Sizing a Battery for Your Perth Home
You've probably seen recommendations like "a 3-bedroom house needs a 10-13kWh battery." Here's why that's an oversimplification, and how to think about what you actually need.
The Problem with Generic Recommendations
Bedrooms tell you almost nothing about energy usage. A couple in a 4-bedroom house might use half as much as a family of five in a 3-bedroom. Your actual usage patterns, appliances, and habits matter far more than floor plan.
What Actually Determines Your Battery Size
Forget about bedroom counts. The right battery size depends on how much electricity you use when the sun isn't shining - primarily evenings and overnight. During the day, your solar panels typically cover your usage directly.
Evening Usage: The Key Factor
Most households use the majority of their electricity between about 5pm and 10pm. This is when you're cooking, running the TV, using devices, and often running heating or cooling. This evening block is usually the biggest chunk of what your battery needs to cover.
The amount varies enormously between households. A family that cooks elaborate dinners with an electric oven, runs the aircon, and has everyone home watching screens will use far more than a couple who eat out frequently and go to bed early. Your actual evening usage might be anywhere from 4kWh to 15kWh depending on your lifestyle.
Overnight Usage: Usually Modest
Between roughly 10pm and 6am, most homes use relatively little - typically just the fridge, standby loads, and perhaps an air conditioner in summer. This is often 2-4kWh for most households, though it can be higher if you're running air conditioning overnight or have a pool pump on a timer.
Morning Usage: The Transition Zone
Early morning (6am to 9am) is interesting because your solar panels are starting to generate, but perhaps not at full capacity yet. Depending on your roof orientation and how much you use in the morning (showers, breakfast cooking, getting ready), you might draw from the battery, from solar, or a mix.
If your panels face east, they'll produce earlier. West-facing panels won't help much until later. This affects how much morning usage your battery needs to cover.
The Variables That Change Everything
Air Conditioning
This is the single biggest variable in Perth homes. A ducted system running during summer evenings can easily add 8-15kWh to your daily usage. Split systems are more modest, but still significant. If you want your battery to power evening aircon, you need substantially more capacity than if you're willing to pre-cool during the day using solar.
Pools
Pool pumps typically use 5-10kWh per day, but the timing is controllable. If you run your pump during peak solar hours (say, 10am to 2pm), it's powered directly by solar and doesn't affect your battery sizing at all. Smart pool owners schedule pumps this way specifically to avoid needing a larger battery.
Electric Vehicles
An EV can use 10-15kWh per charge session, potentially more than your entire evening household usage. If you charge from the battery, you'd need a much larger one. Most EV owners in Perth charge during the day when solar is generating, or overnight from the grid when rates are lower - either approach means the EV doesn't significantly affect battery sizing.
Hot Water Systems
Electric hot water uses 3-5kWh per day. Heat pump systems use less. Gas or solar hot water effectively removes this load entirely. If you have electric hot water that heats overnight, that's additional battery capacity you'd need. Many people switch hot water timing to heat during solar hours instead.
Understanding Your Own Usage
Rather than relying on generic recommendations, look at your actual data. If you have a smart meter (most Perth homes do), you can access interval data through Synergy's online portal. This shows your usage in 30-minute blocks, which reveals your actual patterns.
Pay attention to:
- Your total usage between 5pm and 10pm on typical days
- Your overnight usage (10pm to 6am)
- How much these vary between summer and winter
- How much varies between weekdays and weekends
A battery that covers your typical evening and overnight usage will handle most days. On unusually high-usage days (hot evenings when you run aircon for hours, for example), you might draw some from the grid - and that's fine. Sizing for your absolute peak usage would mean paying for capacity you rarely use.
Common Sizing Mistakes
Sizing Based on Total Daily Usage
If you use 25kWh per day, you don't need a 25kWh battery. Much of that usage happens during daylight hours when solar is generating. A battery half that size, or less, might cover your actual needs perfectly well.
Sizing for Absolute Energy Independence
Some people want to never draw from the grid. This usually requires an oversized battery that sits partially empty most days, costing more without delivering proportionally more benefit. Drawing a small amount from the grid on occasional high-usage days is usually more economical than oversizing your battery.
Ignoring Seasonal Variation
Summer and winter usage patterns differ dramatically. If you size your battery for the hottest summer evenings when the aircon runs constantly, it'll be oversized for the other 300+ days of the year. Think about what makes sense across the full year, not just the extreme days.
Questions for Your Installer
"What data did you use to recommend this size?"
A good installer will have asked for your bills or usage data first. If they're recommending without this, they're guessing.
"What percentage of my evening/overnight usage would this cover?"
This should be a specific answer based on your actual usage patterns.
"How would the economics change if I went one size up or down?"
Helps you understand whether you're getting good value for the capacity.
"What assumptions are you making about my aircon usage?"
Make sure their assumptions match your actual habits.
"How would an EV or pool affect this recommendation?"
If you're considering either, understand how they'd change the equation.
A Sensible Approach
Rather than asking "what size battery does a 3-bedroom house need?", ask "what are my actual evening and overnight energy needs, and what's the most cost-effective way to meet them?"
Get your usage data. Talk to installers who ask good questions about your lifestyle and habits. Be wary of anyone who gives you a firm recommendation without understanding your specific situation. And remember that a slightly smaller battery that fits your actual needs well is better value than a larger one that sits partially empty most days.