Cientista

Energy Efficiency and Renewable Energy Bulletin

December, 2002

Quantities, Units and Symbols … used in the RE Industry
The following is a brief overview that can be used as a ready reference.

Energy – Symbol … E

E=Pxt

Units … kWh or Wh (Watt-hrs) MJ (megajoules)

1 kWh = 3.6 MJ for Battery Storage - Ah (ampere-hours) are normally used. Ah is a measure of charge. It is not strictly an energy quantity as it does not account for voltage.

Standard metric prefixes
Name Symbol Multiplier … micro milli centi kilo mega giga tera  m c k M G T 10-6 10
-3

0.000 001 0.001 0.01 1,000 1,000,000 1,000,000,000 1,000,000,000,000

for Solar Radiation - MJ/m² is often used.
The use of ‘watts per hour’ or ‘amps per hour’ or even ‘watts per day’ are NOT correct as they do not make sense. USE THE CORRECT UNITS

10-2 103 10 10 10
6 9

Other Quantities Temperature – Symbol … T Units … °C ( degrees Celsius ) Irradiance – Symbol … G Units … W/m² ( watts per square metre ) Irradiation – Symbol … H Units … MJ/m² ( megajoules per square metre ) kWh/m² ( kilowatt hours per square metre )
Sometimes referred to as ‘insolation’ in older texts.

12

Electrical Quantities Voltage – Symbol … V
Units V ( Volts )

V=IxR I=V÷R R=V÷I

Current – Symbol … I Resistance – Symbol … R
Units …  ( ohms )

Units … A ( Amperes, Amps )

Peak sun hours – PSH - are often used to represent the irradiation at a site for one day. 1 PSH = 1 kWh/m2 Lead-acid Battery Electrolyte Density
Is the ratio of the density of the electrolyte to that of water and as such, has no units. If a battery has an electrolyte density of 1200 kg/m³ the specific gravity (SG) of the electrolyte is 1200 kg/m³ / 1000 kg/m³ = 1.2 Often, this is referred to as 1200 points. Fluids expand and contact with temperature - the SG should be corrected for cell electrolyte temperature.

Power – the rate of energy transformation
Symbol … P Units … kW or W ( Watts ) - True Power.

Symbol … VA