How to size your solar system for an electric vehicle in Perth

Adding an electric vehicle to your home is the largest single increase to residential electricity demand most Perth households ever make. A typical EV charged entirely at home adds 3,000–5,000 kWh/year to your consumption — roughly doubling the electricity usage of a small-to-medium household.

If you're getting an EV and considering solar, or already have solar and are getting an EV, here's how to account for the charging load in your system size.

How much electricity does an EV need?

EV electricity consumption depends on the vehicle's efficiency and how far you drive:

| Vehicle type | Typical efficiency | Annual consumption (15,000km) | |---|---|---| | Small/medium EV (e.g. BYD Seal, MG4) | 14–17 kWh/100km | 2,100–2,550 kWh/year | | Large EV or SUV (e.g. Tesla Model Y, Kia EV6) | 17–22 kWh/100km | 2,550–3,300 kWh/year | | Large performance EV | 22–30 kWh/100km | 3,300–4,500 kWh/year |

Typical Perth household EV usage at 15,000km/year: approximately 2,500–3,500 kWh/year.

This is the additional electricity demand your solar system needs to cover.

How many extra panels does an EV need?

Calculation:

1. Find your EV's annual kWh consumption (above)
2. Decide what fraction you want solar to cover (typically 60–90%)
3. Divide by Perth annual PSH (5.0 hours/day × 365 days = 1,825 hours/year) and by system efficiency (~79%)

Example:

• EV consumption: 3,000 kWh/year
• Target solar coverage: 80%
• Solar needed: 3,000 × 0.80 = 2,400 kWh/year
• Panels needed: 2,400 ÷ (1,825 × 0.79) ≈ 1.66 kW additional capacity

In practice: A 2kW addition to an existing solar system (approximately 4–5 panels) can cover the EV charging needs of an average Perth commuter. If you're starting from scratch, adding 2kW to what you'd otherwise size means moving from, say, a 6.6kW system to an 8–9kW system (requiring three-phase or a split-system approach).

When to charge: solar hours vs overnight

There are two main EV charging strategies for Perth solar households, and they have very different economics.

Strategy 1 — Daytime solar charging

How: Set your EV to charge during 10am–2pm when solar production is highest. Use your car's built-in timer (most EVs have this) or a smart charger that can be scheduled or solar-triggered.

Tariff: Effectively free if the car's charge rate stays within your solar export surplus. A 7.2kW home charger (the standard EV home charger rate) draws significantly more than typical mid-range solar excess — you'll be drawing some grid in parallel. A lower charge rate (3.6kW) during solar hours reduces grid draw.

Practical constraint: Many Perth workers are away from home during solar hours (9am–3pm). This strategy works best for: work-from-home, stay-at-home households, EV owners who can charge at their workplace and only need a top-up at home, or EV owners with a second car using peak solar hours.

Strategy 2 — Midnight / off-peak overnight charging

How: Charge between midnight and 6am using grid electricity on standard A1 tariff (33.26c/kWh) or on Synergy's EV Add-On Time of Use tariff.

Synergy EV Add-On tariff (as of 2026):

• Super off-peak rate (11pm–6am): approximately 18.13c/kWh for EV charging
• Available as an add-on to residential tariffs

Cost comparison:

• Solar charging (effective cost): 0–5c/kWh (depending on what you'd otherwise export)
• EV Add-On midnight: approximately 18.13c/kWh
• Standard A1 overnight: 33.26c/kWh

Solar charging is cheapest. Overnight EV Add-On charging is significantly cheaper than standard A1 for overnight charging. The best outcome for solar households is to combine both: charge during solar hours when home, and use the EV Add-On midnight rate for any remaining charge needed.

The solar sizing decision: EV upfront or later?

Include EV in solar sizing now

If you're getting an EV within 12–18 months of your solar installation:

• Size the solar system to include estimated EV demand (add 2–3kW to your baseline size)
• Ensures the system is appropriately sized from the start
• Single installation and Western Power connection process
• Maximum STC rebate before the scheme steps down further

Add panels later

If EV timing is uncertain:

• Install a baseline solar system now sized for current consumption
• Many inverters allow adding additional panels later if the inverter has unused MPPT capacity or the inverter can be upgraded
• Confirm with installer at initial installation whether future expansion is possible without full system replacement

Practical note: Most residential installations use a 5kW inverter with 6.6kW of panels (the maximum panel-to-inverter ratio allowed). Adding panels later requires either a second inverter (adds cost and complexity), replacing the existing inverter with a larger model (cost plus waste), or accepting that the existing panels are already at the inverter's limit. This is why it's worth planning for EV demand at initial installation if you know it's coming.

WA-specific: EV charger options and Western Power

A standard home EV charger (7.2kW AC) draws approximately 7.2kW during operation. In suburbs with a 1.5kW export limit, this creates an interesting dynamic: the charger draws far more than any solar can export, so during solar hours, the EV effectively absorbs all solar production plus imports the remainder. This is actually ideal — you're maximising self-consumption of solar and the 1.5kW export limit becomes less relevant.

In unconstrained suburbs, a solar system producing 5kW+ during midday with a 7.2kW charger running effectively directs most solar into the car and imports only the balance — a straightforward self-consumption scenario.

EV energy consumption figures are approximate and vary by model, driving conditions, and climate. Perth PSH figure of 5.0 is the annual average based on BOM data. Synergy EV Add-On tariff rates may change — verify current rates at synergy.net.au.