Why turning on the dishwasher at 9pm isn’t helping the national grid

Sunday, 21 September 2025

Time-of-use plans are increasingly changing the way Kiwis use electricity.

ANALYSIS: If there is one easy win that would make electricity cheaper and less polluting, and reduce the chance of power cuts, it is a change in behaviour by consumers that smoothed out demand for electricity across the day.

Not only would that reduce the need for generators to fire-up expensive coal and gas-fired turbines to meet the usual morning and early evening peaks in demand, it could also help reduce the need for billions of dollars in extra investment in distribution infrastructure to meet the peak loads.

“As New Zealand electrifies, demand for electricity will increase. This increased demand will generally need to be met with new supply. But an important element of meeting this at the lowest cost to consumers will be the effective use of demand-side flexibility,” the Electricity Authority said earlier this month.

“This is where electricity consumers use more electricity during the times when it is cheapest to produce.”

There are a raft of companies developing innovative technologies to try to help achieve that.

Vector says it’s seeing power demand shift, but not smooth out.

These range from “smart” home switchboards being manufactured by Kiwi-founded start-up Basis, to home batteries often bought in conjunction with solar systems, and modern takes on ripple control that allow hot-water heating to be controlled remotely by power retailers or lines companies.

The biggest single advance has been the introduction by electricity retailers of “time of use” plans that reward people for using appliances during off-peak rather than peak times, and which it will be mandatory for them to offer from June next year.

The first such plans came onto the market in 2015 and they have become much more common since 2021, according to electricity system operator Transpower.

Surprisingly though, it doesn’t appear to have been anyone’s job to measure whether peaks and troughs in demand are smoothing out over time or determine exactly what needs to be done to achieve that.

On the suggestion of the Sunday Star-Times, Transpower has undertaken some analysis.

A report it published on Wednesday found there had been a modest reduction in its peak-to-average load ratio over the past 10 years, “suggesting a slightly flatter demand curve”.

The mean peak-to-average load ratio across the entire year — which is one measure of whether demand has been “smoothing out” — decreased slightly from 127.6% in 2015 to 124.7% in 2024 in the North Island, and from 123.4% in 2021 to 121.1% in 2024 in the South Island, it found.

Two small bumps in demand are visible in this data grab from Transpower’s South Island network, one as households appear to respond to price incentives, and the other caused by ripple control.

Clear evidence that time-of-use plans are starting to have a meaningful effect on the overall electricity network was provided by Transpower in a separate monitoring report at the end of August.

It showed two small but clear bumps in demand just after 9pm and 11pm in one part of its network in the South Island.

The explanation would appear to be customers waiting until 9pm, when electricity can be cheaper under some power plans, to turn on some devices such as washing machines, dryers and dishwashers.

Transpower attributed the separate post-11pm spike to lines companies turning back on ripple-controlled systems’ hot-water cylinders at that hour.

But in at least parts of the network, time-of-use plans may be creating some unfortunate incentives.

Another Transpower chart saw demand reaching its daily high on another part of its South Island shortly after 9pm, as customers suddenly switched back on appliances after the supposed early-evening peak period.

Simon Mackenzie, chief executive of lines company Vector, says that is a common occurrence in areas of Auckland.

“What we are seeing is a shifting of demand, but not ‘smoothing’,” he says.

This chart shows larger spikes in demand building up on parts of Vector’s network over recent years after 9pm, as customers turn on appliances to take advantage of cheaper power.

When customers respond to retail tariffs and incentives, instead of lowering the overall peak demand, that is shifting the peak to a different time, a spokesperson explains.

“In some cases, it even makes the peak higher than before, just at a new time instead of the traditional 6pm to 8pm time.

“Current regulations limit how much we can see and manage on the network to reduce and smooth out peak demand. This is especially challenging for residential customers.”

Mackenzie says New Zealand is lagging behind.

He notes that in Germany, since the start of 2024, all large household appliances such as EV chargers and heat pumps have had to be set up so the local lines company can turn down their consumption to no more than 4.2 kilowatts during a power squeeze.

Margaret Cooney says there is an opportunity to do things smarter.

“This helps prevent blackouts, keeps the system stable and makes it easier for more people to connect to the grid as the country moves toward cleaner energy,” Vector’s spokesperson says.

One business with a huge incentive to smooth out electricity demand is British-owned Wellington electricity retailer Octopus.

Independent retailers such as Octopus are reliant on buying in power from generators, so they often bear the brunt when prices spike on the wholesale market during times of high demand and low supply.

Chief operating officer Margaret Cooney says there has been a slight reduction in residential electricity demand over time.

But despite Transpower’s seemingly mildly encouraging analysis, she says there’s been a trend towards more of that demand coming in the “peaky times” outside of the middle of the day — the opposite of what would be needed to limit the need for new investment in generation and power lines.

The evidence that consumers are willing to change the times at which they use electricity to align with price incentives is positive, she says.

But getting the incentives right, to achieve a smoothing of demand, is the next challenge, she says.

Cooney forecasts lines companies will follow electricity retailers in introducing time-variable charges to reward customers who give them some control over their power usage, in order to achieve that.

“It’s about making sure that you've got a smoother load profile, and I think we'll see some evolving network tariffs as a result.”

Whether progress is being made or not, with so much to be gained by reducing peak loads on the electricity network, one question may be why the opportunity isn’t being better researched and efforts to achieve that better coordinated.

“The biggest thing that would be driving consumer behaviour at the moment is pricing plans with ‘a free hour of power’ or ‘half-price nights’ and such things,” Cooney says.

“You can automate a lot of the big, chunky loads, like EVs, and control them, and that's where we've got an opportunity to do things a bit smarter.”

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