*By "international" is here meant "as listed in the International Electrotechnical Vocabulary (IEV) of the International Electrotechnical Commission (IEC)  (see the IEC Electropedia website.) The UK, other EU and over one hundred other nations
use the IEV terms as standard.

1. Watt  An international* unit measuring the rate of energy demanded or generated.

(a) In electrical engineering...........                        1 watt   =  one volt times one ampere.

(b) In mechanical engineering                           746 watts  =  one horsepower

In a complete public  electricity system involving generation, transmission and distribution to consumers the watt is typically used in used in defining :-

(a) full load or other electricity output rate of generators, usually in Gigawatts. (GW)

(b) The rate of public electricity demand for balancing purposes. (It  must by law be balanced or matched  instantaneously and continuously by generation, also in Gigawatts. (GW)).

(c) If the Demand GW exceeds the immediately available Generation GW the system frequency (Hz)  falls below the 50Hz statutory norm and electric motors all over the country slow down and in an extreme case trip out, as does all lighting. Electric clocks may run slow.

(d) If the Demand GW falls short of the immediately available Generation GW the system frequency (Hz) exceeds the 50 Hz statutory norm and electric motors all over the country overspeed, trip out and/ or can be damaged in a severe case. Electric clocks may run  fast.    

(e) It is for reasons (c) and (d) above that National Grid Electricity Transmisson  plc (in their role of Great Britain  System Operator) are charged under the terms of  their transmission licence with conformity to  the statutory permissable limit of frequency(Hz ) and  limited to a temporary 1 per cent either side of the norm. Much other costly engineering equipment may be damaged or wrecked in case of either under or over frequency. Over  or under frequency periodicity must be made good to restore the overal annual system average to 50Hz.

Conclusions:  The combined GW hourly rating pattern of available generating plant in the system must match that of Demand over the whole year. (Particularly see GW of wind turbines for a given GWh output over the whole year.)

2. Watt-hours  An international* unit measuring the amount of electricity in demanded or generated over a given time, in practice most often annually.

This unit is most usefully applied to the amount of saleable electricity (i.e. net of losses ) demanded or generated per annum as it relates directly to the overall economics of the electricity system.

3. Load Factor An international* unit normally expressed as the following annual ratio ( or as a  percentage):-

watt-hours actually demanded or generated per annum

watt-hours demanded or generated per annum  running continuously at full load  


4. Base Load  An international* term for the percentage of  annual peak electricity demand GW below which in any given year demand never falls.

For  the UK mainland public electricity system of which National Grid Electricity Transmission plc has, since since April 2005,  been System Operator, the Base Load has been approximately 40 per cent and the corresponding amount of annual GWh has been approximately 55 per cent of the total annual GWh electricity demanded by or generated for that system.