Charging an EV at home: timing, tariffs and efficiency

Start with your driving, not the charger. A typical weekday round trip from Point Cook, Werribee or Altona Meadows into the city uses only part of a modern EV's battery, and most cars then sit in the driveway for most of the night. That is usually enough time to replace a day's driving at a modest charging rate.

That changes the question. You don't need the fastest charger on the market. You need enough charging in the hours the car is home, at the times power is cheapest or your solar is producing.

Check your car's energy use in its trip computer for a couple of weeks. That figure, and when the car is parked at home, shapes every other decision.

Electricity plans differ in when power is cheaper. Some have time-of-use rates with a cheaper overnight or midday window, and some retailers offer plans aimed at EV owners. Your bill shows what you are on, and the Victorian Government's Victorian Energy Compare site can compare plans using your smart meter data.

Most cars and wall chargers can schedule charging to start and stop at set times. Set it once to suit your tariff and leave it. If both the car and the charger have schedules, use one of them; two competing schedules are a common reason for a half-charged car in the morning.

Revisit the schedule whenever you change plans. A setting that suited an overnight window can be exactly wrong on a plan with cheap power in the middle of the day.

If the car is home during the day, solar charging uses power you would otherwise export. Some chargers can follow your solar, charging only from surplus generation and adjusting as clouds pass. That depends on the charger, the inverter and sometimes an extra meter, so check the model numbers of both.

If the car is at work all week, solar charging becomes mostly a Saturday and Sunday habit. Daytime charging on those days plus a scheduled overnight top-up on weekdays covers most households.

Watch for daytime competition. A heat pump water heater on a midday timer and a car charging on solar can chase the same surplus, so decide which matters more in your week and set the timers accordingly.

A charger draws steadily for hours. On its own that is fine. The problem is overlap: a cold July evening with reverse-cycle heating running, dinner on the induction cooktop and the car plugged in at 6pm, all at once. That is when a modest supply runs out of headroom and the main switch or a circuit breaker trips.

Scheduling the car to start after the evening rush solves most of it. Load management goes further: a sensor on the incoming supply measures what the house draws, and when the oven, heating and hot water are all on, the charger backs off, then picks up again as the load drops. It works alongside a properly sized supply. Where the finished house has big combined loads, such as two EVs, a large ducted system and an induction kitchen, a larger single-phase supply or three-phase can make sense.

Off-peak electric hot water can switch on overnight at the same time as a scheduled charge. Find out when each starts and stagger them if the supply is tight.

A charger's rating is the most it can deliver, not what your car will take. The car's onboard charger sets the ceiling for home AC charging. Turning the charging current down in the car or the charger app lowers the load on the house, at the cost of a longer session.

There is a trade-off at the very low end. Charging at a trickle, such as from a portable charger on an ordinary power point, generally loses a bigger share of energy, because the car's electronics and battery systems stay awake for many more hours. A moderate rate from a wall charger is usually more efficient.

Many cars let you set an everyday charge limit below full. Follow your car maker's advice on daily limits, which is about battery care as much as energy.

Not every unit you pay for ends up in the battery. Some is lost as heat in the cable, the charger and the car's onboard charger, and some goes into warming or cooling the battery. Your meter sees all of it, while the car's app may only show what reached the battery, so the two figures rarely match.

Cold weather adds to the losses. On a July night, with Laverton's mean minimum at 5.2 °C, a car may spend energy warming its battery before it charges efficiently. Charging soon after a drive while the battery is still warm, or timing the charge to finish near departure so the car can warm itself on grid power, both help.

A garage keeps the car out of the worst cold and heat, which helps both the battery and the charging.

A wall charger goes on its own circuit from the switchboard. Energy Safe Victoria's guidance says the Wiring Rules require RCD protection on every final sub-circuit in a home, and Type AC safety switches can't be installed any more, even as a like-for-like replacement.

Running a portable charger from an ordinary power point for hours every night puts a long, heavy load on a point and circuit built for shorter use. If that is your plan, have the point and circuit checked first. Whenever a licensed electrician completes electrical installation work, they must give you a Certificate of Electrical Safety.