Generator Sizing – What It Is And Why You Must Perform It

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Generator sizing is the process of determining the size of generator you will need for your particular set of circumstances. There is a serious industry saying that goes like this, “With generators, size does matter.” Whether you are renting, purchasing or using a generator for emergency purposes only, it is crucially important that you select a unit that is capable of meeting your electrical needs. In this session we’ll make this seemingly complicated process easy to understand. In just a few short moments generator sizing will become a breeze.

I cannot stress this enough; failure to properly size your generator needs will most likely lead to one or more of the symptoms related to generator under sizing:

Symptoms of Generator Under Sizing

 * Intermittent and unreliable performance
 * Constant unexplained shutdowns
 * Inability to handle new loads as they attempt to come on line
 * Unexplained complete system failure
 * Shutdowns as additional loads are added
 * Risks to personal safety and/or property
 * Premature maintenance problems
 * Excessive maintenance problems
 * Shortened system life

What Is The Key To Generator Sizing?

They key to generator sizing is matching the anticipated power to be used to the rated output of the generator. In layman’s terms, that means matching your power needs to the right generator. Here’s an extremely basic barebones example of generator sizing. Let’s say you want a portable generator to light four rooms, each with 100-watt light bulbs. You have a total need of 400 watts. After you factor in a overage cushion of 20 to 25%, (this is recommend to compensate for fluctuating and future loads). Your generator size will be 500 watts. Simple enough?

Starting Load Demands

Running Load Demands

Voltage Demands

Starting Load Demands

Some devices, tools, motors and appliances require more current and power when they are initially turned on, than when they are up and running. This is called the starting load demand. This is especially true for motors and pumps. Once the unit is fully operational, its power consumption will drop and revert to its running load demand. To determine the correct electrical load requirements, always use starting watts, not running watts.

Running Load Demands

The running load demand simply put, is the sum total of all the loads that will be operating simultaneously. You can acquire this figure a number of ways.

1). By using a Clamp on AC Digital Meter. These meters will give you an accurate measurement of your running load demands.

2). By using or Generator Sizing Worksheet. This method will give you an approximate measurement of your running load demands.

3). Hire a licensed electrician. This method will also provide you with an accurate measurement of your running load demands.

Voltage Demands

Most homes are designed to operate on a dual feed, commonly referred to as: 120/240-volt service. This dual feed service makes it possible to operate 120 volt appliances simultaneously along with 240 volt devices such as central air conditioning units, heat pumps, clothes dryers, kitchen ranges, water heaters and well pumps. If you ever expect to power 240 volt devices, you must select a generator with a dual 120 and 240 output.



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