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.
- Evening, 5pm to 10pm
- 4-15kWh
- Usually the biggest chunk your battery covers
- Overnight, 10pm to 6am
- 2-4kWh
- Often just the fridge and standby loads
- Ducted aircon
- +8-15kWh
- Can be added on summer evenings, the single biggest Perth variable
- Use 25kWh a day?
- Halfor less
- A battery that size might cover your actual needs
On this page6 sections
What actually determines your battery size
Forget bedroom counts. It 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
Most households use the majority of their electricity between about 5pm and 10pm: cooking, the TV, devices, and often heating or cooling. This block is usually the biggest chunk of what your battery needs to cover.
It varies enormously. A family that cooks elaborate dinners with an electric oven, runs the aircon and has everyone home on screens will use far more than a couple who eat out often and go to bed early. Your evening usage might be anywhere from 4kWh to 15kWh, depending on your lifestyle.
Overnight usage
Between roughly 10pm and 6am, most homes use relatively little: the fridge, standby loads and perhaps an air conditioner in summer. Often 2-4kWh, though higher if you run air conditioning overnight or have a pool pump on a timer.
Morning usage
From 6am to 9am your panels are starting to generate, but perhaps not at full capacity yet. Depending on your roof orientation and your morning (showers, breakfast, getting ready), you might draw from the battery, from solar, or a mix.
East-facing panels produce earlier. West-facing panels won't help much until later, which affects how much morning usage your battery needs to cover.
The variables that change everything
Timing matters as much as size. Most of these loads can be moved into solar hours.
| Load | Typical use | Effect on battery size | Keeping it off the battery |
|---|---|---|---|
| Air conditioningThe single biggest variable in Perth homes | +8-15kWhducted, can be added on summer evenings | Substantially more capacity if the battery powers evening aircon. Split systems are more modest, but still significant. | Pre-cool during the day using solar |
| Pool pump | 5-10kWhper day | None at all, if the pump runs during peak solar hours | Schedule it for peak solar, say 10am to 2pm. Smart pool owners do this to avoid needing a larger battery. |
| Electric vehicle | 10-15kWhper charge session | Not significant if you charge on solar or overnight from the grid. Charging from the battery would need a much larger one, since a session can use more than your entire evening household usage. | Most Perth EV owners charge during the day on solar, or overnight from the grid when rates are lower |
| Hot water | 3-5kWhper day, electric | Additional capacity if it heats overnight. Heat pump systems use less; gas or solar hot water removes the load entirely. | Switch the timer to heat during solar hours |
Understanding your own usage
If you have a smart meter (most Perth homes do), Synergy's online portal shows your usage in 30-minute blocks, which reveals your actual patterns.
Pay attention to
- Evening
- Your total usage between 5pm and 10pm on typical days
- Overnight
- Your usage from 10pm to 6am
- Seasons
- How much these vary between summer and winter
- Week
- How much varies between weekdays and weekends
Size for a typical day
A battery that covers your typical evening and overnight usage will handle most days.
On unusually high-usage days, like hot evenings running aircon for hours, you might draw some from the grid, and that's fine. Sizing for your absolute peak would mean paying for capacity you rarely use.
Common sizing mistakes
Each one ends in paying for capacity that sits partly empty.
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
Never drawing from the grid usually requires an oversized battery that sits partially empty most days, costing more without proportionally more benefit. Drawing a little from the grid on occasional high-usage days is usually more economical.
Ignoring seasonal variation
Summer and winter usage patterns differ dramatically. Size for the hottest summer evenings with the aircon running constantly and the battery will be oversized for the other 300+ days of the year. Think about the full year, not the extreme days.
Questions for your installer
Before accepting a size recommendation.
Before accepting a size
5 questionsA sensible approach
"What size battery does a 3-bedroom house need?"
"What are my actual evening and overnight energy needs, and what's the most cost-effective way to meet them?"
Do
- Get your usage data
- Talk to installers who ask good questions about your lifestyle and habits
- Remember a slightly smaller battery that fits your actual needs is better value than a larger one that sits partially empty most days
Be wary of
- Anyone who gives you a firm recommendation without understanding your specific situation
Find a battery sized for how you live
Want to take the next step?
Get a quote from an installer from our network that covers your area. You choose whether to act on it. No obligation.