Saving Power at Home & Work

Comparison of Fuel Types

With electricity prices seemingly out of control, as well as electricity supply being somewhat unreliable, a lot of people are looking at various other alternative sources of energy for their needs.

Fuel Prices

We took a quick look at the current prices of various fuel types that may serve as a viable alternative to electricity.

Different fuels are priced in different ways.

In order to properly compare them, we first had to convert them all to a common, comparable price unit.  We chose Rand/kWh which is the way electricity is priced. This is a price per unit of energy (kWh).

Energy is the fundamental element that ultimately is converted into some sort of useful work.  Its the energy stored within the fuel that is required in each case, and a unit of energy from one type of fuel is equivalent to that from another type of fuel. The table below converts each fuel type to the same price unit of Rand/kWh.

The prices used were those available as of June 2014.  Obviously prices may vary depending on your location and other factors, but these are prices we found readily available in Johannesburg without exhaustive searching for the best price.

FUEL Energy Density (kWh/kg)
ENERGY DENSITY by volume (kwh/m3)
Electricity  –  –  – R 1.18
Log wood (stacked – air dry: 20% MC) 4.10 1,700 R1.50 R 0.37
Anthracite 9.20 10,100  – R 3.20 R 0.35
Natural gas (NTP) 10.60  – R 1.14
LPG 12.90 6,600  – R 24.00 R 1.86
Diesel  – 10,000 R 13.50 R 1.35
Petrol  – 8,930 R 13.50  – R 1.51

As you can see the most expensive fuel per unit energy in our list is LPG gas, followed by Petrol and Diesel.  The cheapest is anthracite.

Process Conversion Efficiency

Unfortunately the conversion of energy into useful work is very often  inefficient, despite our best efforts, which means that the cost of fuel per unit energy, as listed above, is not necessarily the price you pay to get the job done.  It could be much worse than that. This is very dependent on the precise process and technology used.  Some fuels and processes are better than others for different purposes.

Lets take heating water as an example:

If you use a conventional electric geyser, the heating element is immersed in water inside the geyser which is a very well insulated tank. All the heat produced by the heating element passes into the water, almost none is wasted.

But if you heated that water using a gas burner, or coal fire, or some other form of combustion process, then its very difficult to ensure all the heat produced by combustion gets into the water. In most systems a large proportion of the heat produced escapes up the chimney or into the surrounding air.  You could lose as much as 50% of the heat!  That means that you could be using twice as much fuel as you thought to achieve the same effect, and thus the actual cost of heating water using this fuel, and this heating process, could be twice what you expected.

Another example is using a diesel generator to generate electricity.  Internal combustion engines typically have efficiencies of about 30-40%.  That means that about 60% of the energy in the fuel you are paying for is wasted, mostly in the form of heat!  So if you are using a diesel or petrol generator to supply electricity to your home or office, then the actual cost of the fuel in comparison to electricity could be three times higher than you thought.  Although, if you use the heat generated by the engine to heat your home or building, as well, then the efficiency improves (because you are gaining more benefit from the fuel’s energy by using the heat that is normally wasted). OffGridDIY have a calculator which will estimate the cost of diesel when generating electricity using a diesel generator.

This shows that you can’t judge the fuel cost without understanding both the fuel price per unit energy content, as well as what process/technology is used for useful benefit.

1 thought on “Comparison of Fuel Types

  1. Good analysis guys. You hear from so many ‘amateur-experts’ about changing everything to gas to save money… but they don’t even do the first bit of analysis. Thanks for adding some real-data to the mix!

    Spot on with your outcomes, just wondering where you got the price of LPG as R24/kg; that looks a tad high to me compared with around R21 delivered – http://www.thegascompany.co.za/products/gas/

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