TUTORIAL PROBLEMS 01
1. A consumer has a maximum demand
of 100 MW at 60% load factor. If the tariff is Rs 20 per kW of
maximum demand plus 1 paise per kWh, find the overall cost per kWh. [1·38 paise]
2. The maximum demand of a consumer is 25A at 220 V and his total
energy consumption is 9750 kWh. If
energy is charged at the rate of 20 paise per kWh for 500 hours use of maximum
demand plus 5 paise per
unit for all additional units, estimate his annual bill and the equivalent flat
rate. [Rs 900 ; 9·2 paise]
3. A consumer has an annual consumption of 2 × 105 units.
The tariff is Rs 50 per kW of maximum demand
plus 10 paise per kWh.
(i) Find the annual bill and the
overall cost per kWh if the load factor is 35%.
(ii) What is the overall cost per
kWh if the consumption were reduced by 25% with the same load factor ?
(iii) What is the overall cost per
kWh if the load factor were 25% with the same consumption as in (i) ?
[(i) Rs 23,400 ; 11·7 paise (ii) 11·7 paise (iii) 12·28 paise]
4. Daily load of an industry is 200 kW for the first hour, 150 kW
for the next seven hours, 50 kW for the next eight
hours, and 1 kW for the remaining time. If the tariff in force is Rs. 100 per kW of
maximum demand per annum
plus 5 paise per kWh, find the annual bill. [Rs 50,258·5]
5. A consumer requires one million units per year and his annual
load factor is 50%. The tariff in force is
Rs. 120 per kW per annum plus 5 paise per unit consumed. Estimate the saving in
his energy costs if he
improves the load factor to 100%. [Rs 13,692
6. An industrial undertaking has a connected load of 100 kW. The
maximum demand is 80 kW. On an
average, each machine works for 60 per cent time. Find the yearly expenditure
on electricity if the
tariff is
Rs 10,000 + Rs 1000 per kW of maximum demand per year + Re 1 per kWh. [Rs 615600]
TUTORIAL PROBLEMS 02
1. An industrial consumer has a maximum demand of 120 kW and maintains a load factor of 80%. The tariffin force is Rs. 60 per kVA of maximum demand plus 8 paise per unit. If the average p.f. is 0·8 lagging,
calculate the total energy consumed per annum and the annual bill. [8,40,960 kWh ; Rs 76,276·8]
2. A customer is offered power at Rs 50 per annum per kVA of
maximum demand plus 5 paise per unit. He
proposes to install a motor to carry his estimated maximum demand of 300 b.h.p.
(metric). The motor
available has a power factor of 0·83 at full load. How many units will be
required at 30% load factor and
what will be the annual bill ? The motor efficiency is 90%. [6,44,307 ; Rs 46,985·35]
3. A factory has a maximum load of 300 kW at 0·72 p.f. lagging with
an annual consumption of 40,000 units.
The tariff in force is Rs 4·5 per kVA of maximum demand plus 2 paise per unit.
Calculate the flat rate of
energy consumption. What will be the annual saving if p.f. is raised to unity ?
[4·69 paise ; Rs 525]
4. The monthly readings of a consumer’s meter are under ;
Maximum demand = 60 kW
Energy consumed = 24,000 kWh
Reactive energy = 15,600 kVAR
If the tariff is Rs 20 per kW of maximum demand plus 3 paise per unit plus 0·1
paise per unit for each 1%
power factor below 85%, calculate the monthly bill of the consumer. [Rs 1960·4]
5. Compare the annual cost of power supply to a factory having a
maximum demand of 500 kW and a load
factor of 40% by having the supply from :
(i) the factory’s own diesel
generating plant.
(ii) a public supply.
With regards to (i), the capital cost of factory’s own generating plant is Rs 8
lakhs, cost of fuel oil is Rs 200
per ton, fuel consumption 0·65 lbs per kWh. Capital charges, cost of repairs
and maintenance, interest and
depreciation 15% of the total capital cost. Salaries and wages of the operating
staff are Rs 15,000 per year.
With regards to (ii), the tariff is Rs 150 per kW per annum of maximum demand plus
2·5 paise per kWh.
Which of the two alternatives is favourable for the operation of the factory ?
[(i) 13·5 paise/unit (ii) 6·8 paise/unit]
6. An industrial load can be supplied from the following
alternative tariffs :
(i) High voltage supply at Rs 65
per kW per annum plus 3 paise per kWh.
(ii) Low voltage supply at Rs 65
per kW per annum plus 3·3 paise per kWh.
The high voltage equipment costs Rs 50 per kW and the losses can be taken as
3%. Interest and
depreciation charges are 15% per annum. If there are 40 working weeks in a
year, find working hours per
week above which high voltage supply is cheaper. [55·42 hours/week]
7. A supply
company offers the following alternative tariffs :
(i) Standing charges of Rs 75 per
annum plus 3 paise/kWh.
(ii) first 300 kWh at 20 paise/kWh
; and additional energy at 5 paise/kWh.
If the annual consumption is 1800 kWh, which tariff is more economical and by
how much ?
[Tariff (i) is economical by Rs 6 per annum]
8. A factory has a
maximum demand of 500 kW, the load factor being 60% during working hours. The
following two tariffs are available :
(i) Rs 8 per kW of maximum demand
plus 3 paise per kWh.
(ii) a flat rate of Re 0·1/kWh.
Determine the working hours per week above which tariff (i) will be cheaper. [44 hours/week]
