On rare occasions Transpower instructs lines companies, including Powerco, to reduce load on the National Grid as part of an emergency shortfall response.

That means immediately reducing the amount of electricity our customers are using.


What’s a National Grid shortfall?

A National Grid shortfall is when Transpower anticipates that electricity usage is going to exceed available supply.

A shortfall can happen for a number of reasons:

  • When it’s very cold and ‘peak demand’ is expected to be too high. 
    (Peak demand refers to when electricity is most in demand – typically in the morning and evening when lots of people are at home using lights, ovens, hot water and appliances. Peak demand is higher when it’s cold and people are using more electricity than usual to power extra heaters.)
  • When lake levels are low, or lack of wind means not enough power is being generated.
  • When there’s a problem with equipment at a power station, or on the transmission network, which means not enough power is available.

It’s important to reduce load before capacity is exceeded to avoid a National Grid blackout.


How Powerco responds

There are two ways we respond:

1. Hot water control (also known as ripple control)

This is where we switch off residential electric hot water cylinders. 

In the event of a shortfall, we use this option first because it means that although you won’t have hot water at home, you’re still able to use lighting, heating, cooking and other appliances.

Once we’ve switched off the heating element, the water in your cylinder will start to cool.

Depending on how long your hot water cylinder stays off, it may take up to six hours for the water in your cylinder to come back up to temperature once we reconnect you.

Bringing your hot water back up to temperature after an outage will not cost you more in power bills overall, when compared to keeping the cylinder at the set temperature with uninterrupted electricity supply.

If you have gas hot water heating, you won’t be affected.


2. Disconnecting your electricity

If turning off people's hot water won’t reduce enough load, we may disconnect your power completely.

We prioritise who is disconnected as best we can. In general, we will disconnect homes in residential areas before we disconnect CBDs, factories or critical services like hospitals.

Who we disconnect will depend on the nature of the emergency, and how much load we have been instructed to shed.


What it means for you

  • Whether we’ve turned off your hot water or disconnected you, you don’t need to report your outage to us or your retailer.
  • If you’re affected by a National Grid emergency, you’ll be able to see it on our outage mapThe cause will show as ‘Loss of bulk supply – Transpower – unplanned’.
  • We will automatically reconnect you once the risk of a shortfall has passed and Transpower instructs us that it is safe to do so.
  • The length of the outage depends on the nature of the emergency. This means we won’t be able to tell you when you’ll be reconnected. 
  • If we’ve disconnected you, switch off appliances in use (like ovens, kettles, heaters) so they don’t come on straight away when your power comes back on.
  • You can help reduce load by conserving power – for instance, refraining from charging EVs, doing loads of washing or drying, or charging devices.

We know a power outage is never convenient, but it always helps to be prepared, view our outage tips here.


How electricity gets to you

Powerco is one part of the electricity supply chain.



Electricity Network


Power is generated at power stations (Aotearoa uses wind, hydro, geothermal and thermal fuel). Transpower, the National Grid operator, is responsible for transmission of high voltage electricity to local distribution lines companies (like Powerco), who own the substations that transform it into lower voltages and send it along the lines (or underground cables) you see in your neighbourhood to your service line, where it connects to your property.

The industry works together in the event of a shortfall to take action to avoid blackouts.