/kWh and FiT of 5 c/kWh, each kWh stored and used saves 20 c. A 13.5 kWh battery cycled 300 days per year saves about $810 annually. With an installed cost of $12,000, the simple payback is 14.8 years—longer than typical battery warranties (10 years). However, with TOU tariffs (peak 40 c/kWh, off-peak 15 c/kWh), savings can reach $1,200 per year, reducing payback to 10 years.
The table below compares typical scenarios:
| Scenario | Battery Size | Annual Savings | Installed Cost | Payback |
|---|---|---|---|---|
| Backup only (Sydney) | 10 kWh | $100 | $10,000 | >20 years |
| Self-consumption (Melbourne, flat tariff) | 13.5 kWh | $810 | $12,000 | 14.8 years |
| Self-consumption (Brisbane, TOU) | 13.5 kWh | $1,200 | $12,000 | 10 years |
For a deeper dive into the financial metrics, see our article on Solar Payback vs. Investment Returns.
Technical Considerations: Chemistry, Power, and Depth of Discharge
Battery chemistry affects sizing. Lithium-ion (LFP or NMC) offers high DoD (80-100%), long cycle life (6,000-10,000 cycles), and high efficiency (90-95%). Lead-acid batteries are cheaper but have lower DoD (50%) and shorter life, requiring larger gross capacity for the same usable energy. For backup, lead-acid may still be viable for occasional use, but for daily cycling, lithium-ion is strongly recommended.
Power rating matters for backup: if you need to start a refrigerator compressor (surge current up to 1,200W), the inverter must handle it. A 5 kW inverter is common for critical loads; a 10 kW inverter may be needed for whole-home backup. For self-consumption, a 3-5 kW inverter is usually sufficient to cover evening loads.
Depth of discharge: For backup, you may discharge to 100% DoD occasionally, but frequent deep discharge reduces cycle life. For self-consumption, many manufacturers recommend limiting DoD to 80% to maximize longevity. The Tesla Powerwall allows 100% DoD; the LG Chem RESU10H recommends 80% for warranty.
Sizing Examples for Australian Homes
Let's walk through two real-world examples.
Example 1: Backup-Focused Home in Adelaide
A 4-person household in Adelaide with a 6.6 kW solar system wants to keep fridge, freezer, lights, and internet running during outages (average 4 hours, but occasional 24-hour events from storms). Critical load is 500W. For 24 hours, they need 12 kWh usable. With a 90% efficient inverter and 80% DoD, gross capacity = 12 / (0.9 * 0.8) = 16.7 kWh. They choose a 13.5 kWh Powerwall 2 (13.5 kWh usable at 100% DoD). However, to cover 24 hours, they may need to reduce load or add a second Powerwall. They opt for a single Powerwall and accept that during extended outages, they may need to run the generator for a few hours. Cost: $12,000 installed.
Example 2: Self-Consumption Focus in Brisbane
A 3-person household in Brisbane with a 10 kW solar system (average daily generation 40 kWh) has daytime consumption of 15 kWh, leaving 25 kWh surplus. Evening consumption is 18 kWh. To cover 80% of evening consumption (14.4 kWh), they need a battery with 14.4 kWh usable. They choose the BYD Battery-Box Premium HVS 15.4 (15.4 kWh usable, 100% DoD). With TOU tariff (peak 38 c/kWh, off-peak 14 c/kWh), they save $1,500 per year. Installed cost: $14,000. Payback: 9.3 years. They also get backup capability for critical loads.
Tools and Methods for Sizing
Several online tools can help. SolarQuotes' battery sizing calculator uses your postcode, system size, and consumption data. Alternatively, you can use a spreadsheet approach:
- Obtain your daily solar generation profile (e.g., from your inverter app).
- Obtain your 24-hour consumption profile (smart meter data or energy monitor).
- Calculate hourly surplus (generation minus consumption).
- Sum positive surplus to find daily excess.
- Size battery to capture a percentage (e.g., 80%) of that excess.
For backup, list critical loads, multiply by desired backup duration, and add a safety margin (20-30%).
For more on the broader economics, read The Complete Guide to Distributed Energy Economics.
Battery Sizing for Different Tariffs
Tariff structure significantly influences optimal sizing. On flat tariffs, the ideal battery size is determined by daily surplus. On TOU tariffs, the goal is to shift as much solar energy as possible to peak periods, which may require a larger battery to store energy from midday and discharge during the evening peak (4-9 pm). Some retailers like Origin Energy and AGL offer virtual power plant (VPP) programs that pay for discharging during peak events, which can increase the value of a larger battery.
For example, with AGL's VPP, participants receive $300-$500 per year for allowing remote discharge during peak times. This additional income can improve payback by 2-3 years. See our article on How to Calculate Solar Payback Period for more details.
Common Mistakes and Best Practices
- Oversizing for backup: Buying a 20 kWh battery for backup when critical loads are only 500W leads to wasted capacity and cost. Instead, size for actual needs.
- Undersizing for self-consumption: A 5 kWh battery may fill up by noon on sunny days, leaving surplus exported at low FiT. A larger battery captures more value.
- Ignoring inverter capacity: A battery with 10 kWh capacity but a 2.5 kW inverter cannot power high-draw appliances. Ensure inverter rating matches peak loads.
- Forgetting about warranty: Daily cycling to 100% DoD may void warranty on some batteries. Read the fine print.
For a comprehensive guide, see Battery Sizing for Home Solar Storage.
Conclusion
The choice between backup and self-consumption sizing hinges on your priorities: resilience or bill reduction. Most households benefit from a hybrid approach—a battery sized for daily self-consumption that also provides backup for critical loads. In Australia, a 10-13 kWh lithium-ion battery (e.g., Tesla Powerwall 2, LG Chem RESU, BYD HVS) is the sweet spot for both use cases. Use the tools and methods above to find the right size for your home, and always get multiple quotes from installers like Solaray Energy or Natural Solar.
Remember, battery economics are improving as costs fall. In 2024, average installed prices in Australia range from $1,000 to $1,200 per kWh. With solar FiTs declining, self-consumption is becoming more attractive. Evaluate your goals, crunch the numbers, and choose accordingly.
Related Articles
- The Complete Guide to Distributed Energy Economics
- How to Calculate Solar Payback Period
- Solar Payback vs. Investment Returns
- Battery Sizing for Home Solar Storage