An Australian researcher has cautioned the increase in energy consumption while pumping water from rainwater tanks, although such tanks can save a lot of water.
"We should ensure we don't create an energy problem while trying to solve a water problem, Professor Stuart White of the Institute for Sustainable Futures at the University of Technology, Sydney, told ABC News.
"The message is that we need to be really careful about this because of the growth in the use of rainwater tanks," he added.
"The message is not that rainwater tanks are a problem," he explained.
According to White, gravity is a useful way to deliver rainwater to taps, but it is not always feasible in urban areas because tanks need to be higher than the roof to provide sufficient pressure for showers.
As a result, most households use small pumps that switch on every time you turn the tap on, even if it's just briefly to clean your teeth or flush the toilet.
"It means you have a pump running at full revs to do a tiny amount of work," said White.
In what is believed to be the first study of its kind, White and colleagues, monitored the rainwater consumption and associated energy consumption of 8 households in Sydney and Newcastle.
They calculated the "energy intensity" of household rainwater systems, which is the amount of energy used to get each kilolitre of water to the tap.
They found in most cases pumping water from rainwater tanks is more energy intensive than getting it from mains water, although lower than getting it from desalination.
White said that the standard suburban system rainwater tank with pump had an energy intensity of 1.5 kilowatt-hours per kilolitre.
"The water that you get out of a tap would be less than 1 kilowatt-hour per kilolitre," he said.
White said that the task now is to work out how to reduce the energy intensity of rainwater systems.
According to him, one option is to use pressure vessels, which can be fitted to a standard pump. These store pressure and avoid using the pump when only a small amount of water is needed.
Another option is to use variable speed pumps, which match the amount of energy used to the amount of water needed.
Still another option are rainwater switches, which uses mains water for small usages, only turning the pump on for larger usages.