Charging basics that change your numbers
Three things move a charging estimate more than anything else: how much energy is lost on the way into the battery, how close your car actually gets to the charger's rated power, and what your electricity actually costs per unit. Here is how each one behaves.
Charging losses: why you pay for more than you store
The energy in your battery is always less than the energy you took from the grid. The difference is lost to heat in the charger, in the car's onboard charger, and in the battery itself as it warms up.
The loss is expressed as a percentage. A 10% loss means that for every 10 units you draw from the grid, about 9 reach the battery. So if you need 36 kWh in the battery, you actually draw 40 kWh — and you pay for 40 kWh, not 36.
| Charging method | Planning assumption | Note |
|---|---|---|
| AC with the portable cable | 15% | Lowest power, so the session runs longest |
| AC wall charging | 10% | The usual planning assumption for home charging |
| DC fast charging | 5% | Highest power, shortest session |
These are planning figures for estimating, not measurements of any particular car. Your real loss depends on temperature, battery age and how full the battery already is.
Charge curves: why rated power is not the power you get
A charger is rated at its maximum output. Your car draws what it can accept, and that number is usually lower. A 7 kW wall charger plugged into a car that accepts 11 kW still charges at about 7 kW.
On top of that, most batteries taper as they fill. Power drops progressively once the battery is mostly full, to protect the cells. This is the single biggest reason a 20-to-80 percent session takes noticeably longer than a flat calculation suggests. The calculator on this site uses 85% of rated power as an average planning figure and tells you that is what it did.
Tariffs: what your electricity actually costs per unit
Australian households are usually on one of two structures. A flat rate has a single price per kWh at all times, so the estimate is straightforward. A time-of-use tariff has a cheaper window and a more expensive one.
If you charge overnight, a time-of-use tariff can meaningfully reduce the cost. If your charge straddles the peak window, the real cost sits between the two rates rather than at either one. Check your plan before using a specific figure. Sources are listed here.
What a charge does not include
- The fixed daily supply charge on your bill, which does not move when you charge more.
- Public charging prices, which vary by network, time of day and membership.
- Any effects of charging on your battery's long-term health, which depends on how often you use fast charging.