Frequently ask Questions


It is not possible to answer this question, even if you are located exactly next door to a power station. It is similar to taking a bucket of water from the sea near an estuary and thinking from which river it has come.

All power stations generate and feed power to an interconnected mesh like structure we refer to as a ‘power grid’. Theses interconnections in the power grid (and thus the name) are made out of high voltage (220kilo Volts or 132 kilo Volts in Sri Lanka) transmission lines. They connect each and every power station in the country. The tall steel tower lines that you see on your wayside when you travel are these interconnections. At certain places within this grid, there are nodes we refer to as ‘grid substations’. They take power in the power grid, convert it to a lower voltage that you can use, and, distribute to your households. This transformation from a high voltage to a lower voltage too takes place in a number of steps. Thus, as all grid connected power stations are interconnected with each other, it is not possible to pin point exactly from which power station you get your electricity. However, as in the example of taking a bucket of water from the sea near a river estuary, if you are near a power plant, we can say that a major portion of the power flow to your household could be from the closest power plant.


As we know from our fundamental science, electricity is a form of energy. Thus, it can neither be created nor destroyed. Further, electricity in Alternating Current form ( AC in short) cannot be stored. Thus, whenever consumers demand certain energy within certain time duration, we have to generate that energy in real time and provide. In other words, the supply and demand of electrical energy must be the same in real time.

This balancing act is done by taking the frequency of the power supply (50 Hz in Sri Lanka) as a guide. For example, when consumers like you demand that they be supplied with 1000MJ (Mega Joules) of energy within any one second time slot, (1000MJ of energy in one second is equal to 1000MW of power), CEB has to provide exactly 1000MW. When we supply the exact amount of the demand, the frequency of supply will be stable (usually at the rated 50Hz). If our supply is less than your demand, the frequency will fall below 50Hz and if we supply more, the same will increase above 50Hz.

However, during certain emergencies, our power plants can get tripped (disconnected or automatically switched off). When a large power station (say 100MW) gets tripped in this manner, this supply- demand balancing act gets disturbed. Now, the consumers are demanding 1000MW but the supply is only 900MW. As a result, the frequency of the power supply starts to drop. (Why and how it happens and who maintains the supply-demand balance during that time is beyond this simple answer.) The engineers who are operating the power system has to act quickly and resupply the shortage. However, this takes some time. Until the supply-demand balance is restored, the frequency starts to fall below 50Hz.

It is not healthy for power plants to operate below a certain frequency. For example, most of the present grid connected power plants in our system cannot operate below 47Hz. Thus, if the low frequency situation is allowed to persist, after a certain time, the remaining power plants too start to get switched off thus aggravating the power shortage further. This could escalate in to a total cascaded failure.

However, all power plant trippings do not end up as total failures as there are certain automatic as well as manual corrective measures to restore the supply-demand balance following a disturbance.


A unit of electricity saved during the peak times has a higher saving potential than during other times. As described in the answer to FAQ2, CEB has to supply the exact electrical power demand of consumers 24×7. At midnight and early in the morning, the power demand from the consumers is very low. Around 2.00 am in the morning, it can fall to its lowest value within a day. During day time, it is somewhat higher and at about 7pm at night, the demand reaches a maximum. The latter is what we call the ‘Night Peak’.

CEB has to follow the power demand in the country by varying its generation accordingly, matching consumer demand MW to MW. During early morning and day time, CEB has a lot of generating options as the demand is much lower than the combined generating capability which is called the installed capacity of CEB. Thus, CEB can afford to generate from the cheapest sources without resorting to expensive ones. CEB starts generation from the cheaper plants first and goes on adding more and more expensive generations as the demand goes up. This is called ‘Merit Order Dispatch’. When the demand drops, reverse activity takes place and CEB reduces generation starting from the most expensive.

During the Night Peak, CEB is running almost all its generators, including the very expensive Gas Turbine Units. The cost of generating a unit from a Gas Turbine can be as much as 10 times that of generating a unit from a Coal Power Plant. Thus, when you switch off a bulb at night, CEB reduces their generation too from the most expensive power plant that is generating at the time. As a result, at night time, a unit saved by you saves more money to the country than during the rest of the time.

There is another reason why the CEB asks you to avoid the night peak. The generators that CEB uses to provide the peak demand called the peaking units, are operated only for a few hours of the day. They remain shut down during the rest of the time. Thus, it is idling investment in a way. If consumers help CEB by curtailing the electricity use during the Night Peak, it not only helps CEB to stop generation from an expensive plant, but also helps to avoid the investment of a complete peaking unit in the future.

We measure electrical energy using kWh. kWh stands for a kilo Watt Hour. Similar to Joules (J) or Horse Power (HP), kWh is a unit of measuring Energy. Watts is the unit of measuring power. A kilo Watt is 1000 Watts and a kilo Watt hour is 1000 Watt Hours.

The difference between Energy and Power can be understood by the following analogy.

Imagine a water tank containing 1000 Liters of water. You have a large tap which can vary its flow. When you fully open the tap, you get water at a higher rate. (say 1Litre every second). The volume of water stored in the tank (in Liters) is analogues to energy (in kWh). The rate at which water is drawn from the tank (Liters per second) is analogues to power (kW). When we draw one kW of power for one hour we have actually consumed one kWh of energy.

If we have a larger capacity tap (or a power plant), we can provide more water (or electricity) to more consumers at the same time. However, what they actually consume is the number of Liters of water (or kWh of electricity). The water flow rate (or power) will decide how fast they consume or the rate at which they consume in Liters every second (or kW).

There is a common misconception among domestic consumers that due to delays in taking the meter readings, consumers are unfairly penalized and are compelled to pay a higher charge than normal as their consumption extends to next higher block. This is incorrect as explained below.

A meter reader is normally expected to visit a premise in 30 day cycles. That is why each tariff block has 30 units. However, it is not practically possible for the meter reader to always visit in 30 days. Thus, to account for each day the meter reader is delayed or is early, the block size in each tariff category is prorated on the basis of the number of days between the arrivals of the meter reader. For example, if the period between two meter readings is 35 days, the limit of the first block would be increased from 30 to 35 kWh, the limit for second block would be increased from 60 to 70 kWh and so forth. This will account for any disadvantage the consumer would have in the increasing block tariff structure.

These computations are made in advance and furnished to the meter readers in the form of a ‘Ready Reckoner’, and thereby the correct charge depending on the interval between the visits and the units consumed is readily available to the meter reader. The entries made by the meter reader would be subjected to verification at the time of data entry, and any error would be automatically corrected in the next bill.

The consumers could verify their bills by using the Bill Calculator provided in CEB website.