Print Friendly and PDF

Are you ready for load shedding?

Power outages are about to become a way of life in South Africa and many homeowners are looking at the option to buy a backup or emergency generator for when the lights - and power - goes out. But what do you need to look for when buying a generator and what type of generator is the right one for you?


We looked at articles across the web and found this one from Turner Morris to be the most comprehensive. While on the technical side, once read you will be more informed on asking the right questions and making an informed decision.

There are certain steps to take when selecting a generator that is right for your needs, but first and foremost you need to determine the size of generator required to power your home during power outages.





What size generator?

Below is a chart that lists various appliances in a home. For the purpose of this article we are focusing on 240 volts, which is the standard voltage for most suburban homes.

Running Load Requirements

Running load requirements are simply a total of all the loads to be operated simultaneously. A way of determining the actual required size of a generator is to refer to the Main Circuit Breaker on your home distribution board. There could be more than one, but by simply adding up the total value of the circuit breakers you will be able to calculate the total usage. This method will ensure a fully functioning supply of electricity to your home in the event of a power outage.

If you are not looking to power your entire home, an estimate of the total running load can be made by adding up the running wattages of all items to be powered at one time. Some appliances may not list the wattage on the nameplate, but may show the rated current in amps and voltage.  The wattage can estimated from these two figures by multiplying them together to get the watts. See formula below:

Watts   =   volts x amps OR Amps =   watts  / amps    

Typical Running and Starting Loads

The graph below shows typical values for the start up of a refrigerator, since most household appliances with motors use more current to start up than to run.

It is clear that your generator needs about 5 times the potential to start up a refrigerator without putting your other appliances as well as your fridge / freezer at risk.

Power Quality & Distortion

Perfectly pure AC power is a sine wave for both the voltage and current. Some types of generators and non-linear loads can alter this perfect sine wave. This distortion is called THD, or total harmonic distortion. If the distortion is severe enough, motors and transformers will operate hotter and over a long period of time this can cause a reduction in life. And some other sensitive electronic equipment may not operate.  An specific example is uninterruptible power supply (UPS) systems powering computers or communications equipment. These types of devices cause some distortion of the AC wave and at the same time can be negatively affected by it. A UPS system powered by an inadequately sized backup generator may continue draining the internal battery rather than switching over to generator power and charging the battery.

To reduce chances for THD problems, the rule of thumb is to select a backup generator kW size at least three times the kW of non-linear loads to be powered. For example, if you have 2000 watts of computers fed by UPS systems and 1000 watts of incandescent lighting to be fed by a generator - first total the power:

2000 W + 1000 W = 3000 W

Then compare the total with three times the non-linear load portion:

3 x 2000 W = 6000 W

The generator needs to be at least 6000 watts in accordance with this rule of thumb.


The following installation methods are available when you want to connect a generator to the main electrical supply in a home and this installation is best performed by a certified electrician:                                                                                                

  1. Automatic transfer swtch (ATS) with UPS (Uninterrupted Power Supply) has no delay before the power is restored. It switches your normal power supply off and your generator on, without you leaving your chair as you watch TV. 
  2. ATS Panel only: this has approximately 15 Sec delay before the power is restored. It switches your normal power supply off and your generator on without you leaving your chair as you watch TV. 
  3. MMF Panel (Manuel Mains Failure) will switch your power supply off and requires that you leave your chair and go to the generator to start it up.                                                                             
  4. Manual change over switch – This requires that you switch the sth electric supply from your main supply to the generator and start the generator yourself. Electric start is optional in this case.  
  5. Direct method. – this is the simplest method. The appliance is plugged into the generator directly via and extension. (Beware not to use a too long or too thin extension cord). In this case just the cost of the generator is what you need to out lay.
  6. Always make sure that the generator is started first and that the power is switched on when the generator is running. Also switch the power off before your generator is restarted. This will eliminate spikes and will protect your appliances.

Petrol  vs  Diesel  fuelled generator

For certain applications a diesel set is a much better option:

a) Longevity - Think of all the18 wheeler trucks capable of 1,000,000 Kilometres of operation before major repairs. Most trucks are diesel powered.

b) Lower fuel costs - Diesel uses less fuel consumption per kilowatt (kW) produced).

c) Lower maintenance costs - diesel have fewer parts. no spark system, more rugged and more reliable engine.

Overall operating costs are typically thirty to fifty percent less for diesel than gasoline units.

Diesel engines running at 1500 RPM and water cooled operate on average for 12,000 to 30,000 hours before major maintenance is required.

Petrol engines running at 3000 rpm and air cooled are normally replaced – not overhauled at 500 to 1500 hours.

Petrol units run hotter due to the higher BTU rating of the fuel, you will therefore see significantly shorter gasoline engine life. Diesels run cooler and last longer.

To summarize:

Use a petrol unit when:

  • Price is important
  • The unit will be used for standby purpose only
  • The unit will be running for 6 hours or less a day
  • Portability is important

Use a Diesel Generator when:

  • The generator is used for prime power supply
  • The unit is used for any mining or similar application
  • The unit needs to be built into a silent enclosure
  • The unit will be used daily for more than 6 hours per day
  • You need a true 15 Kva or larger.

Installing a generator

  1. Use a local electrical contractor to do your installation. Ask for a wiring certificate.
  2. Try a number of contractors for quotes and advice to confirm that the price is reasonable.
  3. Remember that if the contractor does not do the job properly the generator can burn out and no company will warrantee the product.

Tips and hints for maintaining a generator

  1. Refueling – NEVER refuel a petrol generator when the engine is running. A spark from the spark plug can ignite the fuel and start a fire.
  2. Starting up the generator – ALWAYS make sure that the appliances or power tools are switched off before starting up the generator. When the revs have settled to a constant level it is then the time to switch on the appliances.
  3. Extensions – Do not use an extension which is longer than 15 m without increasing the diameter of the cable so as to avoid voltage drops which destroy appliances and tools.
  4. Fuel – NEVER run out of fuel – Always refuel before the tank is empty. (remember to switch off) Just before cutting out, when running out of fuel, the alternator does not produce the correct voltage and can burn out your appliances or tools.
  5. Misconception – generator power is not as good as Eskom power. The power from a generator is as good if not more constant as that of the locally supplied power. Most reputable alternator suppliers offer alternators with less than 5% harmonic distortion. The problem could come in during start up or switch off. This is the time when your generator puts out an unstable current. An AVR (automatic voltage regulator) could eliminate this problem.


back to top