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Congratulations on successfully building your solar phone charger! In case you have no idea what I am talking about, here is my previous article tutorial on how to build a solar phone charger by yourself!

Welcome to the dark side

You might have noticed that it took a noticeable bit of effort to build the DIY solar kit. You might have wasted some materials like wires, recycle bags, glue, tape, mounting board, and etc. during the process. You also need electricity or energy to power lights, a/c, a solder iron, and even your computer to watch my tutorial. Furthermore, the usage of these materials and electricity emits carbon. We term these carbon emissions or carbon footprints. This video perfectly explains what are carbon emissions/footprints:

And if we were to address the elephant in the room, it would take money to build the DIY kit.

My point is,

"We invested energy, carbon emissions, and money into building a solar kit that is ironically supposed to conserve energy, carbon emissions, and money"

The awareness of this statement is the first step to understanding why there is a "dark side" to the so-called clean solar energy. The DIY solar kit on paper may sound very "clean" and "green", but it all depends on how much more energy, carbon emissions, and money we can conserve at the expense of the energy, carbon emissions, and money we invested in. In other words, we want to get the most energy, carbon emissions, and monetary savings at the expense of the least energy, carbon emissions, and money.

Here, we shall term energy, carbon emissions, and money into "ECM" as will be using it multiple times in this article.

How huge is the dark side

Now, imagine if such a simple DIY solar kit could incur a multitude of ECM, how much more ECM could be incurred for a large-scale solar power plant? This is why any business decisions involving the utilization of solar panels at a large scale often involve a series of complex calculations involving ECM. Sadly, many corporates only focus on the "M" in ECM when it comes to this decision-making since the implications of wasting energy and carbon emissions do not affect the present but only affect the far future. Calculations focusing on the "M" part in ECM by using levelized cost of electricity (LCOE) were explained in Chapter 7 of my Shining Light on Solar Cells video series, or you could just watch the video here.

This is why the governments in some countries like Canada and Singapore implemented "Carbon Tax" as a means to make visible the "hidden" social costs of carbon emissions to corporates, forcing corporates to reconsider their business decisions to tailor to lower carbon emissions.

In this article, we will be going through some simple calculations for the "E" and "C" parts of ECM since they are most often neglected.

Energy Payback

Energy Payback for DIY solar charger kit

Energy payback is the duration it takes for the entire solar system to generate the same amount of energy that was used to create the system in the first place. For a simple DIY solar charger kit we created, the solar panel and the USB Buck converter are the major contributors to the energy required to make the kit. Hence, we are only considering them in our calculation and ignoring the rest. Let us term all these the specific energy of the solar system (kwh/m^2). The solar system generates electricity at a particular rate, which we will term rate of generation (kwh/m^2 yr). This represents the amount of energy the solar system generates per year. However, remember we are not using the solar panel 365 days, every day of the year. Hence to account for that, let us assume that we are charging our handphones for 1 hour every day, which equates to 365 hours per year. The energy payback in years is hence:

Energ Payback Equation

The challenging part in this calculation is not the actual calculation itself, but where to get the numbers and how reliable it is. The more reliable the numbers we put into the calculation, the more accurate our answer is. Remember, the main aim of doing an energy payback calculation is to determine if it is a good investment in the first place. So the more accurate your answer is, the better your investment decisions. Let us go through how we get these numbers one by one.

Specific energy for the solar panel

According to the manufacturers' website, the solar panel is made from multi-crystalline silicon cells.

The specific energy of multi-crystalline modules obtained from this reference is 240 kwh/m^2

Specific energy for the USB Buck converter

The energy required for the USB buck converter is quite challenging to obtain. For educational purposes, we are just estimating this value to be 20 kwh/m^2.

Efficiency of solar panel 14.1 % reference
Solar insolation

Assuming the handphone is put to charge for 1 hour every day for a year at 1 kwh/m^2 of sunlight, which means the solar system is receiving 365 kwh/m^2 yr of sunlight. [reference]

Plugging those numbers in, we obtain about 5 years of energy payback time. This also means that I need to charge my handphone once every day for 5 years in order to gain back the same amount of energy that was used to make the system in the first place. While 5 years may seem like a long time, there is a way to hasten the energy payback. If we could charge two phones per day instead of one, we could halve the energy payback time.

The bigger picture

The energy payback for multi-crystalline silicon solar system is usually 1.25 years [reference]. This is because the entire system is operating at a much larger scale with large power plants and large panels. Furthermore, the conversion of sunlight into electricity happens all the time throughout the whole year. Hence, the DIY kit certainly can't match whatever a large-scale solar system could provide.

Carbon Emissions

Carbon emissions studies of different energy systems are usually performed in terms of lifecycle emissions. This means the total emissions of CO2 throughout the lifecycle of the energy system (from mining to installation to maintenance to disposal) per unit energy [g/kwh]. Here is a chart depicting the lifecycle emissions per unit of energy [source].

Lifecycle Graph

As you can see, rooftop solar has only about 41 g/kwh of lifecycle emissions, compared to natural gas which has 490 g/kwh of lifecycle emissions. This means that there is a 449 g/kwh prevention of CO2 emissions when we choose solar over natural gas.

To tailor our DIY solar kit, we need to know how much energy is being used per year by our solar system. Taking into account our solar panel is 0.054m^2 in size, assuming the panels to have 14.1% efficiency, and taking into account our solar system receiving 365 kwh/ m^2 yr of sunlight, we have

electricity generated by the solar system per year [kwh] = 0.027 m^2 X 365 kwh/ m^2 yr X 14% = 2.76 kwh /yr

If we multiply this number by the CO2 emissions prevented, we get about

CO2 prevention per year [gCO2/yr] = 2.76 kwh / yr X 449 g/kwh = 1240 gCO2.

With our solar panels, we managed to prevent 1.24 kgCO2 emissions in one year. To put things into perspective, a solar power plant the size of a football field (about 4000 m^2) would prevent 126 x10^3 kg CO2 emissions per year if replacing natural gas [source].


I hope I have shined some "dark" onto solar cells with some explanation and calculations. The lesson learned here is that we can reap the full "ECM" benefits of solar only when solar energy is harvested in large-scale systems, like this large-scale power plant in Singapore:

This is why building a DIY solar phone charger kit is merely for convenience and educational purposes but doesn't really help in terms of money, environment, and energy payback. Even concepts like putting solar panels above vehicles, integrating them into building windows, and doing fancy stuff only sound good on paper.

I'm an R&D Mechanical Engineer from Keysight Technologies. I design mechanical components that facilitate over-the-air testing equipment. I also have a strong background in photovoltaics. I love creating STEM-related content to inspire and encourage people to pursue STEM, as well as to provide career advice to fresh grads who are looking for their first job
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