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Charging Basics

Charging basics

Alternating current, direct current, phases and kW

In the early days of electrification, the standard for delivering electrical power was Direct Current (DC) – where the power flows in one direction. Nowadays, in the electrical network in NZ, we use Alternating Current (AC) – where the power flow is constantly switching direction, e.g. 50 times a second – transporting electrical charge more efficiently over long distances. 


While most of our household appliances are powered with AC, any device with a built-in battery – like a mobile phone or a laptop – can only be charged with DC. Electric cars also fall into this category. This is why it helps to know about AC and DC if you’re planning to drive an electric car.

What are phases?

In New Zealand the electrical power can be distributed via a three phases network. I.e. there are three lines that carry the electrical current.


For smaller electrical devices like most household devices one phase is sufficient, that is why all regular domestic sockets are single phase.

Bigger devices e.g. in the industry but also ovens in the kitchen are sometimes connected to all three lines as that allows higher power output (high voltage).


Private household are typically only connected to one phase. Therefore it is recommended to also use three phase charging technology (wall box, on-board charger – see below) as it allows higher charging speeds.

Converting AC to DC

In order to charge the battery of your electric car, the current has to be converted into direct current (DC), where the current only flows in one direction. Otherwise it would flow into and out of the battery and therefore couldn’t be stored. 


This conversion can happen in the vehicle by the On-Board Charger (OBC) or outside of the vehicle in DC fast charging stations, which have a much higher charging capacity (up to 100 kW or 80% of load in 30 minutes). 


The OBC is included in your electric vehicle, ranging from single phase – with up to 3,7 kW – to three phase OBCs with 11 kW. However, be aware that the effective charging performance is determined by the weakest link in the charging chain (the household socket used, the cable or the wall box). To obtain the maximum charging performance it is required to connect it to an equally powerful charging device. Therefore, the most efficient and best way to charge at home is always the wall box. Your Opel dealer can help you purchase and install a wall box for your home.


What are kW and kwh?

Watt and Kilowatt (kW) are units of power. I.e. the amount of energy transported, consumed or provided per second. This is why the power of electric devices is stated in watt or kilowatt (1,000 watt = 1 kW). In the past the power of car engines was measured in “horsepower” but today is also stated in kW.

  • Lamp: 10 watts
  • Hairdryer: 1,500 watts = 1.5 kW
  • Corsa-e engine: 100 kW


Kilowatt hour (kWh) is a unit of the amount of energy stored or consumed. It corresponds to litres of fuel in a car with combustion engine, so the capacity of the “electricity tank” – meaning the battery – is stated in kWh.


Kilowatt hour is therefore used as a billing unit for energy delivered to consumers by electric utilities, too. 

  • An electric heater consuming 1,000 watts (1 kW) and operating for one hour uses one kilowatt hour of energy
  • The same amount of energy (1 kWh) is used, if a television consuming 100 watts is operating for 10 hours continuously

How charging an electric car works

When charging an electric vehicle from a normal AC household socket, AC is converted to DC inside the vehicle – but this takes time. You can speed things up by installing an AC Wall box. This is faster than a normal household socket because it has a higher energy transfer rate (up to 7.4 kW). The fastest option, however, is a DC charger. This is because AC from the grid has already been transformed into DC which can be fed directly into the battery. DC chargers are currently only available for public use but are a super convenient and easy way to top up when you’re on the go.

Charging solutions for your needs

By keeping these power basics in mind, you’ll find it much easier to work out the best solution for charging your electric vehicle – whether at home, on the go, or on longer trips.