The variable load problem has introduced the following terms and factors in power plant engineering:
(i) Connected load: –
It is the sum of continuous ratings of all the equipment’s connected tosupply system. A power station supplies load to thousands of consumers. Each consumer has certain equipment installed in his premises.
The sum of the continuous ratings of all the equipments in the consumer’s premises is the “connected load” of the consumer.
For instance, if a consumer has connections of five 100-watt lamps and a power point of 500 watts, then connected load of the consumer is 5 *100 + 500= 1000 watts. The sum of the connected loads of all the consumers is the connected load to the power station.
(ii) Maximum demand:
It is the greatest demand of load on the power station during agiven period.The load on the power station varies from timeto time.
The maximum of all the demands thathave occurred during a given period (say a day) is the “maximum demand”.
| Maximum demand is generally less than the connected load because all the consumers do not switch on their connected load to the system at a time. |
The knowledge of maximum demand is very important as it helps in determining the installed capacity of the station. The stationmust be capable of meeting the maximum demand.
(iii) Demand factor:
It is the ratio of maximum demand on the power station to its connected load i.e.
| The value of demand factor is usually less than 1. |
It is expected because maximum demand on the power station is generally less than the connected load. If the maximum demand on the power station is 80 MW and the connected load is 100 MW, then demand factor = 80/100 = 0·8. The knowledge of demand factor is vital in determining the capacity of the plant equipment.
(iv) Average load:
The average of loads occurring on the power station in a given period (day or month or year) is known as average load or average demand.
Daily average load = 
Monthly average load = 
Yearly average load = 
(v) Load factor:
The ratio of average load to the maximum demand during a given period isknown as load factor i.e.
Load factor = 
If the plant is in operation for T hours,
Load factor = 
| Load factor is always less than 1 because average load is smaller than the maximum demand. |
The load factor may be daily load factor, monthly load factor or annual load factor if the timeperiod considered is a day or month or year. The load factor plays key role in determining the overall costper unit generated. Higher the load factor of the power station, lesserwill be the cost per unitgenerated.
(vi) Diversity factor:
The ratio of the sum of individual maximum demands to the maximumdemand on power station is known as diversity factor i.e.
Diversity factor = 
| Diversity factor will always be greater than 1. |
A power station supplies load to various types of consumers whose maximum demands generallydo not occur at the same time. Therefore, the maximum demand on the power station is always lessthan the sum of individual maximum demands of the consumers. Obviously, the greater the diversity factor, the lesser is the cost of generation ofpower.
(vii) Plant capacity factor:
It is the ratio of actual energy produced to the maximum possibleenergy that could have been produced during a given period i.e.
Plant capacity factor = 
Thus if the considered period is one year,
Annual plant capacity factor =
The plant capacity factor is an indication of the reserve capacity of the plant. A power station isso designed that it has some reserve capacity for meeting the increased load demand in future. Therefore,the installed capacity of the plant is always somewhat greater than the maximum demand on the plant.
Reserve capacity = Plant capacity * Max. Demand
It is interesting to note that difference between load factor and plant capacity factor is an indication of reserve capacity. If the maximum demand on the plant is equal to the plant capacity, then load factor and plant capacity factor will have the same value. In such a case, the plant will have no reserve capacity.
(viii) Plant use factor:
It is ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation i.e
Plant use factor = 
Reference:
“Power system” by V.K. Mehta.