EV charger installation in Mount Eliza: a Zappi on a Powerwall home with a 40A main switch

A Mount Eliza home with a Tesla Powerwall 3, an expansion pack, and around 15kW of solar coupled straight into it. A Backup Gateway 2 outside in its own enclosure. The owner runs the house through Home Assistant and buys electricity at wholesale prices, so he watches the numbers more closely than most.

He wanted a Zappi on the carport post.

The main switch is rated 40A.

The 40A problem

An EV charger is counted at its full rating when you work out what a house can safely draw. No diversity, no allowances, unlike almost every other circuit in the place. Add 32A of charger to a house already running an oven, a heat pump and everything else, and a 40A main switch doesn't have the room.

There were three obvious ways to go.

Refuse the job.

Fit it anyway and hope nobody uses the oven while the car's plugged in.

Or upgrade the supply. New consumer mains, a new switchboard, an inspection, several thousand dollars.

We took a fourth. The charger's maximum current was set at commissioning so total household demand stays inside what the supply can carry.

In practice the owner gives up almost nothing. His car takes 6.6kW at best and he charges overnight, so the ceiling sits well above anything he'd actually use. What he gets is a main switch that's never asked to do something it can't.

A supply upgrade would have been the right call if he were adding a second EV or going all-electric. Not to charge one car overnight.

Where the circuit goes, and why it matters

On a home with a Tesla Gateway there are three places an EV circuit can land, not two. Most quotes only mention one of them.

The backup side means the car keeps charging in a blackout, running down the battery you were relying on for the fridge and the lights. Almost nobody wants that.

The non-backup terminals inside the Gateway sit downstream of Tesla's own metering. The Powerwall sees the car as household load and discharges to cover it, because it can't tell a car from a kettle. The car appears in the Tesla app, and solar diversion has real data to work with.

Upstream of the Gateway, the car is invisible to the Tesla system. The battery never feeds it. Every kilowatt into the car comes off the grid.

Which one is right depends on something no installer can answer for you. If you're on a wholesale or time of use plan and charging overnight when power is cheap, keeping the battery out of it usually wins. If you want the car visible in the app and solar diversion doing real work, the non-backup terminals are the answer, and you accept the battery will feed the car.

That's a decision for the person paying the electricity bill. Ask whoever quotes you which side they're planning to use, and what it means for your battery.

Why the sensor is hardwired

A Zappi normally reads its grid sensor through a harvi, a small wireless sender that sits in the switchboard and talks to the charger by radio. It saves running a cable, and on most homes it's fine.

Not on a battery home.

The harvi is self-powered. It scavenges energy from the current passing through the clamp, and below roughly 2A it hasn't got enough to transmit. A Powerwall holds grid flow near zero for most of the day. That's exactly when the harvi goes quiet, and exactly when the charger needs the reading.

So the sensor was hardwired back to the charger instead, in voltage-rated screened cable sharing the conduit with the supply. An extra run of cable, and the reading is there whether the grid is doing anything or not.

What was installed

myenergi Zappi v2.1, 7kW single phase, tethered, in black. Mounted on the carport post. Dedicated 32A circuit, Type A RCBO, weatherproof isolator, grid current transformer hardwired at the main switchboard.

Installed by Twigg Electric, with a Certificate of Electrical Safety issued on completion.

The result

A charger that works within the supply the house actually has, on a system with a battery and a Gateway that a lot of installers would rather not open.

The interesting part of an EV charger install is rarely the charger. It's the switchboard, the supply, and whatever else is already on the wall.

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