Have you ever wondered whether to boil water using an electric kettle or to use cooking gas? Or maybe you currently use kerosene and wonder what the hype is about gas. My wife and I routinely debated our home’s energy cost with regards to different energy sources, so I decided to do an empiric comparison. Maybe someone else could use this to settle their own debates.
Let me start by addressing two common misconceptions in Nigeria. Firstly, it is untrue that electricity is cheaper than gas and kerosene. I think this misconception may be due to issues around estimated billing for many electricity consumers in Nigeria and the prevalence of energy theft, which makes consumers understate the cost of electricity. As prepaid meters gain popularity, consumers would better appreciate the cost of their electricity usage. Secondly, the fact that gas cylinders routinely need refilling (no domestic piped gas in Nigeria) does not necessarily mean that gas costs more than electricity. Here, psychology tends to act as we see that electricity “does not finish” but the gas in our cylinders may run out, sometimes while cooking something as unforgiving as beans! As the gas market expands, the number of refill stations and small-scale retailers is increasing, which should make it easier to refill gas cylinders. Households may also consider having a spare cylinder as backup for when gas runs out at inconvenient periods.
I have developed a simple Microsoft Excel Spreadsheet for comparing the cost of using electricity, gas, or kerosene to boil a given quantity of water (ignoring heat transfer efficiency). Using a formula common to secondary school physics students (q = -h = mcΔΘ), the amount of heat energy needed to take a given volume of water from a given temperature, Θ1, to a final temperature, Θ2, is calculated. If boiling water, Θ2 would be 100°C. Since electricity is purchased in “units” of kWh (kilowatts-hour), this unit has been used to compare the cost of heating water using the three energy sources. Costs are valid as at September 2020 and can be adjusted by inputting price changes into the appropriate cells. The spreadsheet can be downloaded here.
For those interested in a summary, cooking gas (liquefied petroleum gas or LPG) is the cheapest energy source per kWh (beats electricity and kerosene) even though one litre of kerosene has about 37% more energy than one litre of LPG. Should I also add that LPG is cleaner than kerosene and causes less kitchen stress as pots largely remain clear unlike kerosene soot that blackens pots and can impact your health? This is not an LPG advert, but as electricity costs go up (amidst unreliable supply), it may be smarter to swallow the higher upfront cost of gas cookers and cylinders in return for lower longer-term energy costs.
Image Credit: howstuffworks.com
PS. The National Bureau of Statistics (NBS) published ₦3,723.53 as the lowest cost (state average) of refilling a 12.5 kg LPG cylinder in Nigeria. However, for the applicable month (August 2020), a refilling station in Lagos quoted ₦3,300 for that refill size. Hence, gas (and maybe kerosene) may be cheaper in some parts of Nigeria than the average values published by NBS.
Data Analysis Summary
|Mass, m||1||kilogram, kg|
|Specific Heat Capacity, c||4,184||J/kg.K|
|Initial Temperature, Θ1||20||°C|
|Final Temperature, Θ2||100||°C|
|Temperature Change, ΔΘ||80||°C|
|Heat Required, q||334,720||J|
To raise 1.0 L of water by 80.0°C
|Electricity||₦ 2.98||₦ 3.98||₦ 5.31|
|Gas||₦ 2.04||₦ 2.26||₦ 2.52|
|Kerosene||₦ 2.42||₦ 3.25||₦ 3.73|
Energy Content Assumptions
Liquefied Petroleum Gas (LPG)
Note that LPG in Nigeria is a mixture of propane and butane (70:30 mix ratio assumed)
Propane: 1 kg ≈ 1.96 L ≈ 13.6 kWh
Butane: 1 kg ≈ 1.72 L ≈ 13.7 kWh
LPG: 1 kg ≈ 1.88 L ≈ 13.63 kWh
1 L ≈ 7.22 kWh
Heating value of kerosene between 43.1 MJ/kg and 46.2 MJ/kg
1 MJ = 0.2778 kWh
Density = 800 kg/m3
1 kg ≈ 1.25 L ≈ 12.41 kWh
1 L ≈ 9.93 kWh