Water Efficiency Calculations

Water Efficiency Calculations are used to measure compliance with water efficiency targets for new build dwellings and conversions. These calculations are used to ensure compliance with Part G of UK Building Regulations where a maximum water use of 125 litres per person is required.

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Part G of the UK Building Regulations requires that all new dwellings (including those created by a change of use) achieve a minimum standard of water efficiency. This standard is set at 125 litres per person, per day. Often, this target is set lower by local authorities and/or water suppliers, usually, at 110 litres per person per day. 

Some water suppliers offer discounts as incentive schemes to ensure higher standards of water efficiency are met. Severn Trent, for example, offer developers a discount of up to 100% on infrastructure charges if they can reduce water use to 110 litres per person per day, meaning huge savings can be made. 

How do we calculate Water Efficiency?

The water efficiency of a dwelling is calculated using flow rates and capacities for:

  • Toilets
  • Taps
  • Baths
  • Showers
  • Washing machines
  • Dishwashers
  • Waste disposals
  • Water softeners
  • Rain/greywater harvesting systems

At RJ Energy, we advise that water calculations are carried out at the design stage, prior to the purchase of any fittings. When carrying out the calculation after fittings have been installed you may be required to fit flow regulators or change fittings entirely to achieve a pass. Our bespoke water calculations and report advise on current compliance and any planned fittings which require improvement in order to gain compliance

Water Efficiency - Running Tap

Part G - Water Efficiency Advice

The tables shown below are taken for Part G of Building Regulations and show the advised approach to achieve a target of 125 or 110 litres per person per day. RJ Energy use these tables at the design stage to advise clients on flow rates to achieve when purchasing fittings – staying within these limits for each fitting guarantees a pass on your water calculation. 

Maximum fittings consumption (125 litres)
Water fittingMaximum consumption
WC6/4 litres dual flush
4.5 litres single flush
Shower10 litres/min
Bath185 litres
Basin taps 6 litres/min
Sink taps8 litres/min
Dishwasher1.25 litres/place setting
Washing machine8.17 litres/kg
Optional requirement level (110 litres)
Water fittingMaximum consumption
WC4/2.6 litres dual flush
Shower8 litres/min
Bath170 litres
Basin taps 5 litres/min
Sink taps6 litres/min
Dishwasher1.25 litres/place setting
Washing machine8.17 litres/kg

RJ Energy will provide advise on water efficiency at design stage and can help with further advice when purchasing fittings. Once final fittings have been installed a water calculation will be issued to provide to Building Control which demonstrates compliance. 

Frequently Asked Questions

  • Ensure you have a water calculation carried out at the design stage or at the latest, when purchasing fittings.
  • Follow the maximum fittings guidance in Part G of Building Regulations (as detailed in the tables above).
  • If there is a particular fitting you want but it has a high flow rate then flow restrictors or aerators can be fitted to reduce flow and achieve compliance
  • Low flush toilets (4/2 litre dual flush) are a great way to reduce water use alongside aerating taps and highly efficient showers.

If failure is determined at design stage, we can advise on changes to make which will mean gaining compliance, such as lower capacity baths or high-efficiency showers.

If the calculation is not carried out until the fittings are installed then we can advise on flow restrictors or aerators which can increase efficiency and gain compliance however there is a risk of having to replace fittings if the calculation still cannot achieve a pass. 

We highly advise that the calculation is carried out prior to fittings being installed as we often deal with dwellings where there is limited access to fit flow restrictors, such as concealed shower valves.

Our calculations generally cost £80-100 dependant on the size of the dwelling.