For simplicity, consider food as a vegetarian diet. Plants are autotrophs – produce their own food using the process of photosynthesis to transform water, sunlight, and carbon dioxide into oxygen and simple sugars that the plant uses as fuel and stores the excess. Plants are the primary producers that form the base of the ecosystem. The energy source for photosynthesis is sunlight and hence represents solar energy. When humans consume fruits and vegetables – they get their energy from these forms of ‘excess storage’ by plants – which is solar energy. When a girl presses on a bicycle pedal, the work done by her (while pedalling) uses this ‘solar energy’. She ‘burns’ calories in the process.
When she prefers to use the throttle handle of the solar bicycle instead of the pedals, it means that energy, in watt-seconds, is drawn from the battery that was earlier charged by the attached solar panel. Unlike other machines (vehicles) that work on fossil fuels, the solar bicycle draws its energy from the attached solar panel.
Thus, regardless of whether the solar bicycle is ridden using pedals (human energy) or throttle (energy from batteries), the source is solar. Hence, with solar bicycles, it becomes necessary to equate the energy stored by humans, in calories, with the energy stored in batteries, in watt-seconds.
What is a calorie?
In the nutrition world, when it comes to food, you might have heard remarks such as ‘empty’ calories and ‘full’ calories. Calorie is not a thing hence can neither be full or empty, nor can it be bottled. Calorie is a unit of measurement of energy.
‘Large’ and ‘Small’ Definitions
Due to historical reasons, two main definitions of “calorie” are in wide use resulting in serious complaints and confusion. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of one kilogram of water by one degree Celsius. The small calorie or gram calorie was defined as the amount of heat needed to cause the same increase in one gram of water. Thus, 1 large calorie is equal to 1000 small calories.
Nutrition World – ‘Large’ Calorie
In nutrition and food science, the term calorie and the symbol cal almost always refers to the large unit. It is generally used in publications and package labels to express the energy value of foods in per serving or per weight, recommended dietary caloric intake, metabolic rates, etc. Some authors recommend the spelling Calorie and the symbol Cal (both with a capital C) to avoid confusion; however, this convention is often ignored.
The precise equivalence between calories and joules has varied over the years, but in nutrition it is now generally assumed that one (large or kilogram) calorie is equal to exactly 4184 J, or 4.184 kJ.
Physics and Chemistry – ‘Small’ calorie – Obsolete
In physics and chemistry (thermochemistry) the word calorie and its symbol – cal, is used to refer to the small unit; the large one being called kilocalorie. However, this unit is not officially part of the metric system (SI), and is to be regarded as obsolete, having been replaced in many uses by the SI unit of energy, the joule (J).
Brief History of calorie
There is a controversy on the origin and usage of the ‘calorie’ as a unit of heat in France. The word ‘calorie’ seems to have been coined sometime between 1787 and 1824. Some say that a French chemist, Antoine Lavoisier, who was executed by French revolutionaries in 1794, coined the word ‘calorie’ and used it to refer to a body’s internal heat. He had named the calorimeter (calorimètre) by 1789, and was credited with coining “oxygen” and many new chemical terms, as well as helping define the kg.
Wikipedia says that the “large” calorie, or kilogram-calorie was first introduced, in 1819, by Nicolas Clément, a professor of chemistry at the Conservatoire des Arts et Métiers in Paris. Clément needed a unit of heat for a discussion of how steam engines convert heat into work. Scientists accepted Clément’s definition, and the calorie entered into French physics textbooks in 1841.
The same term, calorie, was used for the “small” unit, i.e., gram-calorie, by Pierre Antoine Favre (Chemist) and Johann T. Silbermann (Physicist) in 1852. German scientists adopted their definition. The small calorie (cal) was recognized as a unit of the CGS system in 1896, alongside the already-existing CGS unit of energy, the erg (first suggested by Clausius in 1864, under the name ergon, and officially adopted in 1882).
In 1879, Marcellin Berthelot proposed using “Calorie”, with capital “C”, for the large unit. This usage was adopted by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887, in an influential article on the energy content of food. Later, in 1928, there were serious complaints about the possible confusion arising from the two main definitions of the calorie and whether the notion of using the capital letter to distinguish them was sound.
The joule was the officially adopted SI unit of energy at the ninth General Conference on Weights and Measures in 1948. The calorie was mentioned in the 7th edition of the SI brochure as an example of a non-SI unit.
Looking at the history, we too, consider the small gram-calorie as obsolete. The smaller unit is not officially part of the metric system (SI), hence we will use the SI unit of energy, the joule (J), instead. This also means that the term calorie would be exclusively used to refer to the large kilogram-calorie.
Measurement – Calorimeter vs. Atwater Indirect System
Initially, a calorimeter was used to measure the calorie content of food. A known amount of dehydrated food was placed in a container that was sealed and surrounded by a known amount of water. The food was ignited after oxygen was piped in. The rise in temperature of the water was used to calculate the calorie content of food. However, this method had problems since it did not accurately model the ‘human digestive system’. Fibre and such other components in food will burn in a calorimeter. But these are not absorbed into our bloodstream and therefore do not contribute calories.
Hence, today, the ‘Atwater indirect system’ is used instead of the calorimeter. The calories provided by the energy-containing nutrients – fat, carbohydrate, protein and alcohol are added up to calculate the total calories. The fibre component of carbohydrate that is not digested and utilised by the body is subtracted from the total carbohydrate before calculating the calories.
The Atwater system uses the average values of 4 cal/g for protein, 4 cal/g for carbohydrate, 9 cal/g for fat and 7 cal/g for alcohol that were determined by burning these substances in a calorimeter. Note: ‘cal’ represents large calories. Thus a label on a chocolate bar that contains 3 g of protein, 29 g of carbohydrate and 12 g of fat would read 236 cal (large kilogram-calorie).
Equating the Commercial Unit (kWh) with Calories
Let us begin with the SI unit of energy – the joule
The joule can be defined by any of the following:
It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied.
It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.
The work required to produce one watt of power for one second, or one watt-second (W X s)
The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb-volt (C X V).
J = kg·m·s-2
We can observe that the transition from small-calorie to the broader universal unit joules was more appropriate for energy. Joules can be equated with watt·second on one side and with large calories on the other.
Equating Watt-sec with calories
In nutrition, one large-calorie is equal to exactly 4184 J
1 cal = 4184 J = 4184 W·s
1 Joule = 1 W·s = (1/4184) cal (5)
Equating kWh with calories
Now 1 Wh = 1 W x 3600 s=3600 J = 3.6 kilojoules (6)
1 Wh = (3600 / 4184) cal = 0.86042 cal (7)
Kilowatt-hour (kWh) is a large (commercial) unit of energy and work, practically used while calculating the consumption of electrical energy.
Now 1 kWh = 1000 W x 3600 s = 3.6 x 106 J
= 3.6 megajoules (8)
1 kWh = (3.6 x 106 / 4184) cal = 860.42 cal (9)
Human efficiency is only 24%
Humans aren’t efficient engines. While riding a bike, we spend a lot of energy on heat generation, balance and other things. Efficiency of cycling is around 24%. We burn 4 joules of energy for each joule delivered to the pedals.
1 Wh (at pedals) = 0.86042 cal (at pedals)
Since Human efficiency is only 24% while cycling
1 Wh (at pedals) = 0.86 * (100 /24) cal (of Human energy exhausted)
1 Wh (at pedals)=3.6 cal
(of Human energy exhausted) (10)
0.2777 Wh (at pedals) = 1 cal (of Human energy exhausted) Substituting, 1 Wh = 3600 J
3600 J (at pedals) = 3.6 cal (of Human energy exhausted)
1000 J (at pedals) = 1 cal
(of Human Energy exhausted) (11)
Thus, a very interesting observation is that for every large-calorie of human energy expenditure, a kilojoule, or 0.2777 Wh, of energy gets delivered to the bicycle pedals.
Human Energy Exhausted – Calories burnt with Exercise
A 68kg, 5ft 6” woman in her 30’s would typically burn per hour when working at an average pace:
Running: 660 cal/hour Swimming: 600 cal/hour
Jogging: 480 cal/hour Cycling: 430 cal/hour
Walking: 240 cal/hour Yoga: 120 cal/hour
Human Energy Exhausted – Calories burnt with NO Exercise
Sitting burns an estimated 75 calories per hour.
In 24 hours = 24* 75 = 1800 calories.
An average person burns around 1800 calories a day doing absolutely nothing. In a year, she burns 365 * 1800 = 0.657 million calories
How many Calories should I eat to lose weight?
An average adult woman expends roughly 1,600 to 2,400 calories per day while an average adult man uses 2,000 to 3,000 calories per day.
To lose weight you have to eat, typically, 500 calories fewer than your maintenance level. If you have a calorie deficit of 500 per day, you’ll lose one pound per week as 3,500 calories roughly works out to one pound of body fat. In a month, you may lose almost 2 kg in weight.
To Lose Weight – Eat Less or Work Out More ?
To Lose Weight = To have a calorie deficit
A person with a sedentary lifestyle can choose to ‘Work Out More’ or ‘Eat Less’.
Working out more is a far better option due to numerous benefits in addition to losing weight.
Example: Cycling One hour at 120W (Manual Mode)
Example: How many calories does a cyclist burn when she averages 120 W during an hour of riding?
-> Cycling 1 hour at 120 W = 120 Wh
Now, 1 Wh (at pedals) = 3.6 cal (of Human energy exhausted).
Therefore, 120 Wh (at pedals) = 120 * 3.6 = 432 cal of Human energy exhausted.
Hence, when a cyclist averages 120W at Pedal (burns 432 cal of human energy) during an hour of riding, she would cover a distance of 18 km.
Hence, A cycle would travel a distance of approx. 150 meters with 1 Wh (= 0.86 cal) of energy (at pedals) or 3.6 cal of (human) energy.
Or, for 100 meters, 2.4 cal of human energy would be expended.
Example: Solar Bicycle – Distance covered with 5Ah
37V, 5 Ah, Battery has capacity of 180 Wh. Let us consider a normal discharge of 120Wh.
A solar cycle travels a distance of approx. 150 meters with 1 Wh (= 0.86 cal) of energy.
Hence, when 120Wh = 120 x 0.86042 cal = 103 cal of battery energy is consumed the distance covered is 18 kms.
In the manual (pedal) mode, for the same distance (18 kms) the human energy expended is 4 times higher at 432 cal.
Example: Walking 1 hour at 240 cal (Human Energy)
When a woman walks briskly for 1 hour, she burns 240 cal of human energy while covering a distance of 4.8 km.
Hence, for 100 meters she burns 5 cal of human energy while walking.
(In terms of Wh, this amounts to (5/0.86) = 5.8 Wh of human energy).
Cycling – More Efficient in terms of Energy & Speed
To travel a distance of 100 meters, a cyclist burns only 2.4 cal. She would expend 5 cal if she walks the same distance instead.
Cycling is at least 2 times more efficient than walking in terms of human energy expended.
While 3.6 cal of Human energy gets exhausted only 0.86 cal (1 Wh) transferred to the pedals. The Human efficiency is only 24%
While 7.5 cal of Human energy gets expended, only 0.86 cal (1 Wh) transferred to the foot (treadmill). Human efficiency is even lesser at 12% while walking.
A cyclist covers 18 kms in an hour. While walking, she would cover only 4.8 kms in an hour. Cycling is nearly 4 times more efficient than walking in terms of speed.
Summary – Cycling – More Efficient in terms of Energy & Speed
To travel 100 metres:
Walking takes 75 seconds. (Speed 4.8 kms/hr); Burns 5 cal of human energy.
Cycling takes 20 seconds. (Speed 18 kms/hr); Burns 2.4 cal of human energy or consumes 0.57 cal (0.66 Wh or 2.376 kJ) of Battery energy.
From the environmental angle, solar cycle in the ‘throttle’ mode, at 5.7 cal/km or 6.6 Wh/km, 23.76 kJ/km, is the most efficient; 4 times more efficient than ‘manual/pedal’ mode. The intermediate – ‘Pedal Assist’ – mode, though not as efficient as throttle, would be best if we wish to add the health angle. No ICE or e-vehicle can come close to this, hence not included in this discussion.
 Joe Schwarcz, How is the caloric value of food determined, McGill – Office for Science and Society – Separating Sense from Nonsense, Sept. 2018
 Wikipedia for definitions; https://en.wikipedia.org/wiki/Calorie
 James L. Hargrove, History of the Calorie in Nutrition, The Journal of Nutrition, Volume 136, Issue 12, December 2006, Pages 2957–2961,https://doi.org/10.1093/jn/136.12.2957
 ALLISON MARSH, How Counting Calories Became a Science, IEEE Spectrum, 29 DEC 2020, https://spectrum.ieee.org/howcounting-calories-became-a-science
Vithal N. Kamat has a Doctorate in Artificial Intelligence from the University of New Brunswick, Canada as a Commonwealth Scholar in 1996. He completed Masters in Control and Instrumentation from IIT Bombay. His current role – reviving a sick industry as a Managing Director of Baroda Electric Meters Ltd. Current interest lies in exploring ways to replace the Human-centric Judiciary with an AI Judiciary, to replace the 24-hour clock with Ghati clock, and to replace ICE vehicles with solar vehicles…