There's a vast amount of whirring and clunking to deliver a deceptively simple product. You might think it begins by damming a river, but actually it goes back much further with engineering, hydrology, geological and financial planning.

Once the dam's built, we need miles of piping, valves, pumping stations, filtration and monitoring systems, and of course, electricity to drive all that. There are administrative and financial systems involving many people beavering in a generally organised way.

If we go back even further in the chain, there's a long story about where and how the copper pipes were made, where the concrete came from, the steel, the plastic, and so on. It feels like an infinitely long sequence, and yet it happens. The swan, as they say, glides across the water, while below there are frantically paddling feet.

When the water arrives in your house's plumbing, the next thing is to heat it. There are several possibilities, depending on what system you have, such as instantaneous gas, gas with storage, electric resistance, solar or electric heat pump.

There are a few variables so it's difficult to give a precise calculation, but running 10 litres of hot water to 50 degrees from a tap will emit nearly half a kilo of CO2 if the electricity is generated from brown coal. That drops to under 400 grams for black coal, or about half the brown coal emissions for natural gas.

Whichever the case, the number is actually larger because every kilo of fuel has to be mined, transported, and fed into a furnace. Then waste ash must be disposed of. This accounts for nearly a fifth of Australia's entire waste stream.

CO2 emissions are much lower for solar hot water. However, there will still be emissions from the electric or gas booster, if it's powered from the electricity or gas grid in mainland Australia. These emissions can be minimised by turning off the booster during most of the warmer months.

An electric heat pump hot water system has almost no emissions during operation when it's powered by rooftop solar electricity.

Response by: Rod Taylor and Dr Mark Diesenforf