Consider some of the different units that are or have been used for measuring energy and power (not an exhaustive list):

Energy

• British Thermal Units (BTU), typically for domestic boilers (although they usually mean BTU/h) – see below
• kilowatt hours (kWh), for electricity (and sometimes gas) bills
• joules (J) – the SI unit – used for food energy (nutrition) on package labels
• calories (cal) (often confused with kilocalories – kcal) also for food energy
• ergs – an obsolete unit from a previous version of the modern metric system
• electronvolts (eV) – used by physicists to measure very tiny quantities of energy

Power

• horsepower (HP) – still sometimes used to describe car engine output
• brake horsepower (BHP) – now rarely used
• tax horsepower – formerly used to calculate tax on cars (e.g. Austin 7)
• Pferdestärke (PS) – a German version of HP for car engines
• chevaux fiscaux (CV) – similarly, a French version of “tax horsepower” (e.g. Citroën 2CV)
• watt (W) – the SI unit, defined as a joule per second (J/s)
• British Thermal Units per hour (BTU/h) – used for heating systems

How – if at all – do all these units relate to each other? To explain this we need to revise a little basic science.

What are energy and power – and how do they relate?

There are various forms of energy (which may be why different measurement units have evolved), but they have one thing in common: energy is what makes things move, or heat, or light up, or make a sound. So it can be thermal (i.e. heat), or electro-magnetic, gravitational, or mechanical etc. But it is all energy. Energy cannot be created or destroyed, but one form of energy can be transformed into another form of energy – e.g. when a turbine generates electricity, which in turn heats a kettle or drives a motor: or when chemical energy stored in a battery is transformed into sound waves from your radio. As it is all basically the same “stuff” – energy – it is helpful if it can be measured in the same way.

Power is the rate at which energy is transformed – e.g. a 60 watt incandescent light bulb converts 60 joules of electrical energy every second into light and heat (mainly the latter by the way). Similarly, it is the rate at which chemical energy stored in petrol is transformed into mechanical energy to drive the car and generate electrical energy to charge its battery. Again, although it can take many forms, it is useful if it can be measured in the same way.

Making comparisons

So if we were to measure energy and power using a single, common unit for each, which units should we choose, and what sort of comparisons would be possible?

For energy, the obvious choice has to be the SI unit, the joule. It has the merit that it is defined in terms of other SI units and does not have to be established experimentally. Energy is in fact “mass × acceleration × distance”, so a joule is defined as the quantity of energy needed to accelerate a mass of one kilogram at a rate of one metre per second squared over a distance of one metre.

All the other possible units listed above have disadvantages. The calorie is the amount of energy required to raise the temperature of 1 g of pure water from 14.5 °C to 15.5 °C at atmospheric pressure of 100 kPa. However, these conditions can only be reproduced in a controlled laboratory situation, and are not generally applicable. Similar objections apply to the British Thermal Unit, which is the amount of energy required to heat one pound of water by one degree Fahrenheit, which also makes it completely incompatible with other SI units. The electronvolt is appropriate for nuclear physics (albeit its value must also be determined experimentally) but it is far too small for normal use.

The “kilowatt hour” (kWh), as seen on electricity and gas bills, is an especially unsatisfactory unit.  As we have seen, a watt is a joule per second. So a kWh is a kilojoule divided by a second multiplied by an hour. There are 3600 seconds in an hour, so a “kilowatt-hour” is in fact 3600 kJ – that is 3.6 MJ.  So wouldn’t it be more sensible to measure the energy contained in electricity or gas in joules – or in this case, megajoules?

Since, as shown above, power is the rate of conversion of energy, the choice of unit for measuring power follows from the choice of unit of energy. “Horsepower”, with all its variants, is not a serious candidate since very few people could actually define it – except to say that an engine with a big number is more powerful than an engine with a small number. “British thermal units per hour” (BTU/h) are unsatisfactory for the same reason as the BTU is unsatisfactory. So it has to be the watt.

Not rocket science

The above relationships are not rocket science, and anybody within the normal intelligence range can easily understand them. So why aren’t they better known and used? I suspect a combination of reasons, such as:

• inertia – people are comfortable with what they are familiar with – so people carry on using different units to measure the same thing.
• the media dumb down to the lowest common denominator of understanding.
• ignorance of basic science (like not being good at maths) is tolerated and even considered fashionable by people who would not admit to ignorance of, say, Charles Dickens or the Battle of Trafalgar.
• mistaken beliefs that traditional “British” units (such as Fahrenheit!) have cultural value.
• fear of incomprehension or even ridicule from their peer group if units are used in an unfamiliar context – e.g. kW for measuring a car’s engine power, or metres in cricket or football.

How can these barriers to understanding be overcome? Basically, by increasing familiarity. Much depends on setting a good example – and this will depend on schoolteachers (especially outside the maths or science lesson), politicians (Tony Blair, to his discredit, did great harm by feigning ignorance of kilometres in 2006), broadcasters and journalists, role models in sport and show business. A lead from the Government would also help.

Reverting to the issue of “calories” on menus (which is where this article started from), it is quite deplorable that a Government agency – the Food Standards Agency – should have encouraged extended use of the unsatisfactory unit, the “calorie” (or did they mean “kilocalorie” or indeed “Calorie”?) instead of the proper SI unit, the joule. They even used the word “calorie” as a synonym for “energy”. In doing so, they are promoting ignorance and misunderstanding, and they are failing even to try to educate people to relate the energy they absorb in food and expend in their physical activity to the energy that is so wastefully used in transport, electricity generation and in domestic heating systems – to the detriment of our planet.

Consultation closes today on the FSA’s proposal that restaurants (including fast food bars) should state the energy value of the food on their menus.  The purpose of this proposal would be to enable customers to relate their energy intake to their daily energy requirement – an important factor in leading a healthy lifestyle.  (In principle, if your energy intake exceeds your energy use you will gain weight – and vice versa.)

The catering industry has been wary of this proposal (not least because many fast food outlets rely on people eating unhealthily!) and the FSA’s proposal is for a voluntary rather than a statutory scheme.  It would be difficult if not impossible to enforce against the thousands of individual fish and chip shops and Chinese or Indian takeaways, so it is mainly targeted at the chains of fast food restaurants that populate every High Street, shopping mall, and motorway service station.

In its submission to the FSA, UKMA has not commented in detail on the (obviously laudable) principle of including energy values on menus, but has recommended that any scheme that is agreed with the industry should use proper measurement units that are compatible with those used in nutritional science.  In particular it has advocated the use of the joule (J) as the primary (or preferably the only) measurement unit rather than the obsolete and unsatisfactory “calorie” – or “kilocalorie” – or “Calorie”.

Unfortunately, the FSA consultation paper set a very poor example by equating the physical concept of “energy” with the misused word “calorie” – for example, writing “calorie intake” rather than “energy intake”.   This is in direct contradiction to the recommendation of the Royal Society – as long ago as 1972 – that “calories” should be discontinued – including in the media.

We give below an extract from UKMA’s submission (text in blue):

“The use of the kilojoule (kJ) vs. the use of the calorie (cal), Calorie (Cal), and kilocalorie (kcal)

We applaud the principle of giving consumers the ability to make purchasing decisions based on the energy content in food. However the consultation document’s proposed continued use of obsolete measurement units presents several issues:

The “calorie” is often confused with, or used in equivalence to, the “kilocalorie”.

A convention is sometimes applied which attempts to avoid the inevitable misunderstanding that this causes. This involves the use of a capital letter ‘C’ when “calories” are to read as “kilocalories”, such that:

1000 calories = 1 kilocalorie = 1 Calorie

Indeed, the consultation document itself is a good illustration of this issue as it uses the word “calorie” erroneously in several instances where the word “kilocalorie” or “Calorie” is intended. e.g. Annex H, 7.3 (text in green):

“Note: “kcal” is used in these statements but “calories” should be substituted if        “calories” are declared as the energy information at point of choice.”

The consultation document acknowledges that …

“36. To aid consumer understanding and contribute to consistency of labelling only one        form of expression (either kcal or calories) should be used in an outlet. “

However, this stipulation will not prevent inconsistency of labelling across different establishments.

In their 1972 report on nutritional sciences, the Royal Society identified the problem of the continued use of calories to describe energy content of food. Its conclusions remain valid nearly 40 years later (text in dark red):

“We are very much aware of the problems that arise because as a result of 30 years of education the public has an awareness of the term ‘calorie’. We cannot see any easy solution to the problem of substituting the concept that man has a requirement for the energy-yielding constituents derived from food, and this is measured in joules, …”

“We recommend that editors of journals should not allow the use of the word ‘calorie’ and list below some obvious alternatives :

calorie intake                                  energy intake

calorie requirement                         energy requirement …”

The Units Of Measurements Regulations, which implements Directive 80/181/EEC, requires that energy should be measured using the SI derived unit, the joule. The fact that the calorie is not an SI unit, and is not listed in the Directive, means that calories can only be authorised for use as supplementary indications, and should not appear more prominently than the primary measurement, in joules (J) or kilojoules (kJ).

Many packaged foods are already labelled in kilojoules (kJ).

Progressive countries such as Australia, have already adopted the kilojoule as the primary unit of energy to indicate energy content of food.

Conclusion

A single unit, the joule, used for all purposes regarding energy (not just food), will both benefit the consumer, and increase the general public’s understanding of the concept of energy in general.

It is for these reasons that we strongly recommend that the opportunity that this consultation presents should be taken to begin the phasing out of the obsolete unit “calorie” in favour of the “joule” (which incidentally is named after the British scientist, James Prescott Joule).

References:

REPORT OF THE ROYAL SOCIETY’S BRITISH NATIONAL COMMITTEE FOR NUTRITIONAL SCIENCES

METRIC UNITS, CONVERSION FACTORS AND NOMENCLATURE IN NUTRITIONAL AND FOOD SCIENCES

Report of the Subcommittee on Metrication of the British National Committee for Nutritional Science

Proc Nutr Soc. 1972 Sep;31(2):239-47.

http://journals.cambridge.org/action/displayFulltext?type=1&fid=980848&jid=PNS&volumeId=31&issueId=02&aid=655296 “

[UKMA submission ends]

Further comment

Some have argued that the general public is familiar with “calories”, and to replace them with joules would be confusing and would reduce the effectiveness of the proposal to include energy values on menus.  This is to patronise the general public and underestimate their intelligence.  It is not difficult, for example,  to remember that the average daily energy requirement of an adult male is approximately 10 megajoules (10 MJ) and hence to relate that to a meal of, say, 4 MJ, or a bottle of wine at 2 MJ.  Moreover, to continue the dumbing down of energy information by using non-scientific units helps to maintain the gulf between the educated scientific community and people who have to rely on the popular media for their information.

The question of how best to measure energy (and also power) is a theme to which we shall return in a forthcoming article.

Technical footnote:

What the above Royal Society quotation does not explain is the reason why the joule is a better unit than the “calorie” (in all its variations).  This is because, whereas the value of the “calorie” is determined experimentally (by heating water), the joule is defined in terms of other SI units.  Thus, since energy = force x distance, a joule is a newton times a metre, or in other words the quantity of energy needed to accelerate a mass of one kilogram at a rate of one metre per second squared over a distance of one metre.  Similarly, a joule can be directly related to the watt (1 W = 1 J/s).  By contrast  the “calorie” is simply an unrelated anomaly that – unfortunately – has gained some currency in the popular media and some parts of the weight-watching industry.  It should be phased out as soon as possible, and the FSA should be helping in this – rather than prolonging its life.