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Size your battery to run on stored solar through the evening, not grid power
Small household
Average household
Large household
Look at your electricity bill or smart meter data to find your average daily usage in kWh. This is the foundation for sizing your battery.
Typical Australian household usage:
The key to battery sizing is understanding how much energy you use when the sun isn't shining (typically 5pm-7am). This is what your battery needs to cover.
Rule of thumb:
Evening/night usage is typically 50-70% of your total daily consumption. For a household using 30kWh/day, that's 15-21kWh overnight.
Most lithium batteries can safely discharge to 90-95% of their capacity. A 13.5kWh battery with 90% DoD provides 12.15kWh of usable energy.
Example: Tesla's Powerwall 3 is rated at 13.5kWh of usable capacity with 90% DoD.
⚠ Powerwall 3 is not eligible for the WA Residential Battery Scheme rebate. Its integrated inverter is not on Synergy's Supported Solutions List. To claim the $130/kWh state rebate, choose a CEC-approved battery that appears on the Synergy SSL.
Your battery should be matched to your solar system size. A larger solar system can charge a bigger battery during the day.
Recommended ratios:
If you want backup power during blackouts, consider which appliances you need to run and for how long.
Essential loads (typical):
A 13.5kWh battery can power essential loads for 3-5 days during an outage
A home battery is not a UPS. Switching to backup power takes a moment, so it must not be relied on for life-support equipment. If someone in your home depends on powered medical equipment, tell your installer and your electricity network operator.
The WA State rebate provides $130 per kWh, capped at $1,300 (10kWh maximum) for eligible Synergy customers. Above 10kWh, additional capacity earns no extra state rebate — only federal STCs:
If you're on a Synergy time-of-use tariff, a battery can shift your usage out of the expensive evening peak and into cheaper off-peak periods, on top of your solar self-consumption.
Schemes like Synergy's Battery Rewards let your battery join grid services and earn you extra income, which can shorten your payback period.
Get matched with local WA installers, no obligation.
Our battery sizing calculator reads your usage patterns and recommends a battery size for your home
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