Explainer: Electricity - where does it come from, where does it go?

Electricity - how do we generate it, how does it get from a hydrostation to our kettles, and where is the technology heading?

Unitec Institute of Technology electro-technology senior lecturer Glenn Nicholson goes through the country's power grid with Jesse Mulligan on RNZ's Afternoons.

What are the main methods of generating electricity?

New Zealand gets roughly 65 percent of its electricity from hydropower.

"Hydro is very weather dependent. It depends on our lake levels, so that can sort of hover between 60 and 65 percent."

Then another 15 percent is generated from geothermal renewable energy sources.

Wind energy - which is also weather dependent - produces between 5 and 10 percent of the electricity supply.

Gas is used to produce about 10 to 15 percent of the electricity.

• Read more: How New Zealand got through the power supply threat by a narrow margin

How is electricity generated?

Hydroelectricity is generated via a storage lake.

Water, stored in a lake, is directed to run down through penstocks, which spins a turbine.

That turbine in turn spins a generator which converts that mechanical rotating energy into electricity.

"It's gravitational potential energy which is stored in the lake. So the lake is acting like a big battery, like a big store of energy."

For geothermal energy, he said steam was pulled out of the ground and used to drive a mechanical rotating machine and that drove a generator.

"So most of our electricity comes from rotating machines, generators."

He said about 80-85 percent of energy was generated from renewable sources, and had been that way for the past 10 years.

"We have a target of getting up to 95 percent by 2030."

But building more hydro was not the answer, he said.

"Building hydro is complex and expensive and there's a lot of environmental impact if you're damming rivers, creating storage lakes.

"We do have some more geothermal potential that we can tap into, which is good news for us."

Nuclear energy was not counted as renewable but had low carbon emissions.

"It's relatively clean energy, but there's always the nasty thing about radioactive waste.

"There's no real political appetite for that."

How does electricity make it into households?

Electricity from the generating station is connected to the transmission grid which is made up of the big steel towers, power pylons, which are operated by Transpower.

"So we generate a high voltage - typically 110,000 or 220,000 volts and we put it out on the national grid.

"The National Grid is the backbone of our electricity system. It's the long-distance transport to get it from where we generate out to the regions where it's going to be used."

About 10 percent of generated electricity can be lost along the way.

It was efficient, he said, but the distance affected the amount of power loss.

"The further you're going to transport it, the more you're going to lose."

Most of the hydrolakes are in the South Island and the bulk of the demand is in upper half of the North Island.

"It's a very resilient system. We know that electricity is an essential service.

"We have backup circuits so that if you do get a point of failure, there's other circuits that you can fall back on and you can shift that load onto another circuit."

He said the grid operator would constantly be looking at risks and potential problem events, and the solutions for them.

What about solar power?

Solar power is still a minor player at about 1-2 percent, but is increasing slowly.

"It's really just started to pick up in the last couple of years where we are getting these grid-scale solar farms being built."

Until now it had only been at a domestic level where people put solar panels on the roofs of their houses and some larger commercial installations.

"But we can expect to see significant growth in that area."

How often are we burning coal?

That depended on the weather, he said.

"The Huntly Power Station is probably our biggest coal use for grid scale generation.

"There have been years where those machines have sat idle for the majority of the time, and then we get cold snaps and we fire them up."

Those turbines can also be powered by gas, so he could not say how much of the time they were running on coal.

"It's not just the generating companies who are potentially burning coal. It's also some of our larger industrial users. They may use some coal to produce their own process heat and also generate some electricity."

Demand and supply: What happens when I turn off my heater?

The generators are automatically regulated, he said.

"Supply and demand do have to match, and if we have too much supply then it affects the stability of the grid. And likewise, if we've got too much demand, it tends to drop the frequency on the grid. So, there's a constant regulation balance occurring between those two."

So when someone switches off their kettle, or the light, even though it's a very small change, that will cause a consequential back off at the generation.

The power to charge electric cars

It would come from the development of more geothermal energy, he said.

"We've got some untapped geothermal ... all around that central volcanic plateau area - Rotorua, Taupō, around those areas."

But wind and solar would also play a big role in helping generate electricity, he said.

He could not say much about the vehicle-to-grid technology just yet.

"I know it exists. How common it is, I don't know. If you've got your EV sitting there plugged in, there is no reason why if you don't need that energy that's stored in your car battery, why you couldn't export it out to the grid.

"And I'm sure that a lot of EV chargers will have that capability now, or if they don't have it now, they'll have it very soon whereby you'll strike a deal with your electricity retailer, where if they need to, they can tap into your EV battery. And they will pay you for the privilege."

New technology

He said transmission and generation went hand-in-hand and a higher demand would mean the need for more grid connections.

"The Holy Grail of generation and transmission is having more storage.

"We can use lakes as storage. You know exactly how much water you've got there and you know exactly how much you can generate. You can't do that with wind or solar."

An alternative still on the books was the Lake Onslow pump storage option, where excess electricity could be used to pump water back up into a lake. Yeah, and hold it there until you need it.

"The other emerging technology is grid-scale batteries - big battery banks."

He said battery storage was going to be "a real game changer".

"If we can have significant amounts of battery storage, what that will enable us to do is basically shave the tops off those peaks and that's going to make our grid a lot, a lot stronger and a lot more resilient."

Cold snaps affecting power

Earlier in May, households were warned that unless they conserved electricity, they could face possible power cuts because of an unseasonable cold snap and low wind generation.

He said that the cold snap overlapped with gas station maintenance work which was meant to prepare it for the winter.

"We had some generation that was out of action, and then we get this cold snap, a very high demand.

"We were a little bit caught out with the earliness of that cold snap."