How to size a home battery for Perth: matching capacity to your evening load

The most common battery sizing question Perth households ask: "Do I need 10kWh or 15kWh?" The honest answer is: it depends on your evening consumption pattern, not your total daily usage. Here's how to work it out.

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The key question: what is your evening load?

Your battery needs to cover the electricity you draw from the grid after solar stops generating. In Perth, solar effectively stops generating useful power by 5:30–6pm (summer) to 4:30pm (winter). Everything after that point comes from either your battery or the grid.

Typical Perth evening consumption (5pm–11pm):

• 1–2 person home: 4–8 kWh
• 3–4 person home: 6–12 kWh
• Large home with pool + heating: 10–18 kWh

Important: this is consumption from 5pm onwards, not your total daily usage. A household using 25 kWh/day might only use 8–10 kWh after solar stops.

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How to find your own evening load

If you have a smart meter (most Perth homes installed after 2015):

1. Log in to synergy.net.au MyAccount
2. Navigate to "Energy Usage" or "My Usage History"
3. Find interval data (half-hourly or hourly)
4. Sum up usage from 5pm to midnight: this is your typical evening load

Do this for a few representative days (one weekday, one weekend, one hot summer day, one winter day) to get a range.

Alternatively, check your inverter monitoring app if you have existing solar: look at grid import after 5pm. This is what your battery needs to cover.

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Three sizing scenarios

Scenario 1: Backup and occasional support

Who this suits: Households mainly wanting blackout protection and modest bill reduction; the battery isn't expected to cover all evening consumption.

Target coverage: 50–70% of evening load

Sizing: 5–10kWh usable capacity

Example: 4-person household with 10 kWh evening consumption. A 6.5kWh usable battery covers about 65% of the evening. The home draws grid for the last few hours. Bill reduction is meaningful but not maximum.

Battery options in this range: BYD Battery-Box Premium HVS 7.7 (7.7kWh), Alpha-ESS Storion 5.1kWh.

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Scenario 2: Full daily cycling for maximum bill reduction

Who this suits: Households on Synergy Midday Saver wanting to capture all solar midday generation and use it in the evening to minimise grid import.

Target coverage: 90–100% of typical evening load

Sizing: Evening load + 15–20% buffer for efficiency losses

Example: 4-person household with 10 kWh evening consumption. Target battery: 10 × 1.15 = 11.5kWh usable. Products near this range: BYD HVS 10.2kWh, or a modular battery stacked to about 12kWh.

For households on Midday Saver, the battery must also be sized to absorb midday solar excess, not just discharge in the evening. If your solar generates 25 kWh/day and you self-consume 10 kWh during the day, the battery needs to capture up to 15 kWh of surplus. A 10–15kWh battery handles most of this; excess is exported.

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Scenario 3: Battery + EV charging

Who this suits: Households adding an EV and wanting to charge it primarily from stored solar.

Target coverage: Evening household load + EV overnight top-up from battery

Sizing consideration: EV charging from battery is only practical for partial top-ups. A 7.2kW EV charger discharges a 10kWh battery in under 90 minutes. For EV-from-battery scenarios:

• Size for evening household load (as above)
• Keep a separate solar-charging window for the EV (daytime solar direct to car)
• Use battery for household, solar for EV

Attempting to run both household and full EV recharge from a standard residential battery is not realistic without a very large (20kWh+) battery system.

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Undersizing is the costly mistake

Size the battery to your evening and overnight use: the energy you buy from the grid between sunset and the next morning's solar.

• Erring small costs more than erring a little large. Adding capacity later usually means a second installation, and the battery you add may need to match the one you have.
• Your evening load is likely to grow. An EV, a heat pump or other electrification adds evening and overnight use over the battery's life.
• Check, don't guess. A battery far bigger than your evening and overnight use can't empty each night, so size it from your bill or smart meter data rather than a rule of thumb.

A practical target: cover your typical evening and overnight use across most of the year, plus the loads you know are coming. Ask your installer whether the battery is modular and what adding a module later would involve.

If your evenings are heavy, also look at the array. A bigger array with the battery often wins, because the extra daytime generation fills the battery as well as covering the day.

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Accounting for battery efficiency losses

Lithium-ion batteries have a round-trip efficiency of approximately 90–95% (LFP chemistry) to 92–97% (some other chemistries). This means for every 10 kWh of solar stored in the battery, approximately 9.0–9.5 kWh is available for evening use.

When sizing, add 5–10% to your target usable capacity to account for this efficiency loss.

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Battery capacity figures refer to usable capacity (which may be less than nameplate capacity depending on the depth of discharge setting). Verify usable capacity specifications with your installer.