Solar + battery + EV: the Perth savings triple stack

Each technology changes a different part of the bill. Solar cuts daytime grid purchases. A battery shifts cheap solar to expensive evening hours. An EV replaces petrol with electricity, with the value depending on kilometres, petrol price, tariff, and charging timing. Combine all three and the savings can compound.

Here's why, and what the numbers look like in a modelled Perth scenario.

The compound effect

Each addition does more than save on its own. It also improves the economics of the others:

Solar alone: You generate cheap electricity but export most of it at a low feed-in rate, because you're at work during the day. Under Synergy's current DEBS scheme that export earns 2c/kWh outside the 3–9pm window and 10c during it, so a typical north-facing system selling mostly at midday earns close to the 2c floor. Self-consumption is typically 30-35%.

Solar + battery: Self-consumption jumps to 60-70%. The battery captures what you'd have exported for a few cents and uses it in the evening when you'd have paid 33c (A1) or 55c (Midday Saver peak). The battery's value depends entirely on having solar to charge it.

Solar + battery + EV: Now your solar has three jobs: power the house, charge the battery, and charge the car. Self-consumption can hit 80-90%. The EV becomes a massive daytime load that absorbs solar generation you'd otherwise export for pennies.

This is the compound effect: each piece makes the others more valuable.

The numbers for a typical Perth household

Let's model a family of four in Joondalup with:

• 25 kWh/day electricity usage
• One car doing 15,000 km/year
• Currently spending $2,000/quarter on electricity (A1 tariff) and $3,000/year on petrol

These are illustrative, modelled figures on Synergy's rates from 1 July 2026, not a real household. They assume a blended export credit of around 7c/kWh; Synergy's current DEBS scheme actually pays 2c/kWh outside 3–9pm and 10c during it, so a north-facing system exporting mostly at midday would see a lower export credit and a slightly longer payback than the table below. Run your own numbers in the Savings Planner.

Step 1: Solar only (6.6 kW)

Step 2: Add battery (10 kWh)

Step 3: Add EV + home charger

In this modelled scenario, the combined savings are the calculator output for the inputs shown above. Treat the table as an example of the method, not a fixed monthly saving: actual results depend on driving distance, charging timing, tariff choice, system size, and household load shape.

Why the EV changes the solar maths

Without an EV, a 6.6 kW solar system exports 70% of its generation at low feed-in rates. With an EV charging at home during the day (timer set for 10am-2pm), you can absorb an extra 8-12 kWh of solar generation daily.

That's 8 kWh that was earning you about 2c/kWh as midday export ($0.16/day) now offsetting 8 kWh you'd have bought at 33c ($2.66/day), or displacing 8 kWh worth of petrol (about $4.80 at $1.85/L).

The EV effectively converts your lowest-value solar output (daytime exports) into your highest-value use (fuel replacement). That's the triple stack.

The tariff play: Midday Saver + EV Add-On

This combination is specifically designed for solar + battery + EV households. Synergy's rates from 1 July 2026:

• Midday Saver base: 8.85c super off-peak (9am-3pm) for daytime loads
• EV Add-On: 19.92c overnight (11pm-6am) for EV charging when solar isn't available
• Peak avoidance: Battery covers the 55.33c peak window (3-9pm)

On this tariff stack, your effective electricity cost across 24 hours is input-dependent. The real blend depends on when you draw power, how much the battery covers during peak windows, and how often the EV charges from solar versus overnight grid energy.

System sizing for the triple stack

Don't size for today. Size for the full stack:

The key insight: oversize the solar. A 10 kW system costs around $1,000–1,500 more than 6.6 kW but generates 50% more energy to split between house, battery, and car. The marginal cost per additional kW of solar is the cheapest energy investment you can make.

Upfront cost and combined payback

Actual payback depends on system size, battery size, tariff, export profile, install price, EV kilometres, charging timing, and household load shape. Use the calculator result with the shown inputs rather than relying on a fixed payback figure.

Long-run savings should be treated the same way: model them from the current inputs, then stress-test them against battery warranty, vehicle use, tariff changes, and replacement timing.

The order matters

If you're building the triple stack over time:

1. Solar first. Always. It usually has the shortest estimated payback of the three, and it enables everything else.
2. Battery second. Once solar is generating, the battery captures value you're currently losing to low export rates.
3. EV + charger third. The car decision is driven by vehicle costs and lifestyle, but once you have solar + battery, the running cost argument is overwhelming.

Installing a battery before solar, or an EV charger before solar, doesn't make financial sense. Solar is the foundation.

Related reading

• Is a home battery worth it in Perth in 2026?: battery-specific payback calculations across tariffs.
• EV charger installation in Perth: install costs, solar charging and Synergy's EV plan.

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Model your own triple stack: Our Savings Planner lets you toggle solar, battery, and EV simultaneously and see the combined savings for your specific usage and tariff. Or try the Calculator for a detailed 25-year projection.

Sources: Solar Choice