The camper trailer book - chapter 31: terminology explained - camper trailer

Lord Kelvin. Pic: source unknown.

What is electricity?

In the mid-1850s, the great scientific pioneer, Lord Kelvin was lecturing on electricity. He asked his class: “What is electricity”?

One student put his hand up, but then stammered out that he’d forgotten. Lord Kelvin turned slowly to the class, and said:

“Gentlemen, you have just witnessed the greatest tragedy of this century. Only two people know what electricity is. One is God, and the other is Mr Smith. God won’t tell us - and Mr Smith has forgotten”.

To this day no-one truly knows, but whilst its exact nature is yet to be fully defined, its behaviour has been surprising well understood since the days of Lord Kelvin and many other scientists of his era.


Alternating and Direct Current: the movement of electrons responsible for the flow of electricity. Direct current flows and performs work in a manner analogous to a band saw: it operates in a continuous direction. Alternating current works much as big cross-cut saws that operate by being pulled to and fro. Preferred abbreviations are now ac and dc respectively (previously AC/DC).

Electricity authorities supply alternating current (although Broome and a few other Australian country towns ran on dc until the 1960s). Batteries supply direct current. It is readily possible to convert alternating current into direct current - and vice versa. (An inverter converts direct current into a higher voltage alternating current, a mains battery charger converts alternating current into a lower voltage direct current.)

Solar panels/modules: The solar industry describes individual solar generating units as solar modules, ‘solar panels’ are assemblies of modules. Assemblies of modules are called ‘arrays’.

Standards: In all Australian/New Zealand Standards ‘should’ is a suggestion or recommendation.

‘Shall’ is a requirement that must be followed to comply with that Standard.

Electrical units & terms

Amps: the amount of electrical current that is flowing. It is akin to flow in a pipe. The greater the voltage, the greater the amount of current that will consequently flow. Its common abbreviation is A, (but that of current generally is I).

Amp hour: the amount of electrical current that flows in one hour. A device that generates four amps for five hours thus produces 20 amp hours: amp hour is commonly abbreviated to Ah.

Extra-low voltage: voltage not exceeding 50 Vac, or ripple-free voltage not exceeding 120 Vdc.

Low voltage: voltage exceeding extra-low voltage, but exceeding 1000 volt ac, or 1500 volt dc.

High voltage: any voltage (ac or dc) that exceeds low voltage.

Ohms: The unit of resistance to the flow of an electric current. The unit is either spelled out (i.e. as ohm), sometimes expressed as R, and (traditionally) also expressed by using the Greek symbol for omega (Ω). One ohm is the equal to the resistance to the flow of current through which a current of one amp will flow if one volt is applied across it.

Ohm’s Law: a fundamental electrical law. Volts, amps and ohms are interrelated and that interrelationship is expressed and defined by Ohm’s Law. That Law states that the direct current (dc) that flows in a circuit is directly proportional to the voltage across that circuit. It is valid for metal circuits and some (but not all) liquids that are electrically conductive.

Power/Energy: These terms are often misused. Power relates to the rate at which work is done and is expressed in watts. Energy relates to the amount of work done and is expressed in watt hours.

Resistance: To varying extents, all substances resist the flow of electricity. This resistance generates heat. Resistance can be useful (it is how an electric kettle works) but where heat is not specifically required, it wastes energy. The thicker a cable, the lower its resistance - and the less the energy lost through heat. The term ‘resistance’ is itself often abbreviated as R. It is measured in ohms (see above).

Volts: the pressure that causes electricity to flow: akin to pressure in a pipe (abbreviated V). It is common to indicate whether such voltage is ac or dc - e.g. Vac or Vdc.

Watts: Volts, amps and ohms are interrelated and, when multiplied together, are a measure of energy used, and also of work performed. The resultant unit is a watt (abbreviated as W). Thus one volt times one amp equals one watt. See also Power/Energy on the previous page.

Watt hour: a measure of electricity generated or used in one hour. A 100 watt globe that is running for 30 minutes consumes 50 watt hours. A 12 volt solar module producing four amps for five hours produces 4 (amps) x 12 (volts) x 5 (hours) = 240 watt hours. The correct abbreviation is Wh.

Watt hours/day: the number of watt hours consumed in a 24 hour period. This unit is handy when scaling solar systems etc. The correct abbreviation is Wh/day.

Quantities and numbers

Where the quantity of anything is over 1000, the ‘1000’ can be shown as k - but never K. For example, 3500 watts = 3.5 kW. If that 3.5 kW is consumed in (say) one hour, it is expressed as 3.5 kWh. The number 1,000,000 can be as ‘m’ ( but not ‘M’.) It is thus 1 megawatt, or 1 mW.

By and large, where a unit is named after someone (e.g. watt - named after James Watt) it is generally printed in lower case e.g. ‘w’ (watt) when it is spelled out in full, but is capitalised (as W) when it is abbreviated. It is thus 12 watts or 12 W. The unit volts (named after Voltaire) is thus V when abbreviated.

The unit ‘litre’ is an exception. It is also usually capitalised when used as an abbreviation. This is done to avoid typographical confusion because the letter ‘el’, in most typefaces (including this one) is identical to the number - and maybe mistaken for a ‘one’

There must always be a space between the names and/or symbols of SI units and their associated numerical value by a space: thus 6.0 V - never 6.0V, but the symbols for degree (º), minute (‘) or second of a plane angle (“) are not so spaced - thus 25º, 10’ and 25”.

Much of the above causes ongoing issues for technical writers faced with magazine editors who say ‘it is not their house style’. Here, no engineer ever climbs down - so any such ongoing deliberate usage is an indication that the user may not be that knowledgeable.

Most of the above is based on the International System of Units (often abbreviated to SI). It is in essence, a practical and updated metric system and defined in Australian Standard 1000 - 1970.

The SI system and its usage is fully described in clear English in the Australian Government’s official Style Manual and that Manual’s equivalent in many other countries.

Despite leading the global move to adopt the metric system, the USA, at the last moment had second, and mainly political, thoughts about it. It retained its own variant of the UK Imperial system, but by and large American scientists and engineers (but not Americans generally) also use the SI system.