Scenario guide: EV charging without draining the battery

This is an illustrative scenario based on typical Perth household data, not a specific customer story.

Plug an EV in whenever the driver gets home and it competes with the battery for the same evening hours, pulling from the grid at peak rates instead of solar. Schedule the charge into the same midday window the battery already fills from, and the two stop competing.

This scenario models that approach: a household with a mid-size EV on a typical Perth commute, running a 10 kW solar system and a 13.5 kWh battery. Mid-size EVs typically use 15-20 kWh per 100 km driven. The figures below use 18 kWh/100km (about 5.6 km per kWh) throughout.

Home profile

• Occupants: 2 adults
• Vehicle: 1 EV commuting ~25-40 km/day (~4.5-7 kWh daily charge requirement)
• Other loads: Evening cooking, occasional electric dryer
• Daily total usage: ~25-30 kWh (including EV)

System

• Solar: 10 kW PV
• Battery: 13.5 kWh usable, hybrid inverter
• EV charger: 7.4 kW single-phase (most common in Perth)
• Goal: Maximise solar into both car and house, limit peak-rate grid imports

Stopping EV charging from draining the battery

Charging demand can overwhelm a battery fast if it's left unscheduled. A 25 km commute needs about 4.5 kWh, roughly half a day's solar surplus that would otherwise go to the battery. Four settings keep the car and the battery from competing for the same energy:

1. Start the EV charge at 11am, inside the solar window.
2. Cap the charger at 3-5 kW so surplus still reaches the battery.
3. Let the battery cover the evening: typically 4-6 hours of coverage even after the EV charge.
4. On cloudy weeks, drop the charger to 3 kW, or force-charge the battery overnight instead.

Why midday charging works

During 11am-2pm, a 10 kW system produces 6-8 kW. That's enough to charge the EV at 3-5 kW while still sending surplus to the battery. Starting the charge at 11am gets the EV most of its energy from solar, and the battery still has enough headroom to charge through the afternoon and carry the house into late evening.

Cloudy weeks

Extended overcast periods call for flexibility. Setting the EV charger to a lower rate, 3 kW instead of 7.4 kW, gives the battery more solar access when there's less of it to go around. If the sun doesn't show up for days, an overnight grid top-up keeps both EV and battery covered without paying peak rates for the shortfall. That's forced-charging: scheduling the grid top-up into a cheap window instead of waiting on solar that isn't coming.

Priorities for EV owners with a battery

• EV charging strategy matters more than raw PV size once you have a battery. Scheduling matters most. If you're still deciding on battery capacity with an EV in the plan, start sizing from the night-time load, not the EV.
• A slightly bigger PV array (10 kW vs 6.6 kW) provides winter headroom when both EV and battery compete for solar.
• Most EV chargers have timer functionality. Use it to restrict charging to solar hours.
• On days you don't drive, the full solar surplus goes to the battery, improving evening coverage.

Where next

How to size a battery for your usage

A step-by-step method covering evening usage and seasonal AC demand.

Where to place a battery in a Perth home

AS/NZS 5139 clearance rules, garage vs outdoor walls, and what installers check on-site.

WA battery rebate: are you eligible?

Synergy customers get $130/kWh back, capped at $1,300.

Explore your options

Answer a few questions and see systems and installers that fit your home.