Yes: You can cook githeri with Sh20 worth of electricity!


The Sunday Nation


01 October 2023


Many readers have tried very much to argue that the experiment described in last week’s article was faulty because “there is no way cooking with gas can be cheaper that with electricity!”. In response, I asked all the doubters to design a different kind of test and share their results.

While we wait for the data from other experiments, we can do further analysis to the one that we got last week. The gas cooker burned 550g of LPG and this pumped 4.48MJ (megajoules) of energy into the water over a period of about 30 minutes.

The energy content in LPG is about 49MJ per kilogram. Therefore, burning 550g releases about 27MJ of energy. Out this 27MJ, only 4.48MJ went into the water. The rest (about 22.5MJ) went to waste. That is, over 83 per cent wasted energy. In other words, the gas cooker used in this experiment achieved less than 17pc efficiency.

The electric cooker, on the other hand, consumed 1.069kWh from the wall socket. This is equivalent to about 3.85MJ of energy (1kWh = 3.6MJ). Out of this, 2.258MJ went into the water. This works to 59 per cent efficiency.

Now we see the reason why cooking with gas is much more expensive: gas cookers waste a lot of energy. It is no wonder that the kitchen gets so hot when cooking. Most of the heat from the fire goes to the surrounding air!

This level of inefficiency should not be surprising: petrol and diesel engines are equally wasteful. Modern designs have efficiencies ranging from about 30pc (for petrol) and 40pc (diesel). Yet these are internal combustion machines where the fire is contained inside a closed cylinder; so, heat losses are significantly reduced. Consequently, it is commendable that the open fire of a gas cooker can achieve 17 per cent efficiency.

The efficiency of an electric cooker can be greatly enhanced (thus, making it even cheaper) if we use a pot with a flat base. This increases the contact area between hot plate/coil and the pot, thereby increasing the speed of cooking and reducing the total energy consumed.

I estimated the efficiency with a flat base pot in my house and achieved above 85 per cent. This brings down the cost of cooking further to about Sh10 per MJ. Therefore, it turns out that Engineer Joseph Siror, the CEO of Kenya Power, was probably right when he claimed that you can boil githeri with Sh20 worth of electricity.

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