Best Solar System Size If You Own an Electric Car

If you own an electric car — or plan to buy one — your solar system needs to generate enough energy for both your household and your vehicle. Under-sizing means you'll rely on the grid for EV charging. Over-sizing wastes money on panels whose excess you export for a pittance. Here's how to find the sweet spot for your situation.

How Much Extra Solar Does an EV Need?

The amount of extra solar capacity you need depends primarily on how far you drive each day. An average EV uses about 16kWh per 100km, so the maths is straightforward:

Total System Sizing: Home + EV

A typical Australian household uses 18–22kWh per day. Adding EV charging brings the total daily energy need to:

For most Australian families with one EV, an 8–10kW solar system covers both household and vehicle energy needs with some surplus for cloudy days and seasonal variation.

If You Already Have Solar

Before spending money on extra panels, check what you're already working with:

Step 1: Check Your Solar Export Data

Open your solar monitoring app (SolarEdge, Enphase, Fronius, SMA, or your inverter's app) and look at your daily export figures. The energy you export is energy that could charge your EV instead.

Step 2: Consider Seasonal Variation

Solar generation varies significantly by season. A system that exports 8kWh/day in summer might only export 3kWh/day in winter. Check your winter export figures specifically — if they're low, you'll either need more panels or accept some grid charging in winter.

Step 3: Check Inverter Capacity

If your existing inverter has spare capacity, you may be able to add panels without replacing it. A 5kW inverter can typically support 6.6kW of panels. If your inverter is already at capacity, you might need a second inverter or an upgrade — factor this into the cost.

If You're Buying New Solar

If you're installing solar from scratch and already own — or plan to buy — an EV, factor in the EV from day one. It's much cheaper to install a larger system upfront than to add panels later.

• Minimum recommendation: 8kW system (covers average household + average EV driving)
• Better recommendation: 10kW system (provides buffer for seasonal variation and future needs)
• Two EVs or high usage: 13kW system (maximum commonly allowed on single-phase residential)

The marginal cost of going from 6.6kW to 10kW is typically only $2,000–$3,500 at the time of installation. That extra capacity can generate $600–$1,000 per year in additional EV charging savings — an excellent return on investment.

The Oversizing Sweet Spot

There's a common concern about "oversizing" a solar system, but for EV owners, oversizing is almost always worthwhile. Here's why:

• EVs are flexible loads: Unlike household appliances, EV charging can be shifted to whenever solar is available, making it the perfect sink for excess generation
• Winter buffer: Oversizing ensures you have enough generation in winter when solar output drops by 40–60%
• Future-proofing: Electricity consumption tends to increase over time (heat pumps, induction cooking, second EV). Extra capacity now prevents the need for costly retrofits later
• Low marginal cost: Each extra kW of solar only costs $800–$1,200 at installation time

Quick Sizing Guide

For a quick answer, use this simplified guide based on your situation:

When in doubt, go bigger. The cost of extra panels is modest, the return is strong, and you'll thank yourself as energy needs grow over the 25+ year life of your solar system.