Ten per cent of new cars sold in Australia are now plug-in EVs. Home battery sales are going through the roof. That means a growing number of homes now have both. Or soon will.
And yet, hardly anyone thinks about how the two will work together.
That is a shame. Because if you do not plan it, the default behaviour is simple:
Whenever you charge your car, any charging power required above your instantaneous solar output will come from your home battery1. Most charging is after sunset, and your car battery is much larger than your home battery, so your home battery simply empties into your car.
Your house is left to buy from the grid overnight and some of what you save on petrol, you quietly lose on electricity.
Let’s walk through your choices when adding a home battery to an EV-driving home:
Option 1: Don’t discuss an EV charging strategy with the battery installer
Whenever you plug the EV in, your 7 kW single-phase charger kicks in. Or worse, your (up to) 22 kW three-phase charger.
Your home battery sees a big load, says “righto,” and empties itself into the car. By supper time your battery is flat.
This only makes sense if:
- You have a very big battery. Think 40 kWh or more.
- It has enough power output in kilowatts to run the house and charge the car at the same time.
- Or you drive so little each day that you are only topping up a few kilowatt-hours from a trickle charger.2
Option 2: Ask the installer to wire the EV Charger upstream of the Battery
This is the boring, simple, rational option.
You wire the EV charger so it sits before the battery in the switchboard.
Result:
- The EV can never charge from the home battery.
- It can only charge from the grid or live solar.
For many households, this makes the most sense because:
- Some retailers offer “3 free hours” of grid power in the middle of the day.
- Some EV plans offer charging at 8 cents per kWh overnight.
- If grid power is cheap at the right times, or you only charge from solar, this option wins.
Option 3: Get an EV Charger That Talks to Your Battery
When your EV charger and battery inverter talk to each other you can choose whether to charge your car from the battery or the grid.
In theory, all this requires is an Open Charge Point Protocol (OCPP) compatible EV charger.
In reality OCPP between brands is a headache, so the practical answer today is usually to buy the same brand EV charger as your battery.
A Special Word for Tesla Owners
If you have:
- A Tesla car
- A Tesla Powerwall
Everything works beautifully. But there is a catch. Tesla controls charging by sending commands to the car, not the wall charger3. So when you buy a non-Tesla EV later, all that clever integration disappears.
If you plan to have a mixed-brand garage, you may want to think carefully before locking yourself fully into the Tesla ecosystem for both battery and family fleet.
Tesla EVs and home batteries play nicely with each other, but bringing a different brand of car into the mix can cause headaches.
The Bigger Point
EVs and home batteries are both big purchases. Each makes sense on its own. But together, they are a system. If you do not design that system, it will design itself. And it might not work how you want it to.
Before you sign off on your next battery quote, ask one simple question:
“How will this work with my EV?”
Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.
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One of the reasons I stuck with the Tesla ecosystem. If somebody does only low kms, ‘top up’ charging from the battery probably isn’t harmful, to the battery?
You can use third party apps to help control this? You’ve written about it before here too, apps like NetZero and its automations, and others? I’ve played with NetZero before just for fun and found it works, though probably not the approach suggested for people who don’t want to have to fiddle with the tech side.
Good advice, the reality for me is i have solar and a battery already, long before i will have an EV.
So it will definitely be a retro fit for the vehicle charging when it occurs, hopefully the technology available at that time wont be a challenge to integrate with my current system.
But who knows, perhaps public charging will be so fast and cheap by then it will be easy enough to just go to the servo same as i currently do for my ice vehicles, without worrying about a home charging setup.
Option 3 B get a Zappi charger, with the option to only use ‘spare’ solar power.
Ours uses this option, and doesn’t use grid power, or battery power only solar power after the battery and everything else is accommodated
The option shows as eco,eco,+
There are other options – but why would you
Result = happiness
I realise you’re not a fan, but we have a Myenergi Zappi EVSE wired downstream of our Tesla Powerwall (charging an Ioniq 5). We mostly charge solar only using “Eco” mode, which is smart enough to know not to use battery power (despite the sad lack of OCCP).
For an 80-100% early morning top up prior to a road trip we use “Fast” and pull power from the Powerwall. On the rare occasion that we want to charge on “Fast” without draining the battery, we can set the battery reserve to its current charge level. A little clunky, but rarely needed.
Hi David,
Sounds like a good workaround. Thanks for the feedback.