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 32c (A1) or 54c (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, 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)

| Metric | Value | |---|---| | Annual generation | ~9,500 kWh | | Self-consumption (30%) | 2,850 kWh saved @ 32c = $912 | | Export credits (70%) | 6,650 kWh @ ~7c modelled export = $466 | | Total annual benefit | $1,378 | | System cost (after STCs) | $5,500 | | Payback | 4.0 years |

Step 2: Add battery (10 kWh)

| Metric | Value | |---|---| | Additional self-consumption | 2,800 kWh shifted from export to evening | | Savings per kWh shifted | ~25c (grid rate less export credit) | | Battery annual savings | $700 | | VPP earnings | ~$80 | | Combined solar + battery | $2,158/year | | Battery net cost (after rebates) | $5,600 | | Combined payback | 5.2 years |

Step 3: Add EV + home charger

| Metric | Value | |---|---| | Petrol replaced | 15,000 km @ 9L/100km @ $1.85/L = $2,498/year | | EV charging cost (solar) | 2,700 kWh @ ~5c (marginal solar cost) = $135 | | EV charging cost (overnight) | 600 kWh @ 19c (EV tariff) = $114 | | Net fuel savings | $2,249/year | | Home charger install | $1,500-2,500 | | Combined solar + battery + EV | $4,407/year |

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 32c ($2.56/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:

• Midday Saver base: 8.62c super off-peak (9am-3pm) for daytime loads
• EV Add-On: 19.38c overnight (11pm-6am) for EV charging when solar isn't available
• Peak avoidance: Battery covers the 53.84c 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:

| Component | Minimum | Recommended | |---|---|---| | Solar panels | 6.6 kW | 8-10 kW | | Battery | 10 kWh | 13.5 kWh | | EV charger | 7.4 kW (single phase) | 7.4 kW with solar integration | | Inverter | 5 kW hybrid | 8-10 kW hybrid (3-phase if available) |

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

| Component | Cost (after rebates) | |---|---| | 10 kW solar | ~$6,500 | | 13.5 kWh battery | ~$6,500 | | 7.4 kW EV charger | ~$2,000 | | Total | ~$15,000 |

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's the fastest payback, lowest risk, and 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

• Home Batteries in WA — the honest ROI picture — Battery-specific payback calculations and brand comparisons.
• EV Charging at Home in WA — tariffs and costs — Charging options and tariff strategies.
• WA Rebates Are Changing in 2026 — Federal and state rebate timelines.

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: SolarQuotes, Solar Choice