How to Size a Portable Generator for Home Use

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Portable Generators are a reliable source of power in the absence of utility power. They provide electrical power to supply our critical power needs when the utility company is unable to supply us electrical power due to fault in their transmission system or during maintenance interventions in their power infrastructure or in the worst case of a natural disaster such as earthquake or a hurricane that has destroyed section of the power grid.

What Size of Portable Generator Do I need ?
The size of portable generator you need depends on your power requirement when the need arises to use the generator. Do you require the generator to power all of your electrical appliances at once? Or do you require the generator to power some critical electrical load during power outage? The bigger your power needs, the bigger the size of your generator and the more expensive your portable generator will be!

Running Watts of an Electrical Appliance
The running watts of an electrical appliance is the power it can draw continuously with rated voltage and current. It is usually calculated as:

Running Watts = Rated Voltage x Rated Current.

Note that the above formula will give power in volts-amps or VA but assuming a power factor = 1 which is rarely the case, we get power in watts. This approximation is done to enable easy sizing of a portable generator for home use.

The running watt can easily be calculated by using the rated voltage and current on the name plate of the appliance. Generators are also rated for their running watts. It is the power the generator can deliver continuously at rated voltage, current and frequency. A generator must not be made to continuously carry load beyond its running watts for a very long time otherwise the generator’s life will be shortened and the device becomes damaged in a short time.

Surge Watts or Start up Power of an Electrical Appliance
Certain devices and appliances have an electric motor or compressor in them. They require additional watts to start them. This additional watt may also be referred to as the surge watt of the device. The surge watts required by these devices may sometimes be two or three times the watts required to run the device. Heat producing devices also called resistive loads such as light bulbs, toasters or coffee makers do not require surge watts at start up. A generator must have enough surge watts capacity to handle devices that require surge watts at start up to prevent a nuisance tripping of the main power breaker in the generator.
As shown above, a generator must have sufficient surge capacity to carry loads requiring additional power during start up. Consider a refrigerator that works for one third of the time within a given time cycle. Each time the refrigerator compressor starts, a generator powering the refrigerator must have sufficient surge power for the compressor each time it comes on!

How to Calculate the Size of Portable Generator Required
To properly size a generator, care should be taken to analyse the load the generator is to power so that both running watts and surge watts can be correctly calculated. To calculate the size of generator:

Add up the total running and surge watts for each appliance. Multiply the total sum gotten by a contingency of 15 – 20 % to get the capacity of your generator.

As a guide during the sizing calculation for domestic application;
Surge watts for refrigerators and air conditioners = 2 x running watts 
Surge watts for motors (surface or submersible pumps) = 3 x running watts
Microwave Oven = 1.5 x running watts

Sample Sizing Calculation
Suppose the following loads are to be powered by a portable generator:

Electrical Load
Running Watts
Refrigerator 1 800
Submersible pump 1 600
Lighting loads lot 150
Air Conditioner (1hp) 1 800
Deep Freezer 1 500
Microwave 1 600
Computer 1 300
TV 1 400

Determine power rating for generator  as shown below:
Electrical Load
Running Watts
Surge Watts
Refrigerator 1 800  2 x 800 = 1,600
Submersible pump 1 600 3 x 800  = 2,400
Lighting loads lot 150 0
Air Conditioner (1hp) 1 800 2 x 800 = 1,600
Deep Freezer 1 500 2 x 500 = 1,000
Microwave 1 800 1.5 x 800 = 1200
Computer 1 300 0
TV 1 400  0
4,350 7,800
Total Power Required = 4,350 + 7,800 = 12,150W

Add 15% contingency = 12,150 x 1.15 =13,972.5W

Size of Generator needed 

= 15,000W or 15KVA   standard Size