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Energy Basics

‘Energy’ is measured in Joules (J) and the rate at which energy is generated or used is measured in Watts. One Watt is one Joule per second – (Js-1). The unit of Watts most commonly used when discussing energy consumption is the kilo Watt – i.e. 1000 Watts – or 1kW.


Energy Rating

Electrical appliances are rated in kilowatts. For example, an oil filled radiant heater is rated at 1.5kW. This means that when the heater is switched on it will immediately consume up to a maximum 1.5kW. Where large amounts of energy are generated or consumed, the units used are more likely to be in one of the following formats; Megawatt (1,000,000 Watts or 1MW), Gigawatt (1,000,000,000 Watts or 1GW) or even Terawatt (1,000,000,000,000 Watts or 1TW).


Energy Consumption

Units of energy consumption are usually expressed in terms amount of energy used over a certain period – the standard term for this is kilowatt hours or kWh i.e. the amount of energy consumed over an hour. The 1.5kW heater if left on for an hour with a constant electrical supply will therefore consume 1.5kWh of energy. Electricity is sold by the kWh, which equals 1 unit of electricity. The current average domestic tariff is around 14p per kWh, therefore keeping the electric heater on for 1 hour will consume 1.5 units of electricity and cost 21p.


Energy Generation

Generators are rated in kW or MW, indicating the maximum that can be generated at any moment. If a 1kW generator is operating at full capacity for 1 hour it will generate 1kWh. However, the amount of energy generated will depend on how much useful energy is available to power the generator. It will only generate to its maximum rated level if it supplied with sufficient useful energy. This applies equally to a small diesel generator or a wind generator, the only difference is that a small diesel generator will generally either be full on (with fuel), or off (no fuel) whereas the output from a wind generator will vary with wind speed.


Renewable Energy

Understanding some renewable energy basics will help you to work through what may be possible for your community. Below are some typical questions that arise as people seek to understand how renewable energy works and why they should consider using it.


Main Useful Sources of Renewable Energy:

Source Utilisation Output
Sunlight - heat Solar water heating Hot water
Biomass - wood Combustion – boiler or stove Heat
Sunlight – heat from sun transferred to soil, air or water Ground source heat pump Air source heat pump Water source heat pump Passive solar Heat and hot water
Sunlight - photons Solar photovoltaic cells (PV) Electricity- see Annex for further info
Wind Wind turbine Electricity
Water Hydro Turbine Electricity
Biomass - wood Combustion – boiler + steam turbine Heat and electricity
Biomass – biodegradable matter Anaerobic digestion (decomposition without oxygen to produce methane) to burn in a combustion engine or CHP system Heat Electricity
Wave (wind) Floating or shore based electrical generators converting kinetic energy from waves. Electricity
Tidal Underwater electrical generators converting kinetic energy from tides Electricity

Renewable energy vs traditional energy sources

Renewable energy technologies tend to be less instantaneous than fossil fuel sources. For example, a ground source heat pump can be ideal for providing background warmth, but cannot react instantaneously to provide immediate additional heat, unlike a gas boiler which can be turned up and down on demand.

Renewable energy sources such as wind, wave, tidal, hydro and solar are all carbon free fuels whereas fossil fuels (coal, gas oil etc) produce carbon dioxide (a major greenhouse gas) when burned.


In general, renewable energy sources are less energy dense then fossil sources. This has two practical implications: more space to store or extract the energy source is required and it takes longer to harness an equivalent amount of useful energy.

Fossil fuels have limited reserves estimated to last between 50 and 120 years based on their current and projected consumption. Renewable energy is limitless and, as its name suggests, is renewable and at least one of its forms is available anywhere in the world.

In the long-term, the on-going cost of renewable energy based systems is likely to be lower than those based on fossil sources with fossil fuels widely expected to increase in cost as global demand increases. In addition, as the use of renewable systems increase, economies of scale will mean installation costs will become more competitive with traditional sources.

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