The horse-powered household

If there is one thing that I consider separates our society for all others in the past, it is a profligate use of energy. It is the incredibly cheap (historically speaking), easily accessible, stored, high-grade energy that lets our society do the things we do at the scale we do it.

The huge populations that support huge and diverse cultures of people, the instant connectivity and the insane throughput of material resources we have, all would be impossible if we used and harnessed energy in the same way any human society did less than 200 years ago.

For the next few posts I’m going to explore the world of energy, for two reasons:

  • I am utterly fascinated by energy, how its moves around us, changes form and changes us
  • Most people don’t give much thought to energy, but might find it interesting.

Dense definitions

Carrying on a little from my last post, the first thing we need to do is talk a little about how energy is measured. The international unit used for measuring energy is the Joule. Most people only come across joules outside of a science class on the nutritional information thing on the site of food packaging. Even then its supplied alongside calories which is the thing many people pay attention to instead.

A joule is the amount of energy transferred to an object by moving an object one metre against a force of one Newton. You don’t need to know what a Newton is, but on Earth the force of gravity means that an object weighing 100 grams is exerting a force due to gravity of about one Newton. Want to produce a Joule? Lift a small apple up a metre. That’s the work in one joule.

The William Tell Moment: An additional 1 Newton to an already weighty head proved too much.

We will look at how the joule relates to other units in a minute, but firstly its necessary to create distinction between two related but different concepts: energy and power.

The difference matters when we try to think about energy, but even national newspapers will get this mixed up on a weekly basis. It is poorly understood but I am going to have a try at explaining.

Watt horse?

First lets talk about power and lets start with a unit that should make sense to most people. The horsepower.

The person history gives credit to for developing the horsepower is James Watt. Contrary to popular opinion he didn’t invent the steam engine. James Watt did make the steam engine far more efficient, had a lucrative business model and had the patent office grant him a monopoly for a long time, but he didn’t invent the steam engine. Popular contenders for that honour are Hero of Alexander and Thomas Newcomen.

James Watt did, however, come up with a way of comparing the performance of a steam engine with that of a draught horse, mainly for marketing purposes. The story behind how  an exact figure was arrived at is a little unclear, but in one version it was based on how quickly a draught horse could work at a mill. The units initially used were in foot-pounds per minute over a working shift for an animal (about 4 hours). That is, a horse was able to turn a mill wheel of 12 feet in radius, 144 times per hour, whilst the wheel resisted (by grinding grain) with a force of 180 pounds (800 Newtons). These figures varied, of course, and what we now have is a little different. Broadly speaking there are two definitions of horsepower in use today:

  • Mechanical or Imperial horsepower: 33,000 foot-pounds per minute (rounded up from the above example’s 32,572 ft.lbf/min)
  • Metric horsepower: the power needed to lift the weight of a person (75 kg) one meter in one second.

The horsepower is abbreviated to hp in English, CV in other European countries (Caballo de Vapor – Steam Horse in Spanish and equivalent literal translations in Italian, Portuguese and French) and PS in German for Pferdestärke (Horse Strength), because Germany.

A simple diagram of what a horsepower might look at. The box on the right will be lifted a metre in one second by the power of one horse. Link.

The horsepower is still widely used when discussing the mechanical output of engines. although it generally isn’t used when talking about thermal power, electrical power, electromagnetic or radio frequency power and others. For measuring these we can use the Watt, the unit of power measurement named after James Watt to describe something that he already came up with a unit for.

A Watt can be converted by multiplication to a horsepower and vice versa. One imperial horsepower is 746 Watts, a metric horsepower being 735 Watts. To make the units easy in the next part I am going to call one horsepower 750 Watts, less than 1% more than an imperial horsepower. Someone gave our horse a sugar cube.

In summary, horsepower and the Watt are both a measure of power.

Joules Burn

Now lets go back to energy and the Joule. Recall that one Joule is the energy taken to raise approximately 100 grams one metre up against gravity. Multiplying by 10 means lifting up 1 kilogram takes 10 Joules. Multiplying by 75 means lifting 75 kilograms one meter high expends 750 Joules of energy.

The key thing to note about these 750 Joules is that they don’t care how long anything takes. Want to use a jack to make is easy to lift 75kg and spend 5 minutes jacking? The result is still 750 joules (plus friction in the jack) when that 75 kilograms it listed up 1 metre.

A horsepower is all about time. In the diagram above, a horsepower lifts 75 kg up one metre in one second.

Power is the rate of energy output.

Apply one horsepower for 1 second? 750 Joules of output. 10 seconds? 7500 Joules.

Energy is how much work is done by the power when it is used for a certain time.

Your electricity company doesn’t bill your for Watts. It bills you for energy, expressed in Watt-hours.

A Watt-hour is like measuring labour in man-hours. It may as well be called a horsepower-hour, that way people might understand what they are paying for.

The horse-powered house

The average household in Australia consumes 18 kilowatt-hours, or 18,000 watt-hours per day. Most of that energy is likely to be coming from a turbine of some kind: steam, gas, hydroelectric or wind. Imagine if that energy supplied by draught horses turning a generator like they did a pump or mill wheel? How many horses would that take?

The 18,000 Watt-hours used in a 24-hour day by the average Australian household means that 750 Watt-hours used every hour. This works out to an average power of 750 Watts – exactly the same as the output of a draught horse. Only one horse per house? That’s not so hard to imagine, at least for a fully-detached house.

The reality is that horses need to rest just like all organisms and a four-hour shift was considered the norm in the days of animal power, at least it is what James Watt used in his marketing. Taking that at face value means that to supply our 24 hour requirement for on-demand power we would need 6 draught horses working in four-hour shifts each.

According to the NSW Department of Primary Industries, the amount of land required to support a draft horse is 14 Dry Sheep Equivalent. I know, I know. Simplistically, it’s just a measure of land productivity and carrying capacity.

1 Dry Sheep Equivalent is the amount of land required to support one 2-year-old, 45 kg wether or non-lactating, non-pregnant ewe. In the area where I live now – some of the most productive country in Australia – the average stocking rate is 15.1 DSE/hectare. Using this measure we need about one hectare to supply one draught horse. In order to supply the average Australian house with it’s 6 draught horses for 24-hour electricity we will need 6 hectares.

6 hectares is 60,000 square metres. By way of a rough comparison Solar Photovoltaic panels would would need only about 25 square metres to generate those 18 kilowatt-hours, or under 2000 times less area. In both situations, some form of energy storage is needed to supply energy when higher-than average requirements were made or during a gap in energy production.

The 6 hectares of land that would be needed to keep my house humming on contemporary sunlight – as opposed to the ancient kind in fossil fuels – is quite a bit more than the 1/10 hectare my house sits on. The work done by fossil fuels is equivalent to 10 billion human slaves working 24-hours a day in output, giving us in the western world a standard of living inaccessible to even royalty a couple of centuries ago.

When flipping the switch on the kettle summons the instant power of 3 draught horses out of a skinny little cord from the wall, it’s not hard to understand why the convenience of instant energy is so addictive.

With that thought I’m off for another fortnight. Next time I’m going to explore what a litre of petrol or short ton of coal actually represent and why when compared to animal power their use became a huge windfall.


I’m curious to know how you found this post. Too dry? Too numbers-heavy? Too long? Unreadable?

Leave your thoughts below, I’m keen to not just have a conversation with myself and if this post brings on any questions, let me know that too.

5 thoughts on “The horse-powered household”

  1. Fascinating Steve, I appreciate the history tempered with humour and of course the gorgeous Rosie helping dad out! Keep them coming….

  2. Stephen, since Jean’s posted these on Facebook I’ve become aware of your interests, humour and well researched writing talent. I’ll admit too often time is short so I skim read a bit but the connections, themes and use of a consistent metric on which to build your case ensure I get the messages clear and strong. Keep it up, Stephen.

  3. Hah you make me laugh by science. Well mainly at your voice in my head reading it out. My favourite bits are:
    – references to obscure calculations and stats (you must have a weird browser history)
    – tangents that measure in dry sheep
    – Rosie as prop

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