For the first time Australia’s main electricity grid reached a share of over 70% for renewable energy this week, an achievement that showcases the rapid rise of solar and wind generation, but also highlights several challenges.

Australia is a world leader in renewable electricity generation and has the world’s highest penetration of rooftop solar systems per capita.

At around 1.05pm Australian Eastern Daylight Time on Oct. 24 the grid that supplies the five of the nation’s six states was powered by 72.5% renewables, with solar and wind providing 70.9% and hydro the other 1.6%.

That may sound like a remarkable achievement in a country that until recently was largely powered by coal, but it could have been even higher, possibly above 90%.

The reason total generation from renewables didn’t reach over 90% on Oct. 24 was that some renewables were curtailed, largely due to negative wholesale electricity prices, according to the website Renew Economy.

The rising share of renewables illustrates a wider problem for Australia’s grid, and those in other countries that are also seeking to transition away from fossil fuels.

Australia generates excess solar during the middle of the day, which in turn forces the ageing fleet of coal-fired generators to ramp down, before they ramp up again as the sun sets and renewable generation slides.

There are solutions to this, and all of them are variations on the theme that as the grid is fed by more and more variable renewables, you need fast-acting and variable backups.

In Australia this role has fallen to natural gas-fired peaking plants, battery storage and hydropower.

Natural gas, while cleaner than coal, still emits carbon and ultimately isn’t a long-term solution if the goal is to reach net-zero electricity generation.