Instant boiling water taps...transparent costs for the first time

The information in this updated article contributed to BBC Radio 4 Sliced Bread show on the subject. Click here to stream.

This article is about the type of instant hot water tap that permanently holds a tank of water close to (or even above) boiling point to provide instant boiling water in a kitchen. There is a common perception that these products are cheaper to run than a regular kettle.

I found in my research that the information on the running costs of these competing product types is of very poor quality, makgin it impossible for someone without good energy knowledge make objective comparisons. Some of the most shocking examples are in my other article here.

This article presents the first clear and meaningful comparison of annual energy costs of several instant boiling tap products with kettles. It is based on robust evidence, real world scenarios and transparent assumptions.

The analysis does not cover more complicated variants of these products that have other features e.g. chilling water.

This article is not 'anti' instant boiling taps. Its purpose is simply to present facts, informing householders when making choices on this subject.

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Analysis headlines

For the instant tap that overall is the most efficient (Quooker):

• A household drinking around 10 mugs of tea or coffee a day will save around £10 a year using an ordinary kettle (assuming a little care to reduce kettle overfilling). Only by drinking more than 23 mugs a day would the instant tap method be cheaper.

• For a household drinking around 10 mugs a day overfilling their kettle (typically 50%) then the annual energy costs of a kettle will be about the same as the instant tap. In this scenario the instant tap gradually become cheaper to use for boiling water use that is greater than this amount.

  
  

For the other brands of instant tap I assessed, the energy benefits of the kettle were more significant, (sometimes much more).

Overall, instant boiling taps only result in significant cost savings if you use significant quantities of boiling water. This makes them ideal for busy commercial environments.

The comparative analysis does not take into account the purchase, installation or maintenance costs of instant boiling water taps. Factoring these in would make the results from any such analysis even further in favour of the kettle.

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Background

Instant boiling taps energy use

They use energy is used in three ways:

1. Energy to keep the water hot and ready for dispensing. Although the storage tank under the sink is well insulated, it still loses heat. This needs to be periodically re-heated throughout the day to keep the water hot.

2. Energy to heat up incoming (cold) water that goes into the storage tank to replace recently dispensed hot water.

3. They waste some energy when dispensing hot water. Some (including Quooker) waste as much as 8-9% of the heat energy in this way.

Kettle energy use

They obviously only use energy when actually in use.

When boiling a kettle, the main ways energy is wasted are:

• Heating the body of the kettle

• Heat escaping through the kettle side walls to the room

• Steam escaping into the room through the spout (most of which is in the final stage of boiling)

Despite the above, kettles are remarkably efficient things (85 to 90% efficient according to a number of academic studies*). I (and others I know) have measured a selection of kettles and found their efficiency is very consisten, in the 86-87% range.

The biggest "inefficiency" factor contributing to the running costs of a kettle is overfilling and boiling some water unnecessarily. It is widely accepted that household kettles are typically overfilled by 50%. This wastage adds up in the long term. The Energy Savings Trust suggests that reducing overfilling of a kettle will save the typical household around £10 a year (this matches my own analysis).

With a little care when filling a kettle it is quite easy to reduce overfilling a kettle. Practical tips to come later.

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Energy cost comparisons

The tables below compare the energy costs of using an ordinary jug kettle with four brands of instant boiling water tap.

They show the annual costs for three different scenarios, quantities of mugs of tea or coffee drunk every day:

• 10 mugs per day (slightly more than the national average**)

• 20 mugs per day

• 30 mugs per day

Of course, water is boiled in homes for other purposes than making hot drinks, such as for cooking. For that reason, the total volume of water heated is also shown in the tables.

I assume a 20% overfill of the kettle in these initial tables. Other scenarios are shown later.

Detailed assumptions are shown at the end of this article.

The figures in each table row are:

Row A - the cost of energy used by instant boiling water taps to keep the water hot and "ready and available" throughout the year. This varies for different manufacturers. This is called "standing losses".

Row B - the cost of reheating incoming fresh cold water to replace the boiling water that has just been dispensed by the instant tap.

Row C - the resultant total annual running costs (rows A and B added).

Row D - the resultant average cost per litre of boiling water for each method/product.

10 mugs a day

In this scenario the kettle costs are comfortably cheaper across the board.

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20 mugs a day

In this scenario the costs for the ordinary kettle still slightly cheaper than Quooker but still a good deal cheaper than other tap brands.

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30 mugs a day

In this scenario Quooker nudges into the lead, saving around £6 a year.

30 mugs of tea though, is around 2 gallons of boiling water per day. A bucket full of boiling water...every day!

Even when boiling this much water, the other instant tap brands are still more costly to run than a kettle.

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Conclusions...so far

As you saw above the Quooker works out similar to the kettle when you reach 23 mugs of use (assuming a little care to reduce overfilling).

For the other brands to have equivalent costs to a kettle you need to drink the following amounts:

Qettle: 65 mugs/day (21 litres/day)

Fohen: 53 mugs/day (20 litres/day)

Hyco: 32 mugs/day (12 litres/day)

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How costs work out if you continue overfilling your kettle?

The table below shows the comparative costs if you carry on being the typical household overfilling your kettle by 50%.

As you can see, whether you use a kettle or a Quooker annual energy costs work out virtually the same: 10 cups of tea/day. (Bear in mind the average household boils the equivalent of 8 mugs a day, which would make the kettle slightly cheaper yet again)

Even in this scenario, the kettle is still cheaper than using the other tap brands.

Quooker response to the above

During the engagement with the BBC Sliced Bread team in the Autumn of 2024, the BBC approached Quooker with my analysis. Quooker countered with their own figures, stating the cost equivalence with a kettle is lower, at 7 mugs.

Quooker stated they were assuming a kettle efficiency of 71%. 71% is significantly lower than academic literature on the subject and observed real-world measurements. I therefore challenge their figure, which clearly makes a significant difference in this comparison in favour of their product.

It was seemed very likely from some of Quookers responses that it was very unlikely they had previously made serious cost comparison of their products with kettles, nor actually measured the efficiency of the latter for themselves.

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Significant overfilling scenario

For my own entertainment and curiosity the comparison below is for a householder who habitually overfills by double every time you they use the kettle i.e. always boil double the amount needed.

Some of the instant taps finally win out...at least on in use costs.

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Other instant boiling tap costs

These products are a significant investment, with some costing over £1000. Maintenance costs can be significant requiring a specialist contractor.

None of these costs were factored in to my analysis.

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How to easily avoid overfilling a kettle

Three straight forward ways:

1) Use a kettle that allows you to boil as little as 0.25 or 0.3 litres. Bosch, Tefal and Philips models allow this. Others may allow too. A geeky wander round the Currys showroom kettle aisle is well worth it!

2) If you have a kettle without a good visible level indicator on the side, junk it and get one that has. It will pay for itself in the long run.

3) Ignore the level markings on your kettle fill window. Make your own. Find the largest mug (s) you use regularly and use them to fill your kettle mug by mug. Then mark up the kettle with a Sharpie pen as I've done here. Once you've done that it makes it super easy to fill precisely.....forever.

Blue Peter practicality :-)

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Misinformation

In my early research I frequently came across instant tap marketing information was significantly incorrect or misleading.  The above analysis and these marketing inaccuracies are the subject of a separate blog post on this site and a complaint to the Advertising Standards Agency that was upheld.

One of the manufacturers, Quooker recently acknowledged this ASA complaint to the BBC, confirming that it would be making appropriate changes to its marketing messaging going forwards.

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The geeky assumptions

• Kettle efficiency assumed is 87%, based on:

  • *Academic studies concluding kettle efficiencies range from 85 to 86%.

  • Validation tests on two different kettles with 1 litre of water and a start temperature of 17C. The average figure was 86.6%. I know others who have conducted these tests yielding similar results.

• **A long term study of 250 UK homes measured the actual kettle use over an extended period. The study found average use of 167kWh across the households observed, (all owner occupier homes with diverse occupancy). Assuming kettle efficiency of 87% , and 50% overfill, this implies a daily net boiled volume of 2.6 litres, = around 8 mugs.

• 0.0966kWh energy required to raise the temperature of 1 litre of water from 17C to 100C.

• Standing losses figures were provided directly by the manufacturers.

• Quooker products (and those of some other brands) heat the water above 100C under pressure to 108C, effectively superheating it. This extra heat energy is lost as steam during dispensing, reducing the efficiency to around 92%.

• 17C start temperature and 100C boil temperature assumed in all calculations on instant hot water tap products above. except for Quooker (at 108C).

• 98% efficiency is assumed for the brands of instant hot water taps that do not heat above 100C.

• An assumption in the analysis model is that during winter, heat loss from the instant hot water tap results in a small reduction home heating costs.

• 24.5p/kWh electricity cost, the Ofgem energy cost price cap from October 1st 2024.

• Product tank sizes: Quooker 3 litres, Qettle 4 litres, Fohen 2.4 litres, Hyco 3 litres.

• Mug size 325ml.

• Assumptions and analysis validated by BBC researchers - Autumn 2024.

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