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 13.5kWh?" The honest answer is: it depends on your evening consumption pattern, not your total daily usage. Here's how to work it out.

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.

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.

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), Tesla Powerwall 2 (13.5kWh, lowest usable = ~10kWh for daily use), Alpha-ESS Storion 5.1kWh.

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, Tesla Powerwall 2 (13.5kWh, excellent fit with ~12kWh usable at 80% DoD).

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 10–13.5kWh battery handles most of this; excess is exported.

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.

Oversizing: when more isn't better

Adding capacity beyond your typical evening load gives diminishing returns:

• A 13.5kWh battery on a household with 7kWh evening consumption regularly goes through the night half-full
• Half-full overnight means the battery starts each morning partially charged, which limits how much solar it can absorb the next day
• Unused capacity means you paid for kWh of storage that never turn into savings

The break-even point: a battery sized to cover approximately 80–90% of your evening load in most seasons is the economically optimal size for daily cycling. Perth winter has shorter solar days — sizing to summer needs means some winter evenings may draw a small amount of grid. That's acceptable; oversizing for winter performance makes the economics worse year-round.

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.

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.