Proven: cooking by electricity is cheaper than by gas – by far!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

24 September 2023

Recently, the Chief Executive Officer (CEO) of the Kenya Power & Lighting Company, Engineer Joseph Siror claimed that it is possible to cook githeri with Sh20 worth of electricity. Many people challenged the CEO to present empirical data to support his claim but he never did; so, I got in touch with my former school and we organised to carry out an experiment to investigate this practically.

Now, the essence of cooking is to inject heat energy into food. So, if we can measure the amount of energy that actually gets into the food, we can compare their costs of different sources of heat on a like-for-like basis.

This simple experiment is enough: pour a known mass of water into a sufuria (in our case it was 2kg); take it temperature (19 degrees celcius) and heat it to boil. Measure the boiling point temperature and leave the water on the heat until a significant amount has evaporated away.

In the case of gas, the water reached 95 degrees celcius and it was left on the fire for about 30 minutes. By that time, 1.7kg had evaporated away leaving only 300g in the sufuria. By comparing the mass of the cylinder at the beginning (19.2kg) and at the end of the experiment (18.65kg), we found that 550g were consumed.

At the current gas price of about Sh3,000 per 13 kg cylinder (I have seen prices ranging from Sh,.600 to Sh3,400 depending on brand), it turns out that 550g costs about Sh127.

In the case of electricity, the water reached 86 degrees celcius and it was left on the cooker for 69 minutes. At the end of the period, 750g had evaporated. During the whole period, the voltage and current in the electric cooker were monitored and their values recorded every 60 seconds.

From that data, the average power consumption was calculated and found to be 929 watts. Therefore, in the 69 minutes, the cooker consumed 1.069kWh of electricity. At the current average price of my house Sh32.17 per unit, the total cost comes to Sh34.

Using the specific heat capacities of water, we calculated the amount of energy that went into the water in each heat source: in the case of gas, it was 4.48 mega-joules (MJ) and 2.258MJ in electricity.

To compare like-for-like, the costs are divided by the energy that went into the water; thus, for gas it is Sh28.35/MJ and for electricity it is Sh15.20/MJ. This clearly demonstrates that cooking by gas is almost twice as costly as using electricity.