Large Home Batteries Enhance Rooftop Solar Efficiency
Last week, I discussed how three government initiatives could build on the recent achievements in small battery technology, potentially leading to an additional 22 gigawatts of rooftop solar installations by 2035. This would be in addition to the 47GW projected in AEMO’s current Step Change estimates.
This expansion would span across Australia’s two primary power grids: the National Electricity Market (NEM) and the South-West Interconnected System (SWIS) in Western Australia. Such a development would significantly aid in the decarbonisation of the grid, increasing the share of renewable energy in the power supply by nearly 9%.
Understanding the Potential for Growth
These insights are elaborated in a report we prepared for the Australian Conservation Foundation and the Smart Energy Council, titled “A Re-evaluation of the Potential for Growth in Rooftop Solar and Small-Scale Batteries.”
However, with solar energy often exceeding half of the power supply during midday hours, there are valid concerns regarding the management of this additional solar capacity. Some fear it may lead to increased curtailment of other renewable energy sources.
Changing Perspectives on Battery Systems
Until recently, I shared these concerns. However, my viewpoint has shifted due to the unexpectedly large battery systems that households are opting to install since these systems became eligible for the Small-scale Technology Certificate (STC) rebate.
The accompanying chart illustrates the average size of battery systems registered with the Clean Energy Regulator daily since 1 July, when the STC rebate was introduced. The green bars represent the average size, while the dotted blue line indicates the moving seven-day average, which has steadily increased to 20kWh.
Given the relatively low cost of adding extra capacity to battery systems post-rebate, we anticipate that within the next year, the average size of residential battery systems will rise to 25 kilowatt-hours. Such sizeable residential batteries will revolutionise the utilisation of household solar energy.
Transforming Solar Energy Usage
Currently, a typical household equipped with a 6.6-kilowatt solar system—still a popular choice despite the overall market average being higher—exports a significant portion of its solar output to the grid, often between 70% and 80%. These exports are predominantly concentrated around midday.
However, our modelling of a 10kW solar system, which is likely to become the average size soon, in conjunction with a 25kWh battery, reveals that these midday solar exports would largely vanish.
The chart below depicts the hourly distribution of energy usage for a solar and battery system in a typical all-electric home with one electric vehicle in New South Wales during both summer and winter.
During summer, from 7am to 6pm, the solar system generates sufficient power to meet all household consumption (represented in light green), with most of the surplus (shown in dark green) directed towards charging the battery. Only a minimal amount (indicated in red) is exported to the grid.
In the evening peak period, between 6pm and 9pm, the battery exports a significant portion of its stored energy to the grid (shown in dark blue), while still retaining enough capacity to meet the household’s overnight energy needs (illustrated in light blue).
Winter Energy Dynamics
In winter, the battery’s capacity is often too large to be fully charged by solar energy alone, necessitating a small amount of power import from the grid (represented by the negative value red bars).
Nevertheless, if the battery is fully charged (as assumed in our modelling), it retains ample excess capacity for exporting power to the grid during the evening peak and for covering the home’s energy consumption overnight.
The large batteries now being adopted by households effectively transform rooftop solar generation into a reliable energy source, akin to a baseload generator, with the added flexibility of a gas peaker.
The Future of Rooftop Solar
If 25kWh batteries become the standard when solar systems are installed or upgraded, as we anticipate, it will significantly enhance our ability to leverage rooftop solar energy to replace coal and gas power plants.
