calculate_power_requirement.pdf

7/27/2019 calculate_power_requirement.pdf

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Calculate your power requirements

Before going to purchase an inverter- You have to make a rough estimation of your power

requirements. remember- Inverter is not a Generator. Inverter has its own limitations. You cantpower and run your complete home with a normal inverter. If your power requirement is more

than 3000 VA, then an inverter alone cant cater your demands effectively. No doubt a highpower inverter can run your refrigerator and air conditioners, but how long? Here your batterywill not last more than few hours.

Watt age rat ing of typical home appliances

Appliances Power

Fan (Ceiling) 50-75 Watts

Fan (Table) 25-50 Watts

CFL 18 Watts 18 Watts

Computer (Desktop) 80- 150 Watts

Laptop 20- 75 Watts

LCD TV (32) 150 Watts

Refrigerator 160 250 Watts

Tube Light (4 Feet) 40 Watts

Star ting power and r unning power

Starting and running power requirement of all electric appliances are different. An 18 Watts CFL

takes around 25 Watts power to start and after few seconds it stabilizes to 18 Watts. Someappliances like Refrigerator, Washing Machine etc take almost double power to start ascompared to the normal running power. Hence before selecting an inverter you must calculatethe starting power requirement also.

Difference between VA and Watts

Often people confuse with the difference between Watts and VA ratings of inverter or other

home appliances. Manufacturers use both these terms according to their requirements to confuse

and mislead the consumers. Power drawn by the equipment (Power using equipment like Bulb,


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Fridge, Washing Machines, ACs, Stabilizers etc.) and Power generated by the equipment(Generators, Inverters, UPS etc) can be expressed in Watts or VA (Volt Ampere). In general

Power generating or stabilizing equipment manufacturers use the term VA to specify theirratings and power consuming products use Watts to specify their ratings. Here comes the

difference. Power in Watts is the real power drawn or supplied by an equipment, this is the real

value. VA is the apparent power (more theoretical) which is obtained by multiplying the Voltageand Current drawn/supplied by an equipment. We can equate the VA and Watts ratings:

Power in Watts = Power in VA- Losses

or

Power in Watts = Power in VA x Efficiency of the equipment

The ratio of Power in Watts to Power in VA is termed as Power factor. Unfortunately

manufacturers often dont specify the power factor or efficiency in their products.

Lets see an example. You are purchasing an inverter with 650 VA power ratings. This 650 VA

indicates, it can supply 650 VA power under ideal situations( means efficiency of the inverter is100%). If we take 80% as a typical efficiency figure, the said inverter can supply 80% of 650 =520 Watts only. Now you can understand why inverter manufacturers only publish the VA rating

on their products. If an inverter or UPS is coming without mentioning power factor, you musttake a safe value of 0.6 (60%) as power factor while connecting appliances.

Inverter or Home UPS?

Nowadays we can see a new segment in inverter market- Home UPS. Functionally both inverter

and UPS do the same job- Converting Direct Current from batter to Alternating Current which isused as a backup power generating device in case of mains power failure. UPS as the name

suggest provide Uninterrupted Power Supply to equipment. The equipment will not feel theswitching delay from mains to UPS supply. Switching over time of a typical UPS is 3 to 5 Milli

Seconds only. A standard inverter takes around 500 Milli seconds which is unacceptable for theelectronics devices like computers, Modems, Routers, printers etc.

Home UPS is a combination of Inverter and UPS. Here the switching delay of inverter has been

brought down to UPS standard. Hence you need not to go for a separate UPS for PCs andassociated devices. One thing you must remember- UPSs are further classified in to three-

Offline, Online and Line interactive. Home UPS are normally Offline only, hence they cannot

replace your online UPS requirements to drive sophisticated devices.


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Digital Inver ter

What is digital in inverters? First generation inverters were using power transistors as switching

devices and transistorized circuitry for monitoring/controlling applications. That is an old story.Nowadays inverter technology has changed a lot. A complete transition from old square wave

technology to Pure Sine wave technology with the help of advanced Digital Signal Processing(DSP) techniques. Micro controllers are extensively utilized for prcised monitoring andcontrolling applications. Switching transistors are replaced with high efficient MOSFETs.

Battery

Batter- the vital part of inverter. Performance and life of an inverter is greatly depends upon

battery. We can see several classifications of inverter batteries. Normally high power Lead Acidbatteries are used to power inverters. Lead acid batteries are classified in to different types as per

application, design, technology etc.

Battery Capacity

Capacity of a battery is expressed in terms of Ampere Hour (Ah). It indicates the rate of current abattery can supply for a given duration. If the capacity of a battery is 100 Ah, that battery can

supply 100 Ampere current for 1 Hour or 1 Ampere Current for 100 Hrs, 2 Amps Current for 50Hrs .Capacity of inverter batteries are generally 100 Ah, 150 Ah or 180 Ah.

Why automotive batt eries ar e not suitable for inver ters?

Lead acid batteries used for automotive purpose are termed as High Cycle lead acid batteries.

These batteries are designed to provide high current for a very short duration (To start thevehicles). Automotive lead acid batteries are not designed to be regularly discharged by morethan 25% of their rated capacity. Here the requirement of inverter is totally different. Inverter

require Deep Cycle type batteries to provide continuous power which can be discharged atleast 50% of their rated capacity. Some good deep cycle batteries can be discharged over 80% of

their capacity. Deep Cycle batteries have specially designed thick plates to withstand frequentcharging and discharging.

Tubular batteries

This is the most popular segment of inverter batteries-Tubular batteries have several advantages

compared to flat plate batteries. More complex design increases the manufacture cost but thehigh electrical efficiency and extra long life make tubular batteries suitable for both domestic and


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industrial applications. A good maintained tubular battery can run unto 8 years or more. Tubularbatteries comparatively require less maintenance. Cost of tubular batteries can go up to double of

a normal flat plate battery, however if you have a high efficiency inverter then go for a tubularbattery, you will not regret. Almost all popular battery brands in India like LUMINOUS,

MTEK,SF have specially designed tubular batteries for inverters.

War ranty/Guar antee of Bat tery

First of all understand the difference between Warranty and Guarantee- Under the terms of

Warranty you will get the maintenance and repair of items in free of cost but no replacement. Aproduct under Guarantee will be get replaced in case of malfunctions. Normally we can see

batteries are marketed under guarantee + warrantee combination , ie first few years Guaranteeand after that warranty. Some brands offer even 48 months warranty. As battery is the costly and

vital part of your inverter system, go for a brand which offer long term guarantee/warranty andafter sales support.

Inverter output (Sine wave, Square wave, Stepped Sinewave)


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Our commercial power supply is 50 Hz pure sine wave (see the figure). However most of theinverters dont output pure sine wave. Old inverters generate 50Hz square wave or 50Hz

modified Sine wave (Quasi Sine wave , Trapezoidal wave or Stepped Sine Wave). Then thequestion comes. This wave shape really matters? Yes it does.

All commercial instruments are designed to run on pure sine wave. Characteristics of suchdevices are greatly depend upon the input wave shape. A change in wave shape will affect the

performance of the appliances. Appliances like Motors, refrigerators, Ovens etc will generate fullpower on pure sine wave input only. Distortion in the sine wave creates humming noise intransformers, and audio devices. Some instruments like light dimmers, few battery chargers,

toasters etc require pure sine wave to work. Prolonged operation of appliances in Square orstepped waves will considerably affect the life of such equipment due to the generation of heat.

You may be noticed that audio amplifiers, Televisions, Fluorescent lamps etc make noise oninverter power. This indicates that inverter output is not pure sine wave. Sine wave inverters arecostly as compared to square wave and stepped sine wave inverters, however it is always

advisable to go for a pure sine wave inverter for the safety and effective performance of yourappliances.

Square Wave Pure Sine

Wave

Safety of

Appliances

Less High

Life ofAppliances

Less High

Battery Life Less High

Noise Level High Normal

Heat

generation

High Normal

Suitabilityforsophisticated

appliances

No Yes

Inver ter efficiency and No load power consumption

Efficiency of an inverter is proportional to the amount of power drawn. It can varies from 90% to

50% from full load to light load. Here the notable factor is that inverter efficiency is high onmaximum load. Generally an inverter is in its most efficient capacity when being used at around1/3 to 3/4 of its full rating.


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Power Consumption of inverters

Inverters take mains power supply to charge the batteries. If the charging section of an inverter is

not energy efficient, it will waste electricity even after the battery is fully charged. This willinflate your electricity bill. It is very difficult to check this aspect in shop before purchasing an

inverter. However you can test the same at your home. Switch off all appliances except the mainssupply to inverter, disconnect the battery and measure the power consumption for a specific

period.

Battery backup duration

Here we have an equation to find the battery life on load

Time(in Hr s) = Batt ery Voltage (In volts) * Battery Capacity (in Ah)*efficiency of invert er

Load (in VA)

Let us have a sample calculation

Battery voltage = 12 Volts

Battery Capacity = 150 Ah

Load = Two fluorescent lamps (40 Watts each) and one ceiling fan (75 Watts )= 155 Watts

Inverter efficiency = 80%= 0.8

Backup Time = 12 * 150*0.8/155= 9.29 Hrs (Approx)

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