Water and energy nexus of residential rainwater tanks at an end use level: Case of Australia

• Energy intensity of rainwater tank end use categories empirically quantified.
• Toilet cistern refills more energy intense than clothes washing or irrigation.
• Toilets consumed largest total annual pump electricity in the typical home.
• Low flow rate water efficient appliances adversely impacted pump energy.
• Popular fixed speed pump models are inefficient at supplying indoor end uses.

Internationally, rainwater tank systems have experienced a renaissance in urban areas in recent times as they are perceived to be a low cost source substitution option for many end uses or micro-components (e.g. toilet, clothes washer, irrigation, etc.) of water demand. While there are many studies that have been conducted investigating rainwater tank performance, none have explored their water-energy nexus at a micro-component level. Through a high resolution (i.e. 5 s, 0.014 L/pulse, 1 W/pulse) smart metering study, this research was able to reveal the energy intensity of three rainwater tank end uses in 19 residential homes located in Gold Coast City, Queensland, Australia. Half flush toilet cistern events were found to have the highest variability of energy intensity values between homes (1.05–3.32 Wh/L) and also the highest energy intensity at 1.88 Wh/L. Full flush toilet events had a tighter range (1.02–2.30 Wh/L) and slightly lower energy intensity than half flush events at 1.61 Wh/L. Clothes washer energy intensity values were quite variable (0.90–2.73 Wh/L) but on average were lower than toilet flushing at 1.28 Wh/L. Lastly, irrigation events had the tightest range (0.83–1.66 Wh/L) and lowest average energy intensity of 1.12 Wh/L. The study has significant implications for building code specifications for internally plumbed rain tanks systems, particularly fit-for-purpose pump selection.