the 3 puzzle pieces
With generous Solar & Battery Rebates available, homeowners are turning to solar and battery systems to achieve greater energy independence. But sizing your system correctly can be puzzling.
At Positive Energy Solutions, our goal is to help design solar & battery systems to maximise your long-term self-sufficiency, so that you’re generating, storing and using your own clean energy as much as possible.
In this blog, we’ll break down the three essential “puzzle pieces” for sizing a solar and battery system:
- Array size
- Inverter size
- Battery size
We’ll explain why each matters, share real-world examples, and emphasize the importance of thinking ahead. Getting this right can save you money and avoid making expensive modifications later on.
PUZZLE PIECE #1: array size
The solar array – that’s the solar panels themselves. If the goal is maximum self-sufficiency, you need enough solar panels to generate enough energy to cover your entire 24-hour daily usage. For instance, if your household’s daily energy consumption is 25kWh, you’ll want enough panels to produce at least that amount.
But here’s the catch: don’t rely solely on averages. A medium-size 6.6kW system might generate about 25kWh per day on average, but in Perth’s winter months or on cloudy days, production can drop significantly.
In Perth, solar panel output during the Winter months is around 56% of the annual average daily production.
That’s why oversizing your array is key – it provides a buffer for those lower-output periods, while also accommodating for future increases in power usage, like charging an Electric Vehicle, switching from gas appliances to electric, or simply running the heating & cooling more freely. With solar prices at all-time lows, many of our customers are opting for larger systems in the 10–15kW range.
For example, a 13.2kW system is a common system size these days. It can produce around 50+kWh per day on average. But in Winter, that will dip to about 25kWh – still enough for most family homes. Come Summertime, you’re looking at 70+kWh per day, giving you the freedom to blast the aircon and stay cool without relying on the grid.
At Positive Energy Solutions, we always recommend a site-specific assessment to tailor the array size to your roof space and energy usage. Oversizing smartly ensures you’re prepared for whatever the weather – or life – throws your way.
PUZZLE PIECE #2: INVERTER SIZE
Not so long ago, when installing solar-only was the norm, it was standard practice to choose an inverter size that was 75% of the array size. For example, 6.6kW of panels with a 5kW inverter. It simply didn’t make sense to install 6.6kW of panels on a 10kW inverter. After all, 6.6kW of panels could never generate enough power to make use of a 10kW inverter.
However, those days are over. With a battery connected, you now have two sources of power to draw from. When solar power production falls short, the battery is there to pick up the slack.
So what is the correct way to size an inverter these days…?
To truly maximize self-sufficiency and avoid purchasing grid power, your inverter must be sized to handle your household’s peak loads.
Let’s illustrate with an example. If your total power usage spikes to 8kW (think multiple appliances running at once), a 5kW inverter can only supply 5kW – forcing you to buy the remaining 3kW from the grid. That’s not ideal, especially during the expensive Peak window (3pm – 9pm)
To avoid this, upgrade to a 10kW or 15kW inverter to cover higher loads without relying on the grid.
In a grid outage, the stakes are even higher. A 5kW inverter maxes out at 5kW in backup mode. Exceed that, and the system trips. This makes 5kW inverters suitable for backing up Essential Circuits only (lights, fridge, internet)
A 10kW or 15kW inverter, however, has a lot more punch! These larger inverters are much less prone to tripping from high loads. Some are even capable of Whole Home Backup.
*Note: Enabling backup power requires extra components and labour, which is often listed as an optional extra on your quote. This lets you decide if the added cost aligns with your needs*
And again, don’t forget to think ahead! Future additions like EV charging, induction cooking, and air conditioning could bump up your peak power usage. We often see customers future-proofing with larger inverters for peace of mind.
One more thing – At night, the inverter’s max output can be capped by your battery’s discharge limits, which brings us to our final piece of the puzzle…
puzzle piece #3: battery size
Batteries are what turn solar into a round-the-clock solution, storing excess daytime energy for nighttime use. There are two main factors to consider:
- Capacity (how much energy it can store)
- Maximum Charge / Discharge (how quickly it can deliver & receive power)
For capacity, aim for a battery that can store enough kWh to cover your energy usage from 3pm – 9am. If your daily usage is 25kWh and you estimate 60% of that is used overnight, you’ll want at least 15kWh of battery capacity (25kWh x 60% = 15kWh)
But as with the other pieces, oversizing is smart for multiple reasons which we covered in our previous article:
Buying a Solar Battery? Size Matters – https://positiveenergy.com.au/buying-a-solar-battery-size-matters/
The second factor is Maximum Charge & Discharge capability, which ties back to handling peak loads. Max charge & discharge can vary depending on the battery brand and battery size.
For example, below is a table showing the maximum charge & discharge capability of different size BYD batteries paired with different size Fronius inverters. You’ll notice that smaller batteries can sometimes have disappointingly low charge & discharge capability
Just like your inverter, choose a battery with discharge capability that can match or exceed your peak loads to avoid purchasing expensive grid power and cope during grid-outages
CONCLUSION: FITTING THE PIECES TOGETHER
If you get any one of these three puzzle pieces wrong – array size, inverter size, or battery size – you could end up disappointed with your solar and battery system. It’s a significant one-time investment, so be sure to do it once and do it right.
At Positive Energy Solutions, we’re here to provide expert advice tailored to your specific needs. Contact us today and let’s solve the puzzle of maximizing your self-sufficiency and minimizing your bills!
Rest assured, our approach is honest, informative, no pressure.
