Diesel and petrol costs compared with other fuels

Before comparing running costs of different types of car, it is worth thinking about the cost of petrol and diesel compared with electricity and indeed other fuels. This post is the first part of a series. We begin by considering the raw costs of fuel. Next we consider the efficiency with which we can use those fuels, which affects the real costs of use. The third part looks how taxes affect the real costs.

Before making a comparison, it’s useful to remind ourselves of the units we are using. Lift an apple (100g) up a metre and you’ve done about a joule of work. Do that 3.6 million times and you’ve done 3,600,000 joules of work: that’s 1 kilowatt hour (kWh), the standard unit for supplying gas and electricity to the home.

Here is a direct comparison of the costs to the consumer of electricity, petrol and diesel.

We are comparing all the fuels by the price per kilowatt-hour, even though we don’t buy petrol and diesel by the kilowatt-hour.

What is there to remark on? Perhaps that the different fuel costs per kWh are broadly similar and there is no obvious reason, from this information, leading us to think that electric cars should be cheap to run.

We buy petrol and diesel in litres rather than kWh and we know that diesel fuel is significantly more expensive than petrol. Yet the cost per kilowatt-hour for the two fuels is similar. It’s not hard to see that diesel fuel contains more energy per litre.

Let’s follow the numbers.

A litre of petrol has an energy content 9.4 kWh per litre and petrol currently costs 116.4p per litre. So the price per kWh is 116.4/9.4 = 12.4p per kWh.

A litre of diesel has an energy content of 10.3 kWh and costs 120.6p. So the price per kWh is 120.6/10.3 = 11.7p per kWh. We can see one reason why diesel cars cheaper to run than petrol cars: diesel, despite being 4% more expensive, is about 6% cheaper per kWh.

Buying electricity from the electricity supply at home, we pay about 17 p per kWh. Some electricity tariffs supply electricity more cheaply at night and charge a bit more per day (20p) and less (10p) for the 7 hours overnight, averaging the same 17p per kWh.

Public electric charging stations
Electric cars are in their infancy. Charging at home is a recent step. But public charging stations are even more of an innovation. The prices of the services they provide will no doubt change over time but at the moment it can be much more expensive to charge your car from a station ‘on the road’. Ionity fast chargers charge 69p per kWh, with other providers charging more than twice what you would pay at home. If these costs become applied generally, electricity will certainly not be a cheap fuel. On the other hand Lidl charges 23p p/kWh at the moment, a reasonable mark-up for the cost of providing the service. Tesco is free but one can’t see that lasting.

You can stop here if you like
The rest of this post is concerned with following the numbers through for a wide variety of fuels, not all of them for transport. I’ve put these figures here because I think it’s fun to collate all the fuel information and also so that later on I can branch off into considering domestic heating. You might like to have a quick look at the summary table a bit further down before departing.

Road LPG
LPG stands for liquefied petroleum gas, It liquefies under modest pressures and so can be stored in tanks and used for domestic and commercial heating and for vehicle propulsion.
LPG has an energy content of 7.2 kWh per litre and at a current price of about 65p per litre, that’s 65/7.2 = 9p per kWh.

Heating LPG
Here we find our first consideration of the effect of tax on prices. LPG for heating has much less tax on it than LPG for transport. That brings its costs down to a typical 36 p per litre and with the energy stored being 7.2 kWh/litre, that equates to 36/7.2 = 5 p per kWh.

Red Diesel
Red diesel is a tax-free version of road diesel, used for purposes like powering agricultural vehicles or heating. It is ordinary road diesel that has a red dye to colour it to check it is not used for taxable purposes. Red diesel has the same energy content as road diesel at 10.3 kWh per litre but the reduced tax means that it costs only about 60p per litre. So the price per kWh is 60/10.3 = 5.8p per kWh.

(Natural) Gas
Gas costs about 3p per kWh at the moment. That’s because the oil price is very low due to the covid crisis. My own view, for reasons that I might address eventually, is that the ‘right’ price for gas is 5p per kWh.

Summary of fuel costs
I’ve decided to summarise the fuel costs at this point before you get too bored with repeated calculations. The remaining calculations and some comments on the fuels follow the table.

Heating oil
10.85 kWh per litre and 41p per litre, which gives 41/10.85 = 3.8p per kWh.

Solid fuel
House Coal is £267 per tonne (1000 kg) or 26.7p per kg. Its calorific value is about 7 kWh/kg. So the cost per kWh is 26.7/7 = 3.8p per kWh.
Anthracite is 40p/kg with a calorific value of 9.2 kWh/kg, giving a cost of 40/9.2 = 4.34 p/kWh.
Blaze Smokeless house fuel is 33.5 p/kg, with a calorific value of 5 kWh/kg giving a cost of 33.5/5 = 6.7 p/kWh.
Like all fuels in which there is direct negotiation with the supplier, solid fuels prices depend on your ability to haggle and the time of the year at which you are buying. You can buy smokeless fuel for 28p/kg in the summer, much less than its winter price.

Wood pellets
£265 for 1150 kg, which is 23p/kg. Calorific value around 4.9 kWh/kg, which gives 23/4.9 = 4.7 p/kWh.
My own view is that wood pellets are not, in general, a renewable source of energy. Wood sawdust should be used to make materials like MDF (medium density fibreboard) which is used instead of wood in many products. Those who use them believing that the have some benefit in ‘saving the planet’ should think twice. Burning pellets of wood results in more living trees being cut down and therefore encourages the destruction of the natural environment.

Kiln dried Ash logs, £350 for 750 kg = 47p/kg. Calorific value 5.5 kWh/kg, giving 47/5.5 = 8.5 p/kWh.

Manufactured heat logs
Wickes; £6.50 for 9.5 kg, a pack of 12. That’s 68p/kg. There is no given figure for the calorific value of this product but the forest research reference below indicates that the calorific values of all woods is about 5 kWh/kg. So these Heat Logs cost 68/5 = 13.7 p/kWh. They are an expensive form of heating, particularly when we take into account efficiency in the next post on this subject.

Sources of information
Figures for this post date from January 2020