You don’t get to see this very often – when the spot price of electricity on all the state-based grids in Australia’s main wholesale market hit zero at the very same time.

The incident, captured in this screen shot of the Australian Energy Market Operator’s data page by Macquarie Capital’s Brian Morris and posted on his LinkedIn account, occurred at 1.15pm on Sunday.

This was when cloudless skies and good breezes – combined with low demand – pushed the share of renewables to more than 43 per cent. At the time of the zero pricing, wind was providing 12.3 per cent of supply in the National Electricity Market, while the combination of rooftop solar and utility-scale solar was providing 22.5 per cent. (Graph at top, courtesy of OpenNem.org).

About half an hour earlier, the total output from renewable energy was 9931.64MW , or around 44 per cent – well ahead of total black coal 9014.85MW and nearly three times the output of brown coal 3426.06MW. This looked like a major milestone but in fact is not the first time renewables have nudged out black coal – it did it in April.

The zero pricing reflected just one 5 minute dispatch period, so the actual settlement price for that half-hour period (based on the average of the six dispatches) was not zero – it was $6.44 per megawatt hour in Queensland, $7/MWh in NSW, $6/MWh in Victoria, and $5.39/MWh in South Australia.

But over the past 48 hours, South Australia and Victoria have experienced several settlement periods of negative prices due to the big amounts of wind generation (in the early hours of the morning), and due to the combination of high wind output and strong solar output during the middle of the day.

As Morris said: “You don’t see this too often. How much will solar be worth in the future? Wind vs Solar PPA’s?” (A PPA is a power purchase agrreement)

Indeed, as RenewEconomy has noted before, some utility-scale solar farms find themselves being switched off if it looks like wholesale prices are going negative for a lengthy period.

The responses to the post by Macquarie’s Morris pointed to the opportunities in demand response, battery storage and hydrogen.

“Negative prices (Tas) at 13:15 on a “winter” day,” noted consultant Martin Gill. “Imagine if this was summer and all the planned large grid scale solar farms were online. Highlights future Demand Management schemes must consider rewarding consumers willing to both increase and decrease their electricity use.”

“It is however a boon for storage whether it is large scale battery, pumped hydro or domestic storage,” wrote Craig Duggan, the CEO of Optimal Group Australia.

“Similarly it creates the opportunity to use zero or negative prices electricity to make hydrogen and store that is our existing huge storage networks, our natural gas networks.

“Solar and wind will fare well in this market as they have a zero cost of fuel. They can run through these low priced events and pick up when the price swings up. There us also a place of natural gas or biogas to fuel peaking gas fired power stations when the market price is right.”

The event was also picked up by analysts at Watt Clarity, who provide the data for the popular NEMWatch widget.

“We’ve been thinking through specific cases like this, and more generally “what happens when energy is free” for some years,” it notes.

“In our Generator Report Card, we invested significant time and energy in investigating the widening gap between Service 1 and Service 2 in the NEM – and the implications for what this means for the success (or otherwise) of the energy transition. Dispatch Intervals like this are part and parcel of that difference.

“Many will point at instances like this as demonstration of the need for storage – and, whilst that is true to a certain extend, we’re wary of any particular ‘solution’ being held up as a magic wand.”