Tuesday, September 10, 2013

Calculating power factor from electricity meter reading

Power factor, PF,  in the electrical installation can tell us about how efficient the plant, factory, building, or installation load is operating at.

Low power factor cost money and  in most cases is against the law. In Malaysia, TNB/SESB set a minimum power factor of 0.85 that every consumer must achieve and if yours falls below this value, you will be charged with power factor penalty in a staggered manner.

Calculating power factor without taking your toolbox with you

kVA2 = kW2 = kVAR2

From this equation, we derive the power factor:

In the case of SESB, a subsidiary of Tenaga Nasional Berhad, the power factor penalty is calculated as follows:-

If the Average Power Factor of any consumer in any month is found to be:

(a) Below 0.85  and up to 0.75 lagging, a supplementary charge of 1.5% of the bill for that month for each one-hundredth part (0.01) below 0.85 will be added to the bill for that month; and

(b) Below 0.75 lagging, in addition to the charge above, a supplementary charge of 3% of the bill for that month for each one-hundredth part (0.01) below 0.75 will be added to the bill for that month.


Say, a premise is categorised under Commercial Tariff Class 1, CM1, and billing meters show the followings during January 2013 monthly meter reading:

kWh meter     : 2,660
kVARh meter  : 2,190

The average power factor for that month, according to the formula is

PF  = kWh / [SQRT(kWh2 + kVARh2)]
       = 2,660 / [SQRT(2,6602 + 2,1902)]
       = 0.77

which is below 0.85 but above 0.75, and hence a PF penalty of 1.5% supplementary charge for every 0.01 part applies.

Supplementary charge  =  100*(0.85 - 0.77)  x  1.5% x  Monthly bill
 =  8 x 1.5% x RM890.10
 =  RM106.81

Calculate monthly bill
First 200 kWh  =  200 x 33.0 cents     = RM66.00
Next 2,460 kW  =  2,460 x 33.5 cents  = RM824.10
Hence, monthly bill  =  RM890.10
Total bill  =  RM996.91

The PF penalty is about 12% of the monthly bill and for a larger installation/premise, this could be a lot of money. Plus, if you continue to register PF of below 0.75, the utility company has a provision to stop supplying power to your premise until you rectify the situation.

Reason for posting this? I'm putting this formula up here for ease of future reference. I can refer to it from my mobile device, too. Also, this maybe useful for the accountants and administrators who receive the bills and in need of quick answer on how/why they were penalised.


thomas said...

i always wonder why a lower PF will need extra charge?

de engineur said...

Because low PF equipment use up more magnetising current or 'reactive power'

Jennifier said...

Calculating electricity bills from meter reading is always a daunting task because typical calculations also you can not control you power consumption in this particular type of payment process. For total control over your consumption and electricity bill you can rely on prepaid meters which gives you proper control over your money and electricity at the same time.

de engineur said...

This calculation is a quick check on finding out if your are charged for low power factor penalty.
Applies only to Commercial and industrial tariffs.
-Domestic consumers are excluded

Mohd Khairuzzaman Bin Mohamad Zamani said...

First of all, sorry for replying such an old post. But for the sake of knowledge, I would like to share it. In electrical power transmission, there are 2 wiring method used. They are Single Phase and Three Phase. Usually, SINGLE PHASE supply are used for domestic usage while THREE PHASE supply are used by commercial, industrial consumers as well as big houses. In THREE PHASE supply, there are 3 quantities of power which need to be considered which are REAL POWER, REACTIVE POWER and APPARENT POWER. Real Power is the power which is consumed by electrical appliances in unit of Watt. Reactive power are power due to reactive components such as inductive elements (inductors or capacitors). Apparent Power is the combination of real power and reactive power.

Apparent power is calculated by using this formula:

Apparent power = sqrt((real power)^2 + (reactive power)^2)

Apparent power is used in determining the sizing of electrical power transmission devices such as transformers. High apparent power which is usually caused by high reactive power, causes the electric company (TNB or SESB) need to install a bigger transformer.

Power factor is defined as the ratio of real power over the apparent power. Power factor can have a value in the range from 0 to 1. Higher power factor value is always desirable. In Malaysia, the power factor should be higher than 0.85 lagging. Low power factor is caused by high apparent power, hence reduce the efficiency of electrical power transmission and increase operation cost.

Simply said, having a low power factor is like you buying an express bus and you are using it to go to your office. Such a waste right? Same concept are applied to electrical system.

However, domestic consumers are not obliged to the power factor.

Hope that my sharing helps to understand more. :)

de engineur said...

@Mohd Khairuzzaman - thanks for commenting, that's definitely an invaluable insight.


Looking back at the post, I think I need to do another one to update on the electricity tariff in malaysia. But generally, the formula to calculate p.f. for any given electricity meter reading stays.

Airellchenko Milan said...

Because tranformer uses extra power for same amount of load. So extra power means that extra maintenance for transformer..example a car and a mtorcycle use a diffrent amount of fuel with same destination

Anonymous said...

Because tranformer uses extra power for same amount of load. So extra power means that extra maintenance for transformer..example a car and a mtorcycle use a diffrent amount of fuel with same destination

Mukesh Kapoor said...

Want to know both lagging and leading PF conditions liable to be charged extra as PF penalty.

de engineur said...

@Mukesh Kapoor - in terms of penalty, leading pf is not an issue. You get penalised when the pf is lagging below certain threshold, e.g. in Malaysia consumers (except domestic) that register pf of below 0.85 get penalised. Other countries may impose more stringent requirement.