The power of kilowatt hour – tangible ways of understanding energy

Kilowatt and kilowatt hour

Understanding power and energy

Terms power and energy are often confused or even mistakenly used as synonyms. I this article I would like to clear out the differences between them and, what is more, give you a few meaningful examples of how much power and energy we spend on our daily activities.

First things first. Power indicates an instantaneous effort that is delivered by a device, machine or person. This value is usually expressed in watts [W] or kilowatts [kW] (1kW = 1000W). Summation, or more precisely integration, of power over a period of time yields energy consumed. Essentially, if we considered an average power consumption, energy consumed will be equal that power multiplied by the time of use. Energy, in physics sometimes called work, is expressed either in Joules [J] or kilowatt hours [kWh]. So, 1kWh is the energy that you will consume using an average 1kW of power over a period of one hour. Kilowatt hours are what is on your monthly electricity bill, and you pay for each of them around 21 cents, as a household, or 12 cents, as an industrial user [Source]. Great! But what is the physical meaning of these numbers? Can we somehow intuitively perceive these quantities? Sure, let me provide you with some practical examples.

What can I do with 1000W of power?

A typical iron will take an average of 1000W while in use, whereas electric hair dryer will consume twice as much. 1000W is equal to 1,25 horsepower, therefore you could power around half of a 50cc scooter, but as much as 4 e-bikes (their max power is limited to 250W by law). Same amount of power will allow to charge/power simultaneously 200 smartphones (with a standard 1A charger), nearly 16.5 laptops and around 5.5 desktop PCs.

Average calorie intake is around 2500 calories per day for a man and around 2000 for a women. These means that we need a power of 120 and 95 watts, respectively, to run our bodies. If you look at it from a different perspective with 1kW you could actually power 8.3 males and 10.5 females, no matter how creepy that sounds. But if you think deeper about these numbers, it seems that we need less than a desktop PC. It is assumed that our brain use nearly 400 calories, which is equal to 20W of average daily power use. Scientists estimated that the brain computational power which we have at our disposal is around 36.8 petaflops. Equivalent supercomputer would need around 6 megawatts of power to achieve equal performance. We can then deduce that our brain is 300,000 time more efficient that a state of the art silicon chip. Millions of years of evolution paid off really well!

Our bodies are truly efficient, but in therms of mechanical output we do not seem to be powerful that much. Average biker can give as much as 150-200W of pedaling power, whereas a road bike professional can reach a maximum of 400W. Human body seem to have an absolute peak power output of around 700W, but these numbers are only reachable by a few individuals.This great experiment shown in the video below really opened my eyes to how relatively weak we are comparing to appliances around us.

Astonishing, isn’t it? Performance of that 90kg olimpic champion can be easily matched or even beaten by a small, 330 gram electric brushless dc motor (like that one), with a peak power of nearly 1000W…

What can I do with 1kWh of energy?

As far as we covered the power case let’s focus on the energy. If we have our 1000W available for one hour, which is worth a total of 21cents if you live in an average European household, how much work will be able to do?

With 1kWh we could run a typical household hairdryer for half an hour, which would probably allow a person with long hair to dry it twice. You could charge your phone around 90 times (assuming 3000mAh battery capacity) or take a 10 minute shower. It will allow you to drive for around 2.3km with a modern, combustion engine powered car (assuming consumption of 5l/100km) but as much as 5 km with an electric one (such as Tesla model S).

Lifting an aquarium with 1 cubic meter of water (~1000kg) 367 meters up (assuming 100% efficiency of the crane) would take the same amount of energy is sufficient to bring only 11 liters of water from room temperature to the boiling point – one kilowatt hour. You will need 5 square meters of solar panels on the rooftop in the full sun for one hour to achieve the same goal.

Speaking again of our bodies, we need 2.9kWh and 2.3kWh of energy intake to survive a day without gaining or losing weight. Our Olympic track cyclist champion was only able to produce 0.021kWh before he nearly fainted . Amazing, huh?

Wrapping it up

Thanks to those few simple examples you can see that most energy demanding activities are the one that include heat conversion, such a heating or combustion. If you consider changing our habits in order to save energy (as well as your money and environment) you should consider focusing you attention on these two. For example having a habit to unplug phone charger while not in use will have significantly less impact than for example taking shorter showers or reducing heating at home in winter or turning down air conditioning in the summer. I hope that after reading this article you can have a better “grasp” on what 1kW and 1kWh means in practice and have a better idea on how to better manage it at your household or work.