There's a bit more to deciding what speed home EV charger you need than is apparent at first. We'll work through how to calculate charging times as well as some factors to consider. Finally, we'll give you our recommendations based on some common use cases.
For petrol cars we think about fuel use in terms of Litres per 100 kms. For EVs we mostly use Watt hours per Kilometre.
Driving 10kms per day in a Model 3 will use around 180 x 10 = 1,800 Watt hours or 1.8 kilowatt hours (kWh's) per day.
Let's work out daily energy used based on how far you typically drive in a year. Each day will be different but it gives you a reference point.
Annual kms / 365 = kms/day.
To find your daily EV charging energy usage, multiply your kms/day x Wh/km for the car. A Tesla model 3 doing 41 kms/day = 41 * 180 / 1000 = 7.38 kWh/day
Medium EV - Tesla Model 3
Large EV - Tesla Model S
SUV EV - Tesla Model X
While you've probably never thought about it, the "charging speed" for a petrol car is the speed the fuel comes out the bowser, we could measure this in litres per second.
For EV charging we measure this in kWs. There are three common charging speeds we see for home chargers:
A 7kW wall charger will give you 7kWh of energy for each 1 hour of charging.
We can calculate how long you need to charge by multiplying how much energy you need by how fast you put that energy in your EV.
A Tesla Model 3 doing 41 kms/day needs about 7 kWh per day. On a 2.3 kW charger you'll charge in 3 hours, on a 7 kW charger you'll charge in 1 hour, at 11 kW you'll charge in 40 minutes, assuming you charge every day.
Medium EV - Tesla Model 3 with a 2.3 kW charger
Medium EV - Tesla Model 3 with a 7 kW charger
Medium EV - Tesla Model 3 with a 11 kW charger
Say 8-12 hours, which, unless you're doing lots of kms each day, is much more than what most users will need as we calculated above. Many EV owners could probably make do with a slow charger, or even charge from a regular wall outlet.
Let's consider some other reasons you might want to be able to charge faster.
In order to charge from your rooftop solar, you need the car plugged in whilst the sun is shinning. Allowing for work, family, outings, errands there might be days when the EV is only plugged in for some of the time your solar is generating power.
When you are plugged in, to use all the available solar energy, you need to be able to charge at the same rate as that you're generating solar energy.
New solar PV installations are typically around 8 kWp, for much of the day they'll be producing energy at a rate of between 2 kW and 8 kW. On average, such a system will create 32 kWh energy per day. If you're charging from a regular wall outlet at 2.3 kW, you can only make use of less than a third of that solar energy. On the other hand, if you had a 7kW fast charger then most of the time you could make use of all of the excess solar, and if you're only plugged in for part of the day it might still be enough to meet your EV charging needs.
From another angle: it's not sunny all day every day, there are plenty of days where you get a few hours of sunshine and the rest is overcast or raining. To charge from your solar on these days, ideally you need to be able to capture all the available energy when it's there.
So to charge from your solar you often won't have that much time, it could be as little as 1-2 hours per day, the faster you can charge the better.
But, keep in mind if you move from a single phase (7 kW) charger up to a 3 phase (11 kW +), the minimum increments for charging speed adjustment become larger which may impact the accuracy of solar tracking.
In 2022, most Australian states (other than South Australia and parts of Queensland) provide a 9 hour overnight window when power is cheap via off-peak network tariffs. This gives you a lot of time for charging. Per above, the average user could get away with a slow charger on a regular wall outlet.
South Australia has some of the highest penetrations of renewables in the world and offers a window into the future. Their off-peak window has already been shifted to the middle of the day and is only 5 hours long, the next cheapest period is overnight but is also only 5 hours long.
As the energy system transitions to one primarily made up of renewable generation, energy will be increasingly cheap when there's lots of sun or wind and more expensive when there is not. Equally, moving loads which can be moved (such as EV charging) to times when renewable energy is abundant will play an important role in reducing fossil fuel use for generating electricity, because the supply market will adjust to meet demand.
Whilst you might be able to plug your EV into a charger for 10+ hours per day, the overlap of times when you're plugged in and energy is cheap or renewables are abundant could be much shorter in future.
It might not happen often, but you may from time to time you find yourself low on charge. You forgot to plug the car in overnight? You're doing a weekend outing and you forgot to increase your charge limit from 70% to 100% the night before?
When this happens you might only have 1-2 hours to add as much charge as you can to the vehicle to avoid having to use a fast charger on the highway. Faster charging in these times helps.
Taking all this into consideration, here's our recommendation:
NOTE: if you do install a 3 phase charger, the circuit should be 32 amps if possible, not 16 amps, because if you purchase a vehicle that only supports single-phase charging (which is most of them) you will be limited to 3.6 kW on 16 amps.
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