For Australian households looking to reduce their electricity bills, finding the best balance between rooftop solar and battery storage can be a challenge.
In a new paper published in SCI Sustainability, Adelaide University researchers have crunched the numbers to give householders some practical options to maximize their energy self-sufficiency.
Dr Kirrilie Rowe and Professor Peter Pudney from the Centre for Sustainable Infrastructure and Resource Management, analyzed real electricity consumption data from South Australian households and apartments over a two-year period.
Rather than attempting to predict future electricity tariffs – which are likely to change over the lifespan of solar and battery systems – the team focused on three long-term trends that are expected to continue:
- daytime solar exports are becoming increasingly less valuable as electricity networks become saturated with rooftop solar;
- battery prices are continuing to fall; and
- electricity purchased from the grid during morning and evening peak demand periods remains more expensive than off-peak power.
Using these trends, the researchers developed a method to identify the combination of solar generation and battery storage that delivers the greatest reduction in grid imports for a given investment.
“Our research shows that the trade-off between adding more solar panels or installing a battery depends on how much solar PV you already have,” said Dr Rowe, who is based in the Centre for Sustainable Infrastructure and Resource Management.
“If your existing system is relatively small, adding more solar may be the best option. However, once you reach a certain point, a battery may be the better investment.”
Professor Pudney said that instead of trying to predict future electricity prices, the research focused on how householders can maximize their use of the clean energy they generate themselves.
“The less electricity you need to import from the grid, the more resilient and cost effective your energy system becomes,” he said.
The study identified clear tipping points at which batteries become more cost effective than additional solar panels.
For the most common household solar installations in South Australia - typically between 6 and 10 kilowatts - the researchers found that homeowners should consider installing a battery before considering further solar expansion.
“Some households may not be choosing the most effective mix of solar panels and batteries to cut their reliance on the grid during expensive peak times,” Dr Rowe said.
“For example, our sample of relatively energy efficient households showed that a 6 kW solar system with 4 kWh of battery storage reduced electricity imports by 71%, while spending the same amount on a much larger 18 kW solar-only system achieved just a 49% reduction.
“Even larger batteries paired with small solar systems couldn’t match the savings of a well-balanced, larger solar-plus-battery setup.”
The researchers describe their findings as a practical guide for residential energy systems, helping households identify the combination of solar and battery storage that is neither too small nor unnecessarily large.
“The solar panels are what generate the clean, low-cost energy,” Professor Pudney said.
“The battery’s role is to store excess energy so it can be used later. The ideal battery size depends on how much surplus solar energy you produce and how much electricity you use overnight.”
The researchers say oversized batteries may still make sense for households participating in virtual power plants or using electricity tariffs that allow them to buy and sell energy strategically. However, for most homeowners, achieving the right balance between solar generation and storage is the key to getting the best return on investment.
Cost-effective residential energy storage independent of specific electricity prices is published in SCI Sustainability.