It is difficult to make prices comparable over time. The purchasing power of money changes over time and there are different things to buy, so estimates of price change over a long period of time are an approximation.
This post is based on comparisons, but the data should be treated with caution because of the difficulty of making like-for-like samples. In the days when coal was purchased by almost every household there were significant variations in price due to the quality of the fuel with nutty slack at the cheap end and anthracite at the other, the sources do not always quote the type. Distance from the goods yard could be significant. People on low incomes might purchase a stone (14 lb, roughly 6 kg) for cooking at a much higher unit cost than a household taking half a ton in a single delivery.
Gas is slightly simpler, but there were and are regional variations and in the post war period there was a transition from "town gas" made from coal and "natural gas" from the southern North Sea gas fields. The nature of the gas industry is such that it is easier regulate, whilst there were a large number of coal merchants, there were relatively few gas companies, many of which where owned by town councils. Most companies served a single area and there was limited competition. Within the home, electricity displaced gas as the energy source for lighting by the 1930s. The main uses for as were for cooking and until the advent of central heating in the post ware period, gas fires were a common way of heating a room.
A cursory reading of Hansard suggested three things. First that energy prices are a constant source of public, and therefore parliamentary concern and that this is accentuated in difficult economic times. Secondly, that there is a general distrust of energy suppliers, coal merchants in the 1920s were attracting much the same criticisms as today's gas and electricity suppliers. What politicians of all persuasions seem to want is a regulated energy market which is isolated from global economic turbulence. Sustainable technologies go some way to meeting this requirement.
The householder does not purchase a hundredweight of coal or a therm of gas, he/she buys warmth and the facility to cook. Open fires accounted a for a large proportion of the coal burnt in England, whilst a coal fire is cheerful and comforting, it's thermal efficiency is low, possibly less than 20%, stoves equipped with a back boiler were more efficient, but a large amount of heat still went up the chimney. Modern stove designs seem to be a big improvement on those of the 1950s and 60s. As a gross oversimplification, someone wanting 1 kwh of warmth might have to purchase enough coal to produce 4 kwh if the fireplace was 25% efficient. Gas central heating boilers might offer 90% efficiency. Making assumptions about thermal efficiency produces this graph which shows the effective energy cost of coal and gas assuming thermal efficiencies of 25% and 90% respectively.
The crossover point is sometime in the 1960s, this is when our parent's generation blocked up the fireplaces and installed gas central heating. Gas was not only cheaper than coal, it was cleaner and easier to live with. Another benefit of gas was improved air quality, well into the 1970s thick fogs were a frequent occurrence due to the high proportion of soot particles in the air. This in turn provided a decrease in respiratory disease.
The sustainable energy technologies sit uncomfortably with economics, the transition from coal to gas was largely driven by the cost advantages, my source for this comment is my parents and their friends. If a similar transition is to take place from gas to sustainable sources, the energy consumers, who are now our children, must perceive some economic benefits.
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