June 2021

Purpose. To explain that even the biggest “big batteries” cannot provide grid-scale dispatchable power.


We are informed by AEMO that the energy transition is inevitable. “This system is now experiencing the biggest and fastest transformational change in the world.”

At least 15GW of coal power is expected to close by 2040, to be replaced by some 36GW of wind and solar power.

The intermittent input from wind and solar will have to be “firmed” (backed up) by “dispatchable” power from some combination of gas generation, storage in batteries and storage in pumped hydro reservoirs.

The dual function of big batteries

One of the planned functions of big batteries is to provide almost instantaneous inputs to counter sudden falls in the supply of wind or solar power that were signalled in this note on fluctuations in the wind supply.

The other function that is (hopefully) planned is to provide substantial amounts of power to cover periods of high demand (dinnertime) and periods when there is little or no wind and solar power (windless nights.)  This proposal is not realistic due to the limited capacity of “big batteries’ compared with the amount of power required in the grid.

The limited capacity of “big batteries”

Consider the amount of power stored in the Hornsdale Power Reserve, the official name of the Elon Musk Big Battery installed at the Hornsdale wind farm in 2017. It was billed as the biggest battery storage unit in the world at the time and it occupied a hectare with a cost of $90 million for 129 MWh of power. In 2020 the second phase added 65MWh at a cost of $71 million. That is $1.1 million per MWh compared with $700,000 per MWh for the first phase. This is surprising, assuming that most of the infrastructure supporting the battery (land, cabling etc) that was purchased and constructed during Phase 1 is common to Phase 2. That should reduce the cost especially as we are assured that the price of storage is “plummeting.”

Big battery capacities are often reported in MW and not MWh. The difference is critical because the MW figure indicates the depth of the flow  or the size of the pipe if you want to think about it like that and the figure for MWh indicates the quantity or the amount of power that flows through the “pipe.”

There are reports of two-hour, four-hour and even eight-hour batteries and we need to know the depth of flow in MW in addition to the period of the flow in hours to know precisely how much storage capacity can be delivered.

To be clear about the limited capacity of big batteries, compare the 194MWh of power stored in the Hornsdale Power Reserve with the amount of power consumed in the state of South Australia. The depth of the stream of power in the grid varies over the range of 1000MW to 2,500MW depending on the time of day and the season. Allowing 1,500MW for the purpose of estimation, that translates into a daily flow of 36,000MWh. That is equivalent to the capacity of 185 Hornsdale batteries.

With the cost apparently in the vicinity of $200 million per unit, that amount of battery storage is clearly out of the question even when the number of units is reduced to take account of solar power during the day. Many more would be required to allow for wind droughts that exceed 24 hours, as occurred in June 2020.


In the whole of the National Energy Market covering all the states in SE Australia the “depth” of the stream of power in the grid varies from 18GW (18,000MW) to 37GW (37,000MW) at the peak of demand during summer heatwaves. The figure below shows how the demand rises from the low point in the small hours of the morning to meet the demand at breakfast time, then settles during the day to rise again to the daily peak at dinnertime. The peak of demand lately did not exceed 30GW and so the calculations underestimate the amount of power required at the peak of demand near 37GW.  It is not necessary to be more precise to make the point about the relative amount of power in “big” batteries compared with the grid.

Picture tells story

Allowing an average flow of 25000MW for the 24 hour day, the total amount of power required is 600,000MWh, that is 3000 Hornsdale units.

It is an interesting academic exercise to calculate the cost of batteries to cover wind droughts of varying duration up to (say) the 33 hours experienced on the 5th and 6th of June 2020.  Forget it, even if the cost plummets to half or even one tenth of the cost of Phase 2 at Hornsdale!

Recommendation. Stop talking about batteries providing dispatchable power at grid-scale.