Electrical Terms Used

To help understand the capabilities and design of renewable energy systems it is important to comprehend some of the electrical terms and energy concepts used.

Current

The rate of flow of electricity along a wire is called the current and is measured in amperes or Amps (A).Current that flows in a single direction is known as direct current or DC. A current that changes direction is called alternating current or AC. Batteries store and transfer only direct current.

Voltage

Voltage is a measure of the electrical pressure between the ends of a wire which causes the flow of current and is measured in Volts (V). Electrical systems in cars use a voltage of 12 volts DC; the mains supply in houses 230/240 volts AC; and long distance power lines over 50,000 volts AC. Different systems use different voltages for different tasks.

Resistance

Electrical Resistance is the property of materials to resist the flow of electrical current. Materials such as glass, rubber and plastic have very high resistance and are called insulators; those such as copper, aluminium, and most metals have very low resistance and are known as conductors. Resistance is measured in Ohms (R). A voltage of one volt will cause one ampere of current to flow through a resistance of one Ohm. This relationship is best expressed by the equation known as Ohm's Law:

V = I x R ; or R = V/I ; or I = V/R

V is voltage in volts ; I, current in amps; R, resistance in Ohms.

Power

Power is a measure of how quickly electricity is supplied and is the product of voltage and current. Power is measured in Watts (W). One watt of power is consumed when one volt causes one amp of current to flow. One thousand watts is called a kilowatt (kW)

Power = Current x Voltage Watts (W) = Amps (A) x Volts (V) W = A x V or A = W/V or V = W/A

The power rating of electrical machinery (eg generators, transformers, inverters) is often expressed in volt amperes (VA). The VA rating is higher than the watt rating in many cases, depending on the type of equipment or appliance. One thousand volt amperes (VA) is 1kVA.

PV Peak Power

The maximum power output of photovoltaic modules are rated in standard test conditions of sunlight of 1000W/m² and at a module temperature of 25° C. It is measured in peak watts (Wp). Larger solar arrays are rated in units of one thousand peak watts: kilowatts peak (kWp)

Energy

Energy is a measure of how much electricity is supplied and is the product of wattage and time. Energy is measured in watt-hours (Wh). One watt-hour of energy is consumed when one watt of power flows for one hour. One thousand watt-hours is a kilowatt hour (kWh) For a given voltage energy can also be measured in amp-hours (Ah). One amp-hour of energy is consumed when one amp flows for one hour.

Energy = Watts x Time

Watt hours (Wh) = Watts (W) xTime (h)

or: Amphour (Ah) = Amps(A) xTime (h)

Energy can be stored. Power describes how quickly that energy can be generated or used.

Efficiency

Efficiency is the ratio of output energy to input energy expressed as a percentage. Energy efficient devices use less energy to perform their task.

Direct Current (DC)

Direct Current (or DC electricity) is an electrical current which flows in just one direction. This is the type of current a battery or PV solar cell can produce. A solar PV system will generate DC electricity and this can be converted by a device called an Inverter into alternating current electricity to power conventional lighting and appliances.

Alternating current (AC)

Alternating Current is an electric current which reverses direction periodically. AC current is the way that electricity is distributed through the power network in the UK. Expressed as a sine wave, the current of AC passes through zero when it changes direction, which makes it a safer electrical current.

Frequency (f)

This is the number of times an AC current changes direction (cycles) in one second, measured in Hertz (Hz). The frequency of alternating current in Europe is 50 Hz, while in the United States it is 60 Hz.