The rainwater harvesting system went live at the start of December 2025, replacing the use of mains water for the toilets, the outside taps and the washing machine – and literally halving the mains water consumption in the process. The rainwater does need to be pumped out of the underground tank but a dedicated electricity meter shows the pump and its controller consume just 1 kWh of electricity per 1,000 litres of water delivered – which costs £0.05 compared with £2.61 for importing 1,000 litres of mains water.

The water saving is no surprise, since the ‘always needs to be mains water’ and ‘potential future rainwater’ consumption have been metered separately since the House was built. The low electricity consumption was largely expected by making careful choices when selecting the water pump and its controller (and by including a pressure vessel which gets ‘charged up’ whenever the pump runs, avoiding the need for the pump to start more than a few times a day) – but it is good to have this confirmed.

Back when I was specifying the systems for the House, the consensus in the low energy building community was heavily influenced by a 2010 report by the UK Environment Agency which concluded that domestic rainwater harvesting systems were 40% more carbon intensive than using mains water, with a split of 52% ‘operational’ carbon and 48% embedded. That ‘operational’ carbon was due to the electricity consumption of the rainwater pump – especially for ‘direct’ systems which need the pump for every rainwater draw-off (rather than periodically filling a cistern which then supplies water by gravity). Despite this report, I went ahead and installed parallel pipework systems in the house, for potentially splitting out rainwater usage from mains water usage. To a large extent, this was based on the expectation that the carbon intensity of the electricity supply would reduce over time.

More recent research has shown that things have changed and that rainwater harvesting systems are now typically beneficial from both a carbon- and a cost-saving standpoint. However, it still makes sense to reduce the embedded carbon – for example by avoiding tanks which need to be bedded in concrete – and to choose energy-efficient control systems.

My experience is that the water looks as clear as mains water with no hint of any smell. There is no sign of any algal or bacterial growth where the water gets stored at room temperature in toilet cisterns. The mains water here isn’t especially hard but the rainwater is completely soft so there’s no limescale build-up and washing detergents are more effective. I had wondered if it might be necessary to retrofit an inline filter for the washing machine in particular but based on current experience that is absolutely not necessary.

The filter on the inlet to the underground tank has been getting quite dirty with fine dust coming off the new clay roof tiles (a distinctive ‘Staffordshire Blue’ colour) and needed cleaning ever month or so, but that’s now reducing.

While the saving on the mains water bill is the main appeal, there are wider benefits too:

• Provided the tank isn’t already full, hundreds of litres of rainfall will get stored in the tank, reducing the run-off from the site and helping mitigate the risk of flooding in the wider area
• Less demand is placed on the water supply and delivery network
• The tank holds an on-site reserve of water for firefighting purposes, which is useful since the nearest fire hydrant is literally miles away

There’s some more detail in the Technical Article here.