The right battery is not the biggest battery that fits on the wall. It is the capacity your solar system can charge, your household can use and your budget can support. Start with your evening and overnight electricity use, then test 10 kWh, 20 kWh and 30 kWh options against your solar exports and backup goals.
- Compare usable capacity, not only the battery’s headline size.
- Ask for every rebate, inclusion and exclusion to be shown on the quote.
- Verify current rules with the linked government source before committing.
10 kWh, 20 kWh or 30 kWh: what is the difference?
| Battery size | Often suits | Watch for |
|---|---|---|
| 10 kWh | Small to medium homes with moderate evening use | May not cover high overnight heating, EV charging or long outages |
| 20 kWh | Families with larger evening loads and more solar surplus | Needs enough solar to fill regularly |
| 30 kWh | High-use or all-electric homes seeking longer coverage | Higher cost and more capacity may sit unused |
These are planning categories, not promises. Usable capacity, output power, reserve settings and battery chemistry vary by product.
Use your bills and smart-meter data
Look for the electricity imported after solar generation falls away. If your home imports 9 kWh from 4 pm to 7 am on a typical day, a 10 kWh battery may already cover most of that energy, allowing for reserve and losses. If the home imports 18 kWh overnight, a 20 kWh system may be more appropriate. Seasonal heating and cooling can change the result.
Do not size a battery from quarterly spend alone. A bill shows cost, while sizing needs an energy profile in kWh and, for backup, the power demand of the circuits running at the same time.
Capacity is not the same as power
Capacity answers “how much energy can it store?” Power answers “how fast can it deliver it?” A battery may have enough kWh for overnight use but not enough continuous output for an induction cooktop, heat pump, pool pump and other loads at once. Ask for continuous and peak output, not only storage capacity.
When is 30 kWh too large?
A 30 kWh system can be oversized when the home has limited solar surplus, low overnight demand or a small inverter. Oversizing may also lengthen payback because unused capacity earns no additional savings. Conversely, a high-load home with an EV, electric hot water and evening heating may need more storage than a typical household.
A simple sizing process
- Record evening and overnight grid imports for at least two representative weeks.
- Estimate the solar surplus available to charge the battery.
- Choose how much backup reserve you want.
- Check continuous output against simultaneous loads.
- Compare the financial result of 10, 20 and 30 kWh using the same assumptions.
Start with the decision behind the search
People rarely search for this topic because they want another generic explanation. They are usually close to a decision. They may have received a quote, noticed a rebate advertised online, seen a fault code, or wondered whether their current solar system is still doing enough. The useful answer begins by identifying the decision and the evidence needed to make it.
For a homeowner who wants a battery size recommendation rather than a product pitch, the practical question is how much storage can be used regularly and whether the inverter can deliver the required power. That means a good assessment cannot rely on a product name or a single headline number. It has to connect the system to the property, the electricity profile, the electrical design, the program rules and the support available after installation.
This guide is written for the search intent behind queries such as “what size battery do I need, 10 kWh vs 20 kWh vs 30 kWh, battery sizing calculator”. It explains what to check, which questions to ask, and where a site-specific assessment is necessary. It does not replace an electrical inspection or a formal government eligibility decision.
What information should you collect before comparing options?
Gather the information an installer will need before asking for a recommendation. Start with your address and roof type, then record the age and model of any existing inverter, the approximate solar-system size, recent electricity bills and the times when the home uses the most energy. If you have interval data from a smart meter, provide it. It is more useful for battery sizing than a quarterly bill total because it shows the shape of your demand.
Take clear photos of the inverter, switchboard, meter box, roof access and the proposed equipment location. Do not remove covers. A photo cannot replace a site inspection, but it can reveal whether the quote needs to allow for a long cable run, a crowded switchboard, a three-phase connection or a difficult installation position.
Write down future changes as well. An electric vehicle, heat-pump hot water, induction cooking, ducted heating, a pool pump or a home office can alter the design. A system selected only for today’s bill may be too small later, while buying a great deal of unused capacity now can make the project less economical.
How to read the quote or recommendation
The quote should describe night-time imports, daytime solar surplus, usable capacity, reserve percentage, continuous output, peak output, seasonal demand, EVs, heat pumps and future expansion. If a line is missing, ask whether it is included, excluded or simply not required. A professional quote should also state the exact product models rather than relying on a brand family or a phrase such as “premium panels”. It should separate the pre-discount price from the discounts and show the final amount you are expected to pay.
Look for assumptions about annual generation, export limits, roof orientation, shading and electricity tariffs. These assumptions are not promises. They are inputs to a forecast. The more optimistic the forecast, the more important it is to ask what happens if your actual usage or export rate is lower.
Check the warranty schedule carefully. Product warranties cover the equipment. Performance warranties may describe output over time. Workmanship warranties cover the installation. They may be administered by different organisations. Ask who you contact first and whether labour, travel, removal and reinstallation are included if a component fails.
Common mistakes that create an expensive outcome
The first mistake is choosing from the advertised price alone. The cheapest headline may exclude the inverter, backup hardware, switchboard work, monitoring, scaffolding or difficult roof access. The second is comparing nominal battery capacity with another quote’s usable capacity. The third is treating a government program as an automatic entitlement rather than checking the current product, installer and application requirements.
Another common mistake is assuming that a standard grid-connected solar system will power the house during a blackout. It will normally shut down for network safety unless it has been designed with compatible backup equipment. A final mistake is failing to plan the service path. Solar is an electrical installation that should be monitored and maintained; when something fails, the person who sold the system should be easy to contact.
A practical comparison method
Put competing options into the same structure. Record the system size, usable storage, inverter output, expected annual generation, backup scope, final price and warranty term. Then add the unknowns: export restrictions, switchboard upgrades, network approval, installation timing and the assumptions behind the savings estimate.
Next, separate must-haves from preferences. A household may require backup for a medical device, while another may only want to shift solar energy into the evening. One property may need a battery-ready hybrid inverter, while another is better served by a carefully designed AC-coupled retrofit. There is no single “best” configuration for every roof.
Finally, ask the installer to explain the recommendation in plain English. If the answer depends on your actual consumption, ask which data would change the result. If the answer depends on a rebate, ask which official rule supports it and whether the quote is conditional on approval.
When a professional site assessment is worth it
A site assessment is especially useful when the home has multiple roof planes, shade, a two-storey roof, a crowded switchboard, three-phase power, an older inverter, limited wall space or a large proposed battery. It is also worthwhile when you want backup power, because the backup circuits and changeover equipment need to be designed rather than added as an afterthought.
The assessment should leave you with a clear next step: proceed, change the system size, repair or replace existing equipment, or wait until more data is available. A responsible recommendation can be “this option is not suitable” when the expected benefit does not justify the cost.
What a good next step looks like
The right next step is not always to buy immediately. For a homeowner who wants a battery size recommendation, it may be to gather interval data, confirm a product model, check the current government page, arrange an inspection or ask for a second quote using the same scope. That short pause can expose a missing cost or an assumption that would otherwise be hidden in a headline price.
Before you approve the work, ask the installer to repeat the recommendation in writing. The document should explain why this option fits the property, what could change the price, what the system is expected to do, and who will be responsible for commissioning, monitoring and support. If the answer depends on an incentive, ask for the official program name and the condition that makes the incentive available.
For a Melbourne property, local conditions can matter. Roof access, weather, shade, switchboard layout, distribution-network limits and the age of existing equipment all affect the design. A reliable proposal acknowledges those variables instead of promising an identical result for every home. Once you understand the scope and the assumptions, the choice becomes much easier to compare.
Keep your final quote, product manuals, compliance documents, monitoring login and warranty contacts together. These records make future repairs, upgrades and warranty claims faster. They also help a future installer understand the system if the original business is no longer available.
People also ask
Can an online estimate replace a quote?
No. Online estimates can help with early research, but roof access, electrical condition, product compatibility and network requirements affect the final scope.
Should I accept the first quote that includes a rebate?
No. Confirm the current program rules, compare the same scope with another quote, and check whether the discount is already deducted or merely estimated.
What is the most useful question to ask an installer?
Ask: “What part of this recommendation would change if my actual usage, roof condition or existing inverter is different from your assumption?” The answer reveals how carefully the system has been designed.
How do I know whether I need a repair or a replacement?
A technician should test the system first. Replacing equipment without confirming the fault can waste money, while delaying a safety issue can increase risk.
Sources and verification links
Government and industry sources checked for this article. Program details and pricing can change; verify current conditions before purchase.