Users Guide to Off-Grid Energy Solutions
Electricity and Electrical Terminology Fact Sheet
Electricity is energy that is generated by a number of forms. Electricity has been generated as a form of energy for over a century now. It can be generated from such sources as mechanical (e.g., wind turbines), to thermal (e.g., diesel generator, biomass steam turbines, etc.), to photovoltaic, and used to provide power for a number of applications. Electricity has the unique ability to be generated and transmitted and distributed over long distances, or to be generated and used in situ. It takes any number of forms of energy (solar, wind, hydro, etc.) and converts them into an energy form (electricity) that is directly useable for everything from lighting to transporting trains, from cooling and heating large buildings, to powering small wrist watches.
Photovoltaic cells can generate electricity in virtually all parts of the world, although, of course, solar radiation is required, and the more solar radiation there is, the better the photovoltaic electricity generating prospects. Photovoltaic systems do not need to be connected to the grid. They can provide households, businesses, other users with stand-alone electricity, which is particularly useful in isolated, non-grid connected systems. The costs of PVs are coming down, and the economics of photovoltaic-generated electricity are improving steadily.
However, PV remains limited at the present to relatively low energy demands or loads. Any load above approximately 1 kW (kilowatt) is rarely economically suitable for PV. Rather, at higher loads, more "conventional" sources of electricity generation, whether wind and hydropower, or diesel, are required. Wind tends to be intermittent, so, in isolated situations, with high loads/demands, particularly where reliable power is required, diesel and petrol generating sets (gensets) are still the most viable off-grid solution, even if they are used as back-up in a hybrid situation. The following terms and definitions provide further information on electricity generation.
Terms and definitions
AC - Alternating Current: AC is the standard form of electrical current supplied by the utility grid and by most fuel-powered generators (diesel or petrol "gensets"). The polarity (and therefore the direction of current) alternates. In the USA, standard voltages for small water pumps are 115V and 230V. In most of the rest of the world, standard voltages for small water pumps are 220-240V. In most of the world, the current frequency is 50 Hz ("Herz", or cycles per minute), while in the US, current frequency is 60 Hz.
alternating current (ac): electric current in which the direction of flow changes at frequent, regular intervals
ampere (amp) (A): unit of electric current which measures the flow of electrons per unit time
amp hour (amp hour) (Ah): a measure of total charge commonly used to indicate energy capacity in batteries. One amp hour is equal to the quantity of charge in the flow of one ampere over one hour.
appliance: a tool or other device such as radio or television which consumes electricity
circuit: a system of conductors (i.e. wires and appliances) capable of providing a closed path for electric current
converter: An electronic device for DC power that steps up voltage and steps down current proportionally (or vice-versa). Electrical analogy applied to AC: See transformer. Mechanical analogy: gears or belt drive.
current (amps amperes A):, Electricity current is the rate at which electricity flows through a circuit, to transfer energy. Current is measured in Amperes ("A"), commonly called Amps. Analogy: Flow rate in a water pipe.
DC - Direct Current: DC is the type of power produced by photovoltaic panels and by storage batteries. The current flows in one direction and polarity is fixed, defined as positive (+) and negative (-). Nominal system voltage may be anywhere from 12 to 180V. See voltage, nominal.
direct current (dc): electric current flowing in one direction
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.
electric power: the rate at which energy is supplied from an electricity generating source. It is measured in watts (W)
electricity prices: the price at which electric energy is supplied from an electricity supplier.
fuse: a device which protects circuits and appliances in the system from damage by short circuits
grid: A network that connects a supply of electricity (water, transport, etc.) to a number of users. In electricity terms, a grid usually consists of some form of electricity generator, with electricity taken along a transmission line at high voltage, then stepped down to lower voltage on a distribution system that delivers electricity to end users (households, industries, etc.)
hybrid system: a hybrid system refers usually to the combination of two energy generating applications (e.g., diesel generator with PV system) to provide electricity at all times, or in all critical times. Generally, a hybrid system will be designed to ensure reliability (e.g., hospitals, industrial processes, computer operations), where one energy source (e.g., PV from the sun, wind) is intermittent, ie, not always available.
interconnected system (ICS): Refers to an electricity system that connects one user to the next, and the source of electricity supply to consumers through the network or grid. An ICS is another word for a grid.
kVA: kilovolt ampere (one thousand Volt Amperes), the current flowing in a circuit multiplies by the voltage of that circuit, usually measured on a transformer. A measure of power whereby one kVA typically equals approximately 0.8 kWh (depending upon the phase).
kWh: kilowatt-hour = 3.6 MJ. An energy measure that indicates a Watt consumed or generated in one hour equivalent.
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.
off-grid: A situation when a consumer is not connected to an electricity grid. This is fairly rare in Europe or North America (limited to camping areas, very isolated sites, etc.), but is very common in rural areas, and many near-urban (peri-urban) areas of the developing world where electricity companies and suppliers have been unable to connect domestic, commercial, industrial and institutional consumers. The main thrust of this workbook is to provide "off-grid solutions".
ohm(s): a unit of electrical resistance
open circuit voltage (Voc): the maximum possible voltage across a solar module or array. Open circuit voltage occurs in sunlight when no current is flowing
resistance: the property of a conductor (i.e. a wire or appliance) which opposes the flow of current through it and converts electrical energy into heat. Resistance has the symbol R, and is measured in ohms.
system voltage: the voltage at which the charge controller, lamps and appliances in a system operate, and at-which the module (s) and battery are configured.
total daily system energy requirement: see daily energy energy requirement. The amount of energy requuired to meet the daily electrical load plus the extra energy required to overcome system energy losses.
transformer: An electrical device that steps up voltage and steps down current proportionally (or vice-versa). Transformers work with AC only. For DC, see converter. Mechanical analogy: gears or belt drive.
utility grid: A utility grid is usually a commercial electric power distribution system that takes electricity from a generator (e.g., fossil fuel boiler and generator, diesel generator, wind turbines, water turbine, etc.), transmits it over a certain distance, then takes the electricity down to the consumer through a distribution system. The entire system is referred to as the grid. Synonym: mains. Analogy: Water utility, where water is taken from a reservoir, transmitted through mains pipes and then
VAC: Voltage alternating current
volt (V): a unit of measurement of the force given to electrons in an electric circuit; see potential difference. Analogy: Pressure in a water pipe
voltage drop: Voltage drop is the loss of voltage (electrical pressure) caused by the resistance in wire and electrical devices. Proper wire sizing will minimize voltage drop, particularly over long distances. Voltage drop is determined by 4 factors: wire size, current (amps), voltage, and length of wire. It is determined by a consulting wire sizing chart or formula available in various reference tests. It is expressed as a percentage. Water analogy: Friction Loss in pipe.
voltage, nominal: Nominal voltage is a way of naming a range of voltage to a standard. Example: A "12 Volt Nominal" system may operate in the range of 11 to 15 Volts. We call it "12 Volts" for simplicity.
voltage, open circuit: The voltage of a PV module or array with no load (when it is disconnected). A "12 Volt Nominal" PV module will produce about 20 Volts open circuit. Abbreviation: Voc.
voltage, peak power point: The voltage at which a photovoltaic module or array transfers the greatest amount of power (watts). A "12 Volt Nominal" PV module will typically have a peak power voltage of around 17 volts. A PV array-direct solar pump should reach this voltage in full sun conditions. In a higher voltage array, it will be a multiple of this voltage. Abbreviation: Vpp.
voltage: Voltage is the measurement of electrical potential. Analogy: Pressure in a water pipe
watt (W): The internationally accepted measurement of power. One thousand watts are a kilowatt, and a million watts are a megawatt. A Watt is the power used when one Joule of energy is used every second (ie, 1 Watt = Joule/second; 1 Joule = 1 Watt second; 1 Watt hour/Wh = 3.6 thousand Joules/kJ; 1 kWh = 3.6 MegaJoules/MJ)
watt hour (Wh): a common energy measure arrived at by multiplying the power times the hours of use (1 Watt hour/Wh = 3.6 kilo Joules/kJ; 1 kWh = 3.6 MegaJoules/MJ). Grid power is ordinarily sold and measured in kilowatt hours