Selecting a suitable generator for your application- Part 2

Last time,on my post “Selecting a suitable generator for your application – PART 1”,I’m promised to continue discussion about selecting a suitable generator for your application.In this post, I explained details and shared some tips to get a suitable generator for your application.

What’s this THD?

For ensuring smooth and continuous running of generator it is important to measure its THD –total harmonic distortion. When a significant portion of the load on a generator (or any power source for that matter) is non-linear, all the loads fed by the source will see this distortion.

Example: An UPS system, supplying power to computers or communication equipment, if it is powered by an inadequately sized backup generator then it may continue draining the internal battery instead of switching over to the power of generator and charging the battery. Such a situation can be avoided by selecting a generator at least three times the kW of non-linear loads to be powered.

Points to remember

  • Voltage check: Choose a generator with 240 volt output if there is a need to power 240 volt load. Most appliances operate on 240 volts.
  • Running Load Requirement: Cumulative of all loads that need to be operated together with generator. Take an electrician’s help to find this.
  • Estimate of total running load : Can be calculated by adding up running wattages of all items you want to power at one time.
  •  If there are no wattage mentioned on the appliance nameplate and only rated current in AMPS and voltage is mentioned then wattage can be calculated by multiplying the two:

    Watts   =   volts x amps


    Amps =   watts / amps

How to find out starting load requirements?

This may be a bit ticklish exercise: Some electrical devices like motors require extra power to get started. Same is the case with regard to fan, pump, compressor, saw, etc. This is because these standstill rotating parts need more energy to accelerate.

Basic Physics: Most engine driven generators start a motor with up to 1/5th the horsepower of the engine, if it is the first load connected.

Example: A 2500 watt generator driven by a 5 horsepower engine can start up to a 1 hp electric motor.



What’s running and starting loads?

A freezer or a refrigerator uses more power at the time of starting it than when it is actually running. So in such cases it is advisable not to go by spec-plate to try and match the generator.

The generator will require approximately five times the potential of freezer or refrigerator to start them safely.

Calculate to avoid risks:

  • 1000 Watt = 1 KW = 1 Kva. True for all sets less than 10 Kva. Operate at a power factor of 1.0.
  • 10 Kva generators have a power factor of 0,8. A 20 Kva generator will give 16000 Watt = 16 KW.
  • To find out amount of Amps from a generator: Wattage divided by voltage. E.g.:  2500 Watt    /    220 V   =   11 Amp
  • On 3-Phase unit (380 V) need to take the output. Example: 15 KVa. 15000 Watt x 0,8 (Power Factor) = 12000 Watt ( 12 KW). Divide this by 380 V = 32 Amps. Take 32 Amps and divide this by 1.732 (Root to the 3rd) = 18.23 Amps. This = Amps from a 15 Kva unit per phase.
  • De-rating of generator engine: 1 per cent for every 100 meter above sea level. Done to get actual performance at the point of use.
  • In case of diesel run generators it is better not to have one unless there is a minimum requirement of 75 per cent of its total potential load. Running the generator below its potential load may damage it, needing costly repair works.
  • Avoid refuelling a running generator.
  • Switch off appliances before starting up the generator.
  • Avoid using a wire longer than 15 meters without increasing its diameter to avoid voltage drops that might harm appliances and tools.
  • Fill the fuel tank after switching off .
  • An AVR (automatic voltage regulator) can prevent likely damage to generator due to unstable current at the time of switching on and switching off.

Finally, which is best: a petrol or diesel run generator?

  • Diesel generators use less fuel per kilowatt power it generates
  • Maintenance cost is minimal: no spark system, rough and tough, more reliable engine
  • Operating cost nearly 30 to 40 per cent less than petrol run generators
  • Running at 1500 RPM and water cooled, diesel generators operate on an average for 12,000 to 30,000 hours before major maintenance is required.
  • Petrol engines running at 3000 rpm and air cooled are generally replaced – not overhauled at 500 to 1500 hours.



  1. Good Information.

  2. Hey
    Informative article. Well, generator is very useful device for our domestic as well as industrial purpose.

  3. In selecting generator application, you really have to make sure that it really suits what you need. Good thing you are able to discuss and explain here what should be done better regarding it.

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