How the Blue savings calculator works

How we calculate your estimated savings

The Blue savings calculator estimates your potential annual energy, cost and CO₂ savings based on your shower habits, heating system and energy price.

It assumes System A installation, where Blue supplies preheated water to both the cold inlet of the shower mixer and your water heater. The calculation includes heat loss before the wastewater reaches Blue, a short warmup period and Blue’s electricity consumption.

1. Your annual shower use

We estimate your household’s annual shower water use using:

People × showers per person per week × 52 weeks × shower duration × flow rate

For example, 3 people showering 7 times per week for 8 minutes at 8 litres/minute use approximately 69,888 litres per year.

2. Energy needed to heat your shower water

The calculator uses these default assumptions:

• Cold water temperature: 10°C
• Shower temperature: 38°C
• Required temperature increase: 28°C
• Heat capacity of water: 4.186 kJ per litre per °C

For the example above, heating the shower water requires approximately 2,275 kWh of useful heat per year.

We then account for the selected heating system to estimate the energy purchased:

These figures cover shower water heating only.

3. Heat recovered by Blue

The calculator uses the following heat-recovery efficiencies at the default shower flow rate of 8 litres/minute:

• Blue 10XE: 52%
• Blue 21XE: 70.6%

These percentages describe heat-exchanger performance. They are not the percentage saved on your energy bill.

First, we assume the shower water cools by 5°C before reaching Blue. Water leaving the shower at 38°C therefore reaches the heat exchanger at approximately 33°C.

Blue’s efficiency is applied to the temperature difference between this wastewater and the incoming cold water:

Preheated water temperature = cold water temperature + (wastewater temperature − cold water temperature) × heat-recovery efficiency

For Blue 10XE:

10°C + (33°C − 10°C) × 52% = 21.96°C

With System A, the remaining heat required is equivalent to warming the shower water from 21.96°C to 38°C, instead of from 10°C to 38°C.

This gives a thermal saving of approximately 42.7% while heat recovery is fully operating.

We also allow 0.5 minutes per shower for warmup, during which the model assumes no heat recovery. For an 8-minute shower:

42.7% × (7.5 ÷ 8) ≈ 40.0%

For the example household, Blue 10XE therefore recovers approximately 911 kWh of useful heat per year, before accounting for the heating system’s efficiency and Blue’s electricity consumption.

4. Blue’s electricity consumption

The calculator includes:

• Active power: 42 W for both Blue 10XE and Blue 21XE
• Standby power: a conservative assumption of 1 W
• Active time: the full shower duration, including warmup
• Standby time: the remaining hours of the year

The default calculation assumes one Blue unit remains powered throughout the year, with no additional pump runtime after showering.

For the example household, this equals approximately 14.7 kWh of electricity per year.

The results account for this consumption:

Energy saved = avoided water-heating energy − Blue electricity consumption

With Blue = remaining water-heating energy + Blue electricity consumption

For Blue 10XE with an electric boiler:

For gas and district heating, the energy results combine heating energy and Blue’s electricity in kWh. Financial and CO₂ savings account for each energy source separately.

5. Your estimated financial savings

Annual financial savings are calculated as:

Avoided water-heating energy × heating energy price − Blue electricity consumption × electricity price

For electric boilers and heat pumps, the electricity price you enter applies to both the heating system and Blue.

For gas boilers and district heating, the price you enter applies to heating energy. Blue’s electricity costs use the calculator’s configured electricity tariff, which defaults to €0.30/kWh.

For the electric-boiler example with Blue 10XE:

• Net electricity saved: approximately 915 kWh/year
• Electricity price: €0.30/kWh
• Estimated annual saving: €275

The calculation excludes installation costs, maintenance costs, standing charges and payback periods.

6. Your estimated CO₂ savings

The calculator applies a CO₂ factor to the avoided heating energy, then subtracts the emissions associated with Blue’s electricity consumption.

Default factors are:

For the electric-boiler example with Blue 10XE:

915 kWh × 0.27 kg CO₂/kWh ≈ 247 kg CO₂ saved per year

These are calculation assumptions; actual emissions depend on your electricity supply and heating network.

Important assumptions

The calculator provides an indicative estimate for System A installation using the assumptions described above. Heating-system efficiency and heat-pump COP are treated as constant.

The heat-recovery efficiencies of 52% for Blue 10XE and 70.6% for Blue 21XE remain fixed in the model. If the configured flow rate changes, water use changes, but the calculator does not automatically adjust these efficiencies.

Actual savings depend on installation, pipework, water temperatures, flow rate, shower habits, heating-system performance and energy tariffs.

The results are estimates and do not guarantee actual energy, financial or CO₂ savings.