Should you charge your battery from the grid overnight? Perth Midday Saver maths

Most Perth solar battery households assume the battery should only ever charge from solar. On Synergy's Midday Saver tariff, there's a more nuanced strategy available: charging the battery from the grid overnight at off-peak rates and discharging it during peak hours.

Here's when this makes financial sense and when solar charging remains the better approach.

The Midday Saver arbitrage opportunity

Midday Saver tariff rates (effective 1 July 2026):

• Super off-peak (9am–3pm): 8.85c/kWh
• Peak (3pm–9pm): 55.33c/kWh
• Off-peak (all other hours, including overnight): 24.34c/kWh

The overnight → peak spread: Purchase from grid overnight: 24.34c/kWh Discharge to avoid grid during peak: saves 55.33c/kWh Net saving per kWh cycled: 55.33c − 24.34c = 31c/kWh (before battery round-trip losses)

After round-trip efficiency losses (~8%): approximately 28.5c net saving per kWh

Comparison with solar charging: Solar charges battery at effective cost of 8.85c/kWh (super off-peak rate if you'd otherwise export at that rate) → discharges at 55.33c peak Net saving per kWh: approximately 46c

Solar charging is still more profitable — but overnight grid charging at 24.34c still generates a positive return if you have unused battery capacity that solar won't fill the next day.

When overnight grid charging makes sense

Perth winter with limited solar

Perth June–July solar generation is significantly reduced (3.5 PSH vs 7.0 in summer). A battery that would fully charge from solar in summer may only reach 50–60% charge from solar in winter. If you're going to import from the grid during peak hours anyway (because the battery ran out), pre-charging from the cheaper overnight rate is better.

Winter decision rule: If your solar is unlikely to fully charge your battery the next day (forecast overcast or short winter day), pre-charging overnight at 24.34c is worthwhile.

Summer decision rule: If solar will fill the battery (clear day with good generation forecast), don't force overnight grid charging — you'd be paying 24.34c for electricity you'd otherwise get for free from solar.

Multi-day cloudy periods

Extended cloudy weather in Perth (rare but possible in winter) means solar generation may be inadequate for several consecutive days. Pre-charging from overnight off-peak ensures the battery is available for peak-hour discharge each day, avoiding a situation where the battery sits empty through peak hours.

Extended peak-hour events

Some days have unusually high consumption during 3pm–9pm (a dinner party, a hot summer afternoon where the AC runs harder than usual). If you forecast high peak-hour consumption, ensuring the battery is full going into peak — including via overnight top-up — maximises savings at 55.33c/kWh rates.

When NOT to force overnight grid charging

Summer clear days: Solar will fill the battery in the first few hours of generation. Overnight pre-charging wastes 24.34c on electricity that would arrive for free at 8.85c.

When the battery is already full: If you cycled conservatively the previous evening and the battery is 70%+ charged overnight, pre-charging to 100% costs money with minimal benefit.

When peak consumption is low: If your household has low consumption during 3pm–9pm (away from home, no cooking, moderate AC), the peak-avoidance saving is smaller. At very low peak consumption, the overnight charging cost may not pay back within that evening's cycle.

How to implement overnight charging

Most modern battery systems can be scheduled to charge from the grid:

Tesla Powerwall (via Powerwall app): Set a "Reserve for Power Outages" level, then enable "Time-Based Control." In Time-Based Control, you can set the battery to charge during off-peak periods.

BYD Battery-Box / other batteries via compatible inverter: Fronius GEN24+, SolarEdge StorEdge, and similar hybrid inverters have time-of-use scheduling via their apps. Set "grid charge" enabled during overnight off-peak window.

Home Assistant or third-party automation: More precise control — trigger grid charging based on forecast solar generation, battery SoC, and peak hour timing.

Simple approach: Many battery apps allow you to set a target SoC (e.g. "charge to 80% from grid by 3am"). For Perth winter, setting a nightly schedule achieves the arbitrage passively.

Worked winter example

Scenario: Perth July day, forecast 60% cloud cover, 4 PSH estimated. 10kWh usable battery, currently at 30% (3kWh remaining) at midnight.

Without overnight charging:

• Solar generates ~5kWh into battery (reaches ~80% charge)
• Peak discharge: 8kWh from battery, evening grid import: minimal
• Outcome: reasonable

With overnight charging (charge to 90% by 6am):

• Charge 6kWh from grid at 24.34c = $1.46
• Solar generates ~5kWh into battery (already full by midday; must export at 2c)
• Peak discharge: 9kWh from battery
• Saving from peak avoidance: 9kWh × 55.33c = $4.98
• Net benefit: $4.98 − $1.46 − export cost lost = approximately $3.30

In this winter scenario, overnight charging generates positive value. On a clear summer day, the solar would have filled the battery without grid assistance and the overnight charging cost would have been pure waste.

Synergy Midday Saver tariff rates effective 1 July 2026. Battery scheduling options vary by brand and system configuration. Verify current tariff rates at synergy.net.au before implementing a charging strategy.