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generator sizing...

The following steps are a guideline to assist you in establishing the optimum size generator for your use.


To correctly size a generator for home or business use, you will need to determine your power requirements by calculating the load (‘load’ is the current required to run your appliances) of your home or business. This can be achieved in one of three ways.


The first and easiest way to measure the load is to use a ‘tong tester’ with a digital readout. A tong tester is a tool used to measure ac/dc current in amps. The jaws of the tong tester are normally spring loaded and can be placed around the live cable of the mains supply (between the meter and distribution board). With the tong tester in place, you should now turn on every appliance you wish to power with the generator. If you only require certain circuits to be powered by the generator, isolate them by turning off the circuits you will not require at the distribution board. Once all appliances are turned on and your home or business is at full load, the tong tester will display the current drawn (in amps). Make a note of the amperage reading displayed on the tong tester.


Generators are sized in kilowatts, so you will now need to convert your amperage reading into kilowatts using the following formula; Volts x Amps x Power Factor = Watts.


In Thailand the domestic single phase supply is 220v ac. Power factor involves the efficiency in the use of the electricity supplied to the system. For most generators calculations we use a power factor of 0.80, for example; if your amperage reading was 80amps on full load, the calculation would read as follows:


Volts (220v ac) x Amps (80amps) x Power Factor (0.80) = Watts (14,080)


Given this example, the load requirement for your home or business is 14 kilowatts.


Diesel powered generators operate most efficiently when they are running at 65 to 80% of their maximum rated power output, this also allows for future growth in power demand. A 14Kw load is approx 80% of the maximum load rating of a 17Kw (21Kva) generator which is therefore the optimum size generator to power your home or business (to calculate Kva from Kw, the formula is; Kw / 0.80 = the value in Kva).



The second method, if your home or business is supplied with a traditional style kilowatt hour meter with a rotating disc, is to calculate the amps used by using a stopwatch as the disc rotates.


If you have a meter with a rotating disc, first read the constant value on the face of the nameplate (shown as Kh). This value is the number of watt-hours equivalent to one rotation of the disk.

Now, start the desired appliances, heating or air conditioning as in the previous example with the tong tester.

Using a stopwatch while watching for the black mark on the meter's disk, measure the time it takes for one or more disk rotations. If the disk is rotating rapidly, better accuracy will be attained if you time more than one rotation.

Finally, take the three values and use the equation below to calculate the watts seen by the electric meter.


     Kh     = meter constant

     Rev    = revolutions of disk used

     T       = total time from stopwatch for revolutions of disc


     Watts = Kh  x  Rev  x  3600



Here is an example calculation for a meter with a reading of Kh = 7.2 per disc revolution. The time measured for 5 rotations of the disk was 24 seconds. Thus, Rev = 5 and T = 24 seconds. Calculating the electrical demand as follows:


     Watts = Kh  x  Rev  x  3600                            T


              = ( 7.2 watt-hrs/rev )  x  ( 5 rev )  x  ( 3600 sec/hr)

                                            24 seconds


              = 5400 watts.



The third and final method is a simple calculation of the power requirement for each and every appliance to be placed on load:


· Identify the appliances the generator will power. - If the generator is to power all appliances in your home or business, make a list of every appliance. If only certain circuits are required, make a list of every appliance on those circuits.


· Determine the wattage for each appliance. – Determine the wattage for each resistive load on your list (toasters, coffee makers, light bulbs, convection ovens, televisions etc are examples of resistive loads). Typically, resistive loads do not contain an electric motor. If the wattage does not appear on the nameplate of an appliance, a rough guide of wattage ratings for common appliances can be found here.


· Identify electric motors/pumps the generator will power. – Identify pumps and motors on the circuits to be powered by the generator.


· Determine the wattage for each motor/pump. – Determine the wattage for each reactive load on your list (air conditioners, refrigerators, freezers, well pumps, air compressors etc are examples of reactive loads. Typically, reactive loads contain an electric motor). Reactive loads are little more complex. They can draw 2 to 4 times more power to start than they do to run. When calculating your power requirements for reactive loads, always use starting watts, not running watts. A rough guide to the starting watts for reactive loads can be found here.


· Total the wattage of appliances, motors/pumps to be powered. – Add the total wattage of all resistive and reactive loads on your lists to determine your power requirement.


· Convert watts to kilowatts. – Generators are sized in kilowatts. For your convenience a list of simple conversion formulae can be found here.

Please use common sense and be aware of the danger of electrocution at all times. If you are in any doubt, seek the advice of a qualified electrician.