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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 10kWh battery with 90% DoD provides 9kWh of usable energy.
Example: a 10kWh battery with 90-95% DoD gives about 9-9.5kWh of usable energy. Batteries are modular, so check the usable figure on the spec sheet for the size you are quoted.
⚠ Not every battery qualifies. Tesla's Powerwall 3, for one, is not eligible for the WA Residential Battery Scheme rebate. Its integrated inverter is on Synergy's Supported Solutions List but not approved for the battery scheme. To claim the $130/kWh state rebate, choose a CEC-approved battery paired with an inverter that is on the Synergy SSL and approved for the battery scheme.
Size the battery to your evening and overnight use first, then make sure the array can fill it. With a battery connected, a hybrid inverter can carry a bigger array, up to the inverter maker's maximum, and a 5kW hybrid still keeps DEBS. Undersizing is the costlier mistake: adding capacity later usually means a second installation.
Array needed to fill the battery:
If you want backup power during blackouts, consider which appliances you need to run and for how long.
Essential loads (typical):
A 10kWh battery can power these essential loads for about 2-4 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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