Fast read
Sizing a battery for a large family home requires matching its capacity to your daily energy use and solar system. A household using 25-30 kWh daily often starts with a 10-15 kWh battery to cover evening needs. If you plan to charge an electric vehicle overnight, you'll need a much larger capacity, possibly over 20 kWh, or a modular system you can expand. Analysing your electricity bill is the crucial first step to determine your specific requirements.
How Big Is Big Enough for a Family Solar Battery?
Choosing a solar battery is one of the most significant decisions you’ll make in your home’s renewable energy journey. For a large, active family, the promise of slashing electricity bills and gaining energy independence is incredibly appealing. But it also brings a wave of uncertainty. How big is big enough? Can you get one that’s too big?
We understand it can feel overwhelming. You’re trying to balance cost, future needs, and current energy habits. This guide is here to walk you through the process calmly and clearly, helping you understand the key factors so you can confidently choose the right battery size for your family.
How much energy does a large family home actually use?
Before you can size a battery, you need a clear picture of your energy appetite. A “large family home” in Australia, typically with four or more people, often uses significantly more electricity than average.
While the average Australian home might use around 16-20 kWh per day, a large family with multiple appliances, ducted air conditioning, a swimming pool, and teenagers who never turn anything off can easily consume 25 kWh, 30 kWh, or even more. Households with five or more people average around 25.4 kWh daily.
The most crucial step you can take is to look at your recent electricity bills. Find the “average daily usage” figure, usually shown in kWh. This number is the foundation of your battery sizing calculation. Pay attention to how it changes between summer and winter, as heating and cooling can cause significant seasonal spikes.
The ‘Goldilocks’ principle: Why not just get the biggest battery?
It might seem logical to simply install the largest battery you can afford, but that’s often not the most effective or economical strategy. A solar battery works by storing the excess, unused solar power your panels generate during the day.
The key is balance. Your battery needs to be small enough that your solar panel system can reliably charge it full most days. There is little value in having a huge 20 kWh battery if your 6.6 kW solar system can only spare 8 kWh of excess energy to fill it after running the house during the day.
The goal is to find the “just right” size that covers most of your evening and overnight usage without being oversized and underutilised. This ensures you get the best possible return on your investment.
A starting point for your calculation
To get a good preliminary idea of a suitable battery size, you can use a couple of trusted industry rules of thumb.
First, consider your night-time usage. For most families, about 30% of energy is used during daylight hours while the sun is shining, and the remaining 70% is used at night. To calculate your overnight need, multiply your average daily usage by 0.7.
Example: Your bill shows an average daily use of 28 kWh.
28 kWh x 0.7 = 19.6 kWh.
This means you need approximately 19.6 kWh of stored energy to get through an average night.
This calculation immediately tells you that a standard 10 kWh battery would cover your expensive evening peak usage but might not last the entire night. A battery in the 13-15 kWh range would be a closer match for full overnight independence on most days.
Another common guideline is to match the battery capacity to your solar system size, often by multiplying the system’s kilowatts (kW) by 1.5 or 2.
**Example:** You have a 10 kW solar system.
10 kW x 1.5 = 15 kWh.
This suggests a battery around 15 kWh would be a good fit, as the solar system should be able to generate enough excess power to charge it.
The big question: Do you have an electric vehicle?
The rise of the electric vehicle (EV) is the single biggest factor changing how families size their home batteries. An EV is essentially a very large battery on wheels, and charging it at home can double a household’s daily electricity consumption. A typical EV requires 15-20 kWh to travel 100km.
If you own or plan to buy an EV, you have a few options:
Go big on battery storage: If you intend to charge your EV from your home battery overnight, you’ll need to factor this extra consumption into your calculations. For a family using 25 kWh and adding a 10 kWh daily charge for the car, the total storage needed approaches 35 kWh. This often means installing multiple battery units, for instance, two Tesla Powerwalls provide a combined 27 kWh of usable capacity.
Use smart charging: The most economical option is to use a smart EV charger that can be programmed to charge only when there is excess solar power being generated during the day. This avoids draining your home battery entirely overnight.
Consider future technology: Emerging Vehicle-to-Home (V2H) technology will allow your EV to act as your home battery, powering your house during peak times. While not yet widespread in Australia, it is a significant future consideration.
Putting it all together: A real-world example
Let’s imagine a family of five in Perth with a pool and ducted air-conditioning. They have a 10kW solar PV system installed.
Their electricity bill shows their average daily usage is 30 kWh. Their overnight usage is roughly 70% of that, which is 21 kWh.
Scenario 1: No EV. A battery with around 13-15 kWh of usable capacity would be a great fit. It would eliminate the majority of their grid usage overnight, leading to significant bill reductions. A modular battery, like a Sungrow SBR, could be a smart choice, allowing them to start with a 12.8 kWh capacity and add more modules later if their needs change.
Scenario 2: They buy an EV. They drive about 50km per day, requiring about 10 kWh of charge. Their total overnight energy need is now 31 kWh (21 kWh for the home + 10 kWh for the car). A single 15 kWh battery is no longer sufficient. They would need to look at higher capacity solutions. For example, a Sigenergy SigenStor is an all-in-one system that integrates EV charging and can be scaled with 5 kWh or 8 kWh battery modules, allowing them to build a system large enough to meet this high demand.
Questions to ask your installer
When you’re ready to get quotes, you are armed with the knowledge to have a confident conversation. Be sure to ask any potential installer:
- What is the usable capacity of the battery you are quoting? This can be lower than the advertised “nominal” capacity.
- Is this battery system modular?
- Can I add more storage capacity in a few years?
- What will this battery actually run during a blackout, and for how long?
- Can you show me a projection of how this specific battery size will cover my nightly usage based on my energy data?
Ultimately, choosing the best battery size is a personal decision based on your unique circumstances. By understanding your energy consumption, considering your future needs, and balancing it with the capacity of your solar system, you can make a choice that empowers your family with clean, reliable energy for years to come.
If you’re feeling ready to take the next step, Your Energy Answers can connect you with a network of trusted, SAA-accredited local installers who can provide a detailed analysis and tailored quotes for your home.
