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Electricity Tariff Impacts on Timing of EV Charging

September 19, 2023

As our user base and data sets grow so too does our confidence that EV charging will help more than hinder the grid as it transitions to renewables. Give EV owners some basic price signals and the tools to respond to them and they will charge their EVs in a grid-friendly way.

We’ve previously published some EV load charging profiles which show a healthy inverted duck curve - a beautiful match for energy systems with large amounts of solar.

Different regions in Australia have adopted time of use tariffs in different configurations. By looking at the load profiles on a per-region basis we can see just how effective they are in influencing charging behaviours.

Time of use tariffs + solar charging can effectively:

  1. Keep EV charging away from early evening peak demand periods
  2. Shift EV charging to the middle of the day to soak up excess solar
  3. Shift EV charging into the middle of the night to improve network utilisation

With smart charging tools, this can happen dynamically to respond to fluctuations in demand and supply.

The data

Before we get into the detailed graphs, some important notes about the data behind them.

EV load profiles:

  • We don’t currently have any knowledge about the tariff that our users are on.
  • We track individual users’ time zone at the state/capital city level and have used this to group users in these graphs.
  • We currently only automate charging from solar, any grid-based charging schedules are set by individual users.
  • The graphs include all users, the majority have rooftop solar but many don’t and use Charge HQ only for advanced scheduling. On average, users that do have solar will rely on topping up from the grid for about half of their EV charging.

Time of use periods

  • Most distribution networks offer tariffs on a time of use basis. They set both the periods and rates for each window.
  • The end user contracts their electricity supply with a retailer, that bundles the network costs along with generation and other admin costs.
  • The time of use windows adopted by the energy retailers often vary from those set by the distribution networks. There are currently several popular EV energy tariffs offered with off peak periods starting at midnight despite few (if any networks) adopting the same.
  • Most states have multiple distribution network operators with slight differences in ToU tariffs. Off peak and shoulder periods for each state have been identified from Wattever and attempt to summarise the more common offers.


  • South Australia (Adelaide time zone) offers a daytime solar soak tariff, making it cheaper to charge between 10 am and 3 pm than overnight. The effectiveness is evident in the hard shoulders seen on the EV load profiles.
  • The next cheapest network tariff is the shoulder, starting at 1 am which is not currently found in other states and the impact is evident.
  • In Western Australia (Perth time zone), like South Australia a daytime solar soak network tariff is offered, making it cheaper to charge between 9 am and 3 pm than overnight. The effectiveness is evident in the hard shoulders seen on the EV load profiles.
  • Whilst the network tariff appears to provide an off peak period starting at 9 pm, an EV retail tariff is offered with the off peak period not starting until 11 pm.
  • In Victoria (Melbourne time zone) 9 pm is a common start time for off peak tariffs and some networks are starting to offer daytime solar soak tariffs. The effect is visible but much less pronounced than in SA & WA.
  • The lower solar generation in Melbourne through winter and higher overnight loads compared with other states are very evident.
  • In NSW (Sydney time zone) the Ausgrid distribution network (where most of our users are) has an off-peak period starting at 10 pm where a small load increase is seen. At midnight a far larger increase is apparent. We think this is a combination of a) retailer EV tariffs offering super off peak periods starting at midnight, and b) users taking midnight as the default safe option to avoid charging in peak price or demand periods.
  • Queensland (Brisbane time zone) has the least notable step changes in overnight demand, we suspect due to a mix of a) EV circuits on controlled load b) lower uptake of Time of Use retail energy offers and c) the greater contribution of rooftop solar charging through winter.

Retail energy tariffs are a powerful lever in influencing when EV charging occurs. They will be most effective if they strike the right balance in optimising for network utilisation, renewable generation and wholesale energy costs.

See also