Choosing the right battery size is one of the most important decisions in a solar battery system. Size too small and you run out of stored energy before sunrise. Size too large and you are paying for capacity that rarely gets used. This guide explains how to work out the right size for your home in Perth and why the timing of that decision matters more than most people realise.
Why Getting the Size Right Matters
A battery that is too small will drain before morning, leaving you drawing from the grid at peak rates overnight. A battery that is too large charges and discharges at a shallow depth, reducing how hard it is working for you relative to what you paid.
The goal is to size your battery so it covers your typical evening and overnight usage, fully charged from the day’s solar, without significant excess capacity sitting idle. That said, there is one important reason to err slightly on the larger side, which we cover later in this guide.
Step 1: Find Your Daily Energy Usage
Your starting point is your Synergy electricity bill. Look for your daily average consumption, shown in kilowatt-hours (kWh) per day. Most Perth households fall somewhere between 15kWh and 30kWh per day, depending on home size, occupancy, and whether you have a pool, ducted air conditioning, or an electric vehicle.
If you do not have a recent bill handy, check the MyAccount portal on the Synergy website. Your usage history is available by billing period and you can calculate a daily average from there.
Your daily usage figure is on every Synergy bill. It is the single most useful starting point for sizing your battery correctly.
Step 2: Work Out Your Evening and Overnight Usage
Your solar panels produce energy during the day. Any solar that is not used immediately either goes into the battery or gets exported to the grid. The battery’s job is to store that surplus and use it once the sun goes down.
The relevant figure is not your total daily usage. It is specifically how much energy your home consumes from roughly 4pm through to when you wake up the next morning. For most households, this evening and overnight window represents 40 to 60 percent of total daily consumption.
Example: if your home uses 20kWh per day and roughly half of that happens in the evening and overnight, you are looking at around 10kWh of overnight demand that the battery needs to cover.
Rule of thumb: take your daily kWh usage and multiply by 0.5. That is a reasonable starting estimate for the battery capacity you need to cover a typical Perth night.
Rough sizing estimates for common Perth household usage levels:
These are estimates only. A proper assessment by a Positive Energy Solutions energy consultant will account for your actual usage profile, time-of-use patterns, and any planned changes to your home energy use.
Step 3: Factor in Your Electricity Tariff
The tariff you are on has a meaningful impact on how your battery should be used and how much capacity makes sense for your situation.
Synergy A1 (Standard Tariff)
On the A1 tariff, the import rate is the same at all times of day. Your battery’s main job is to store solar surplus and displace grid electricity at night. A battery sized to cover your overnight usage gives you the best return on your investment.
Synergy Midday Saver
The Midday Saver tariff offers heavily discounted electricity between 9am and 3pm. This opens up an additional strategy not available on the A1 tariff: charging your battery from the grid at the low-cost midday rate and then using that stored energy during the more expensive morning and evening peak periods.
This means that even if your solar panels do not fully charge your battery on a given day, you can top it up from the grid at a low rate and still displace expensive peak-rate electricity later. For homes with flexible loads, Midday Saver also allows you to shift appliance use to the low-cost window, reducing the demand on the battery.
Midday Saver enables energy arbitrage: charge low-cost, use when expensive. This can meaningfully improve your battery’s financial performance and may allow a slightly smaller battery to cover your needs effectively.
The interaction between panel size, battery size, and tariff choice is worth modelling before you commit. Positive Energy Solutions can run both scenarios for your specific usage.
Step 4: Consider Your Solar System Size
A battery is only as useful as the solar that charges it. If your solar system is undersized relative to your battery, the battery may not reach full charge on a typical day, particularly in the shorter winter months.
As a general guide:
- A 6.6kW solar system in Perth generates an average of 25-30kWh per day in summer and around 15-20kWh in winter
- A 10kW system generates roughly 38-45kWh per day in summer and 25-30kWh in winter
- The portion available to charge a battery depends on how much is consumed during the day
DC Coupling and Panel Oversizing
DC-coupled battery systems connect the solar panels and battery through a shared inverter. This is more efficient than AC-coupled systems and allows panels to be oversized above the inverter’s rated capacity. For example, a 10kW inverter with 13kW of panels can still capture more solar across the day, including the early morning and late afternoon periods when output per panel is lower.
Oversizing panels above the inverter capacity is a common and effective approach for maximising battery charge in Perth conditions. Your Positive Energy Solutions energy consultant will assess how much solar can be realistically installed on your roof and design the system accordingly.
If you are adding a battery to an existing solar system, your historical Synergy export data will show exactly how much surplus is currently going to the grid and how much a battery could capture.
Step 5: Understand How Backup Changes the Sizing Equation
If whole home backup is a goal, the key constraint is not just battery capacity. It is inverter size.
In a grid outage, your battery inverter limits how much power can be drawn at once. A 5kW inverter can supply 5kW of continuous power in backup mode, enough to run lights, a fridge, and some appliances, but not an air conditioner and electric oven at the same time.
Whole home backup requires an inverter rated at 10kW or above for most Perth homes. That requirement comes before you even consider battery capacity.
Load Management in Backup Mode
Even with a correctly sized inverter and battery, whole home backup requires load management during a grid outage. Running multiple high-draw appliances simultaneously, such as ducted air conditioning, an oven, and a washing machine, can overload the system and cause it to trip. Planning which loads you will prioritise during an outage is an important part of getting value from a backup system. The monitoring app that comes with your battery system makes this straightforward, giving you real-time visibility over what is drawing power and how quickly the battery is depleting.
If backup power is important to you, tell your Positive Energy Solutions energy consultant upfront. Backup capability affects which inverter and battery combination is right for your home. It is much easier to plan for at install time than retrofit later.
For three-phase homes, the situation is more nuanced. Some inverter brands can only back up one phase from a single unit, which limits which circuits can be protected. Your energy consultant can walk you through the options for your specific setup.
For a detailed breakdown of how each brand handles backup in Perth, see the Positive Energy Solutions backup power guides for Tesla Powerwall 3, Fronius GEN24 PLUS, Sungrow, and SigEnergy SigenStor.
Monitor Your System and Adjust Your Habits
Every battery brand that Positive Energy Solutions installs comes with a user-friendly monitoring portal, accessible through an app on your phone or via desktop. These platforms give you live and historical visibility over your solar generation, battery state of charge, and household energy consumption.
In practical terms, this means you can see:
- How quickly your battery is depleting at any point in the day or evening
- Your total household power draw at any point in time
- How much solar you generated and how much went to the battery versus the grid
- Your energy self-sufficiency over days, weeks, and months
This information is valuable beyond just curiosity. Customers who actively use their monitoring app are better placed to shift habits, for example running the dishwasher earlier in the evening when the battery is fuller, or noticing that the total draw spikes when certain appliances are running and adjusting when they are used.
Over time, monitoring your usage patterns through the app is one of the most effective ways to reduce how much you draw from the grid and get more value from the battery capacity you have installed.
Common Perth Household Profiles and Typical Sizing
Every home is different, but the following profiles cover the most common situations Positive Energy Solutions encounters across Perth.
EV charging is handled separately from the household battery. The most cost-effective approach is either to charge during the Midday Saver low-cost window (9am to 3pm) when electricity rates are low, or to switch to the Synergy EV add-on tariff which offers lower-cost rates from 11pm. Running your EV charger through the household battery is not the recommended approach, as the battery is sized for household loads. Your Positive Energy Solutions energy consultant can advise on the best EV charging setup for your situation.
Why It Pays to Size Up Now, Not Later
Battery systems are modular. Most of the brands Positive Energy Solutions installs allow you to add more battery modules after the initial installation. In theory, you could start smaller and expand later.
In practice, this approach costs significantly more in the long run.
The WA Battery Rebate Is a One-Time Claim
The WA Home Battery Scheme rebate can only be claimed once per property. If you install a smaller battery now and want to add another module later, the additional module attracts no rebate. You pay full retail price for that extra capacity.
The Federal Cheaper Home Batteries Program
The federal Cheaper Home Batteries Program provides a discount at the point of installation, applied by the installer. From 1 May 2026, this discount follows a tiered structure based on the total capacity installed:
- 0 to 14 kWh: full support rate applied
- 14 to 28 kWh: support rate applied at 60%
- 28 to 50 kWh: support rate applied at 15%
The first 50kWh of installed capacity is eligible. Like the WA rebate, this discount is claimed once at installation. Additional modules added later receive no federal support and are charged at full retail price.
The tiered structure means the strongest financial incentive sits at the 14kWh mark, where you capture the full support rate. Above that threshold the per-kWh value of the federal discount tapers, though a meaningful discount still applies up to 28kWh.
The Combined Impact
Between the WA rebate and the federal Cheaper Home Batteries Program discount, the effective cost of battery capacity at installation time is materially lower than the cost of adding modules later. Sizing up at install time, even slightly beyond your immediate needs, almost always works out lower-cost than expanding later at full retail price.
If you are deciding between a 13kWh and 16kWh battery, both fall within the full federal support tier. The incremental cost of going larger at install time is almost always lower than adding that capacity later without any rebates. Your Positive Energy Solutions energy consultant can model both scenarios with current pricing.
This is also worth considering if your circumstances are likely to change. Planning to get an EV in the next few years? Expecting more people at home? Building a pool? Sizing up now locks in the discounted rate for capacity you will eventually use.
Next Step: Get a Proper Assessment
Battery sizing is not a one-size-fits-all calculation. Your Synergy bill, tariff, solar system size, roof space, backup requirements, and future energy plans all feed into the right answer for your home.
Positive Energy Solutions offers a no-obligation energy consultation for Perth homeowners. We will review your actual usage data, model the scenarios that suit your situation, and give you a clear recommendation on battery size with the numbers to back it up.
The best battery for your home is the one that is right-sized for how you actually use energy. Book a consultation with Positive Energy Solutions today.
Related guides
- How Much Can You Save with Solar and Battery in Perth? 2026 Breakdown
- What Your Solar and Battery Installation Day Looks Like in Perth: A Step-by-Step Guide
- Adding a Battery to Your Existing Solar System in Perth: 2026 Costs, Rebates, and Inverter Compatibility Guide
- Before Your Solar Is Installed in Perth: What to Expect From Sign-Off to Installation Day
Frequently Asked Questions
How do I find out how much energy my home uses?
Check your Synergy electricity bill for your daily average consumption in kWh. You can also log in to MyAccount on the Synergy website to view your usage history by billing period. Alternatively, speak to one of our Positive Energy Solutions energy consultants, who can pull your usage data and provide a personalised analysis and recommendation.
What happens if my battery runs flat overnight?
In normal circumstances the system automatically switches to drawing from the grid, so there is no interruption to your power supply. In backup mode during a grid outage, if the battery runs flat there will be no further power supply until either the grid is restored or your solar begins generating again the following morning. This is why load management during an outage is important, and why the monitoring app is a useful tool for keeping an eye on battery state in real time.
Can I add more battery capacity later?
Yes, most battery systems are modular and can be expanded. However, the WA battery rebate and the federal STC discount both apply at the time of installation only. Adding modules later means paying full retail price for that additional capacity. It is almost always more cost-effective to size correctly from the start.
Does a bigger battery mean I need a bigger solar system?
Not necessarily. Your solar system needs to be large enough to charge your battery on a typical day, but available roof space is also a factor. If your roof is small and limits how many panels you can install, the Midday Saver tariff can supplement battery charging by allowing you to top up from the grid at low-cost midday rates and use that stored energy during peak periods. This energy arbitrage approach can make a larger battery viable even where rooftop solar capacity is constrained. Your Positive Energy Solutions energy consultant will assess what can realistically fit on your roof and design the system accordingly.
Does my tariff affect how big a battery I should get?
Yes. Midday Saver is typically the better tariff for battery owners, as it allows you to charge from the grid at a low rate during the 9am to 3pm window and use that energy during the more expensive morning and evening periods. This works best when you avoid drawing from the grid during on-peak hours, which would reduce the financial benefit. On the standard A1 tariff, the battery relies solely on solar surplus for charging, so sizing needs to account for that more carefully.
What is the minimum battery size to get the WA rebate?
The WA Home Battery Scheme applies to systems up to 10kWh in capacity. Systems larger than 10kWh are still eligible. The rebate just caps at the first 10kWh of installed capacity.
Is 13kWh enough for a typical Perth family home?
For a family of three or four with average usage around 18-22kWh per day, 13kWh will typically cover most overnight demand but may not get you all the way through on higher usage days. Many Perth families find 16-19kWh gives more consistent overnight coverage. Your Positive Energy Solutions energy consultant can run the numbers based on your actual Synergy data.
How do I know if my battery is working efficiently?
Every battery system installed by Positive Energy Solutions includes a monitoring app that gives you live and historical data on solar generation, battery state of charge, and household consumption. If you notice your battery is consistently running flat before morning, or that switching on certain appliances causes a significant spike in draw, that is useful information for adjusting your habits or discussing a capacity upgrade with your energy consultant.
About the Author
Jason Wiktorowicz
Sales Manager, Positive Energy Solutions | Perth, WA
Jason has been at the centre of Western Australia’s energy transition for over a decade. As Orchestration Lead on Project Symphony — WA’s largest virtual power plant pilot — and Stream Lead on Project Encore, he worked directly with AEMO, Western Power and Energy Policy WA to shape how distributed energy resources connect and operate on the SWIS. That work included developing and rolling out CSIP-AUS, now mandatory for all new WA grid connections. He also led Emergency Solar Management (ESM), the first program of its kind in WA, VPP for Schools (VPP4S), which provided direct market benefits through Supplementary Reserve Capacity, and REBAR, the WA Residential Battery Rebate scheme.
His earlier career spanned Synergy, Infinite Energy and Curtin University’s Sustainability Policy Institute, where he co-authored peer-reviewed research on net-zero urban precincts, solar integration and peer-to-peer energy trading, published in the journal Urban Planning in 2018.
At Positive Energy Solutions, Jason leads sales with a focus on system design accuracy and long-term performance. Positive Energy Solutions installs solar and battery systems, EV chargers and heat pumps, and holds partner accreditations with Fronius, SigEnergy, iStore and SunPower. Connect with him on LinkedIn.
