Users Guide to off Grid Energy Solutions
Power and Energy Fact Sheet
Energy is the product of power and time, measured in Watthours (Wh). Energy is generated by the sun, and is stored in a variety of forms. It is locked into biomass through the process of photosynthesis. Burning biomass releases energy, as does decomposing biomass (e.g., digesting it through anaerobic processes as in biogas). Energy is stored in the oceans where the movement of the earth and gravity release it through tides, where the earth’s movement and gravitational pull of the moon and sun generate tides, temperature variations and the earth’s spin generate waves and wind.
Power is the flow of energy at any one time and is measured in Watts. Useful power is generated from renewable energy resources through solar conversion (photovoltaics, solar thermal electricity, solar heating), through harnessing water (hydropower for electricity, wave and tidal generation through turbines), through harnessing the wind (wind pumps for water, wind turbines for electricity), and a variety of other energy sources.
Equipment and appliances are rated on a basis of Watts, and the amount of energy generated by equipment, or the amount consumed by equipment and appliances is measured in Watthours. The following terms and definitions provide further detail to the notion of power and energy.
Energy and Power Concepts
Energy
Energy is referred to as the ability to do work. Energy is measured in units called joules, J, or in watt hours as shown below. One kilojoule, kJ is equal to a thousand joules, and one megajoule, MJ, is equal to a million joules.
Watt hours (Wh), are a convenient way of measuring electrical energy. One watt hour is equal to a constant one watt supply of power supplied over one hour (3600 seconds). If a bulb is rated at 40watts, in one hour it will use 40 Wh, and in 8 hours it will use 320 Wh of energy. Electric power companies measure the amount of energy supplied to costumers in kilo
watt hours, Wh (or thousands of watthours, kWh). Note that one kilowatt hour (1kWh) is equal to 3.6 megajoules.
Power
Power is the rate at which energy is supplied (or energy per unit time). Power is measured in watts. One watt is equal to one joule supplied per second
SOME USEFUL ENERGY CONVERSIONS 

watt hours x 1000 
= kilowatt hours 
kilowatt hours x 1000 
= megawatt hours 
megajoules + 3.6 
= kilowatt hours 
= peak sun hours 

kilowatt hours x 3.6 
= megajoules 
langleys x 0.0116 
= kilowatt hours 
= peak sun hours 

langleys x 0.0418 
= megajoules 
POWER CONVERSIONS 

watts + 746 
= horsepower 
watts x 1000 
= kilowatts 
kilowatts x 1000 
= megawatts 
Terms and definitions
calorie: 4.1840 Joules (kilocalorie/kcal = 4.184 kilo Joules/kJ)
daily energy requirement: the amount of energy that a household, an institution, or any establishment requires to meet the sum total of its energy needs. Example: The daily energy requirement for a typical European household is approximately 45 kWh comprising demand from refrigerator, 20 light bulbs, a dishwasher, a washing machine, a clothes dryer, a TV, other household appliances. Typical energy requirements for an average East African rural household is on the order of less than 2 kWh comprising primarily heat energy (from biomass) for cooking, some kerosene for lighting. These estimates do not include the human power required to accomplish a number of tasks.
direct radiation: radiation coming in a beam from the sun which can be focused
efficiency: Efficiency is the percentage of power that gets converted to useful work. Example: An electric pump that is 60% efficient converts 60% of the input energy into work  pumping water. The remaining 40% becomes waste heat.
electricity: Energy that is generated by a number of forms, ranging from mechanical (e.g., wind mill), to thermal (e.g., diesel generator), to photovoltaic, and used to provide power for a number of applications.
energy: Energy is the product of power and time, measured in Watthours. 1,000 Watthours = 1 Kilowatthour (abbreviation: kWh). Variation: the product of current and time is AmpereHours, also called AmpHours (abbreviation: AH). 1000 watt consumed for 1 hour = 1 kWh. See power.
energy calculations: any set of calculations typically used to determine the amount of demand or load that one has (e.g., the number and voltage required for appliances, equipment, etc.), and then defining the electricity that needs to be supplied to meet that demand/load, taking into consideration all losses in the system.
energy calculations: any set of calculations typically used to determine the amount of demand or load that one has (e.g., the number and voltage required for appliances, equipment, etc.), and then defining the electricity that needs to be supplied to meet that demand/load, taking into consideration all losses in the system.
hydropower: power generated by the flow of water. Water moving by force of gravity, either vertically (as in streams and rivers) or otherwise (as in tides and waves driven by the gravitation force between the earth and move, by the wind, etc.). Water power
insolation: incident solar radiation. A measure of the solar energy incident on a given area over a specific period of time. Usually expressed in kilowatthours per square metre per day or indicated in peak sun hours
Joule: A joule is the energy required to push with a force of one Newton for one metre. A foot pound is the energy required to push with a force of one pound for one foot (1 joule = 1 Newton meter, 1 joule = .737562139 foot pounds, 1 foot pound = 1.355817967 joules)
kilocalorie (kcal): a measure of energy equivalent to 4.187 kJ/kilojoules, or 1.63 Wh/Watt hours
load: The amount of demand placed on an energy system. In the case of most electricity, load could be the set of equipment appliances that use the electrical power from the generating source, battery or module, and the amount of electricity (the load) that those appliances require. Load is often used synonymously with "demand". Load is usually expressed in "watts", so that, for example, if a refrigerator has a rating of 1 kW, the load is cited as being a 1 kW load.
power: The rate at which work is done. It is the product of Voltage times Current, measured in Watts. 1000 Watts = 1 kilowatt. An electric motor requires approximately 1 Kilowatt per Horsepower (after typical efficiency losses). 1 Kilowatt for 1 Hour = 1 kilowatthour (kWh).
RE: renewable energy. Energy from sources such as the sun, wind, water, waves, tides, biomass, that are renewable, that is, that can be renewed within a relatively short period of time. [for good explanations see: http://www.aesms.com/overview/overview.html]
RET: Renewable energy technologies. Technologies that utilise renewable energy resources such as the sun, the wind, biomass, geothermal, water, etc. [for good overview, see: http://www.aesms.com/overview/overview.html]
SEC: specific energy consumption, equivalent to the amount of energy consumed for a specific time in a specific area or for a specific purpose.
solar energy: energy that is received from the sun.
total daily system energy requirement: see daily energy requirement. The amount of energy required to meet the daily electrical load plus the extra energy required to overcome system energy losses.
wind power: power generated by the movement of the earth's rotation, and the temperature variations around the globe. Wind has been harnessed for power for thousands of years (e.g., sailing boats) and for mechanical power for a thousand years (wind mills).
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