What is Energy

Zelé*  Everyone!  Pe Sô Pe Pon*!

At the time I am writing this post, sitting is my office in the UK, there is a beautiful sun out there and it is very tempting to just leave what I do and go enjoy the day outside.   Especially when accustomed to the particularly flexible nature of the British summer. It would be an understatement to say that sunny days are all but guaranteed. Totally the opposite of the African continent, where solar resources are abundant.

Over the past few years, African civil societies have grabbed the opportunity to harness the solar energy to improve Electricity access.  From the 5 kW residential rooftop systems to the 1 MW solar plants supporting industrial buildings or feeding the grid, the number of solar installations has dramatically increased across the continent (My report on utility scale projects and solar plants over 5 MWs solar is NOW available when you sign up to our newsletter).

Looking at the knowledge gained and documented from these projects, I could say there are 3 key components that are essential for a successful solar project: a good and robust design, an effective implementation and a supportive commercial and regulatory framework.

So what about them?

Before giving you more detailed information on these key elements, I would like to briefly revisit the notion of Energy and Power and their units to clear any confusion that could subsist in tormented minds.

The concept of ENERGY

From the scientific definition, Energy is the ability of a system to do work. It is the quantity to get something to move, light up, burn, explode, and so on. Due to the fact that it is converting from one form of energy to another it comes in different forms: electrical energy, chemical energy, nuclear energy, kinetic energy, etc. In the power sector, the most common units of measurement are kilowatt hours (kWh), megawatt hours (MWh), megajoules (MJ), and terajoules (TJ).

The concept of POWER

Power is the rate at which energy is being used. The units of measure for power we will mostly use are kilowatts (kW) or megawatts (MW).

To borrow the analogy mentioned in David McKay’s excellent book “Sustainable Energy – without the hot air”, Energy and Power are like water and water-flow from taps. Imagine you want to drink a volume of 1 litre of water. When you turn the tap on, you create a flow of water. Depending on the effectiveness of the tap, you could get this quantity in a minute if the flow is 1 litre per minute, or you could get the same quantity for 30 seconds if the flow is 2 litres per minute.

The flow is a rate at which the volume of water is delivered and the equation here is:

Volume = Flow x Time

Young Boy drinking Water in Africa

Energy is similar to Volume and Power is similar to Flow. We then have:

Energy = Power x Time

It is important to note that the notion of Power as a rate is special as it does have its own unit, contrary to other rates that are usually in “per unit of time” like speed, flux, heart rate or dimensionless like tax rate (remind you of something?).

What does 40 W represent?

For a quick equivalence between the units, 40W ≈ 1 kWh per day

In other words a 40W light bulb kept switched all the time in a small hut would use 1 kilowatt per day.

40W Lamp


Hopefully now, the confusion between the two concepts has completely disappeared.  In case there is still some doubts, maybe the analogy with distance and speed could help. Energy, like distance, is a quantity, whereas power is a rate like speed. Like the relationship between energy and power, the relation between distance and speed is written as:

Distance = Speed x time.

Having clarifying the meaning of the two notions, we could now refocus on the key elements of a successful solar project that will be developed in the next section of this article, coming up very soon.

Until then, take of yourself and look after our planet…Switch this device off when you’re done!!


*Zelé: “Good Morning” in Bafang, *Pe Sô Pe Pon: Welcome in Batie. 2 languages spoken in West Cameroon.




  • Energy is the ability of a system to do work
  • Power is the rate at which energy is being used :
  • Energy = Power x Time



  1. Energy Science, Principles, Technologies, and Impacts, 2nd Edition – John Andrews, Nick Jelley
  2. Photovoltaic Solar Power Course – Prof John I B Wilson, Heriott Watt University
  3. Sustainable Energy – Without the hot air – David JC MacKay



  1. Zelé mon frère! Interesting read, mais à notre âge les formules mathématiques c’est les maux de tête assurés! Lol Good Luck!!

  2. Good shot brother. Solar energy is surely the technology we must focus on in Central Africa. I will be happy to learn more from your blog.


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