Energy, Power and Efficiency

The following section is a review of the energy concepts that must be understood to install and design solar energy systems.


Energy is referred to as the ability to do work. For example, energy is required to boil tea, to move a vehicle between two points or to make a radio work. When boiling tea on a woodstove, the energy source is chemical energy stored in firewood. When driving a car, the source is chemical energy stored in petrol. When operating a radio, the energy source is chemical energy stored in dry cells.

Energy is measured in units called joules, J, or in watt-hours (see below). Because one joule is such a small amount of energy, words that name large numbers of joules are commonly used. One kilojoule, kJ, is equal to a thousand joules, and one megajoule, MJ, is equal to a million joules. Charcoal, for example, contains about 32kJ of energy (or 32,000J) per gram and petrol contains about 45kJ per gram. During the course of a clear day at the Equator, about 23MJ of solar radiation energy falls upon an area of one square metre.

Watt-hours (Wh) are a convenient way of measuring electrical energy. 1 watt-hour is equal to a constant 1 watt of power supplied over 1 hour (3600 seconds). If a bulb is rated at 40 watts, in 1 hour it will use 40Wh, and in 6 hours it will use 240Wh of energy. Electric power companies measure the amount of energy supplied to customers in kilowatt-hours, kWh (or thousands of watt-hours). In this book, energy is always referred to in watt or kilowatt-hours. Note that one kilowatt-hour is equal to 3.6 megajoules.


Power is the rate at which energy is supplied or consumed (or energy per unit of time). Energy can be supplied or consumed at a high rate or at a low rate. For example, it takes roughly the same amount of energy to travel 10km walking as it does to travel 10km running. The difference is that, when running, more energy is being used per unit of time than walking. Similarly, the amount of energy required to boil a pot of water is constant; the time it takes to boil the water depends on the power or the rate at which the energy is supplied. More power is required to boil a pot of water in 2 minutes than is required to boil the same pot in 10 minutes.

Power is measured in watts (W). One watt is equal to one joule supplied per second. As in the case above, large amounts of power are given the name kilowatts, kW (thousands of watts), and megawatts, MW (millions of watts). As an example, an incandescent light bulb might use 40W, while a radio uses about 5W, and an electric cooker might use 2000W. A human being riding a bicycle produces about 200W of power, while a typical automobile engine produce about 25kW. On a clear day, solar power arrives upon a flat surface at a rate of about 1000W (1kW) per square metre.


Efficiency is the ratio of output energy to input energy expressed as a percentage. Mathematically, it is expressed as follows:

Efficiency (%) = output energy/input energy X 100

Energy-efficient devices use less energy to perform a given task than energy-wasting ones. For example, some types of stoves use less fuel to cook than others. Similarly, some types of cars use petrol more efficiently than others. Fluorescent lamps consume less energy than incandescent lamps to produce the same amount of light.

Energy Conversions

watt-hours X 1000 = kilowatt-hours kilowatt-hours X 1000 = megawatt-hours

megajoules/3.6 = peak sun hours or kilowatt-hours kilowatt-hours X 3.6 = megajoules

langleys X 0.0116 = peak sun hours or kilowatt-hours langleys X 0.0418 = megajoules watt-hours X system voltage = amp-hours

Power Conversions

watts X 746 = horsepower watts X 1000 = kilowatt kilowatts X 1000 = megawatts

Appendix 2

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