In short
Working out the costs from the generation data.
The latest data
The Electricity Authority publishes each month a few weeks after it ends, so this shows the latest data, not live output.
The story
Six chapters on how close New Zealand is to a fully renewable grid, what stands in the way, when going all the way gets cheaper, and what solar is really worth.
1Mostly renewable alreadyHydro and geothermal have carried the grid for decades. Wind and solar are now adding to them.
Where New Zealand's power comes from
Share of grid generation each year. The last bar is the past 365 days of data.
2Dry years bring back coalWhen the lakes run low, Huntly burns more coal and gas to make up the gap.
New Zealand's hydro lakes only hold a few weeks of winter demand. In a year with little rain or snow melt, the gap is filled with gas and coal, most of it at Huntly.
Hydro and fossil generation each year
Fossil generation each year, in TWh. Years when hydro made at least 1 TWh less than usual are outlined: they stand out as fossil peaks. Hover a bar for that year's hydro.
3More hours on almost no fossil fuelThe grid now spends long stretches with gas and coal under 100 MW.
Hours each year with gas, coal and diesel together under 500, 250 and 100 MW. The last bar is the past 365 days of data.
4Batteries are arrivingNew Zealand's first grid batteries connected from 2023.
Battery output over time
Daily mean battery discharge into the grid, in MW. The Electricity Authority's data shows what batteries send to the grid, not what they take when charging.
The biggest is still to come. Genesis is building a battery at Huntly Power Station: 100 MW and 200 MWh due in 2026, and another 100 MW and 200 MWh around 2029 (Genesis to the NZX, ESS News). It plugs into the coal station's existing grid connection. At full size it covers about an eighth of the fossil capacity the grid leans on at its evening peaks, so the cost model in chapter 5 counts it. It does nothing for a dry year: 400 MWh lasts two hours, and a dry year is short of water for months.
5When does 100% get cheaper?The cost of covering the driest year on record with renewables, against keeping coal and gas on standby.
Getting rid of the last fossil fuel means covering the worst dry year without it. That takes enough extra renewable generation to replace the water the lakes don't get, and enough fast capacity to replace the gas and coal plants at the evening peaks. In wetter years, much of that extra energy isn't needed.
What has to be replaced
Yearly cost of each way through the weather on record
NZ$ million a year, in 2025 dollars. Each renewable line is enough of that one source, plus batteries, to get through every year from 1980 to 2024 with no fossil fuel, sized by the lake model in chapter 6. Renewable and battery costs fall a little each year; fuel and carbon are held at the chosen prices.
What the answer depends on
How this is worked out, and sources
6What if we put solar on hydro lakes?What solar is worth when the lakes run low, from 45 years of rain, wind and sun.
Running the lake model.
The hydro lakes through 45 years of weather, with no fossil fuel
Energy stored in the hydro lakes each week, in TWh, if every day from 1980 to 2024 had today's demand and no coal or gas.
What each build costs
NZ$ million a year to build and run, in 2025 dollars, annualised at the cost model's rates. Each gets through every year from 1980 to 2024 with no fossil fuel. All of them also need the same batteries for the evening peaks, costed in chapter 5.
How the lake model works
About the data
Generation comes from the Electricity Authority's generation output by plant: half-hourly metered output of every grid-connected plant since August 1997, published monthly. It leaves out rooftop solar and other small generation inside local networks, so solar and the total read lower than MBIE's national figures. Huntly's Rankine units can burn coal or gas, and the data counts them as coal.
Renewables here are hydro, geothermal, wind, solar, wood and biogas. Fossil fuels are gas, coal and diesel. Battery discharge is shown on its own and is not counted in total generation.
A spell is a run of consecutive half hours that meet the threshold. Gaps in the data of up to an hour are bridged when the half hours either side qualify; longer gaps end the spell. Years run in New Zealand time.
Built by Alex, and the source code is on GitHub.
Contains data from the Electricity Authority's Electricity Market Information website.