Sizing Guide for a Home Standby Generator

Whenever a power failure occurs, emergency home back-up generators are an alternate source of power. The home’s electrical system is disconnected from the electrical supply that powers the house form the utility company. This is accomplished with the use of a transfer switch. The home’s electrical system is then connected to the back-up generator. The generator will not energize the utility’s wires, because the automatic transfer switch separates the home’s electrical power from the public utility lines.  In many municipalities this is a required building code. When there is an electrical outage, all of this happens automatically. Conversely, if you are using a manual system, then the disconnection and switching needs to be done manually.

The accurate sizing of a backup generator is extremely important and quite simple. The generator will not provide enough power in the event that it is undersized. On the other hand, fuel will be wasted in the generation of power that is not needed, if the generator selected is oversized. The sizing of generators allows an electrician to configure the generator to power all of the circuits in the home or to specify a smaller sized generator to only power up a select number of circuits.

The sizing of generators is typically done in a unit of measure referred to as a kilowatt (KW). Appliances are usually rated in amps, amperes, volt-amperes, or in some cases watts and kilowatts. During an emergency, determining which electrical circuits and appliances should remain powered, is the initial step in the process of sizing a home standby generator.

To Power The Whole House Or Not, That Is The Question…

Large appliances utilize the most power. If you happen to have an electric stove/oven, that single appliance may use as much as 8500 watts. If you have central AC (HVAC) in your home, that unit will use 6500 watts or more. Just these two appliances combined will exceed the maximum output of most standby generators. Therefore, you can see why it is important to size your standby generators with a little bit of thought. Tough decisions may need to be made if you are on a budget and need to keep down the size of your generator. For example, you may start to look at the stove as a convenience instead of necessity. However, on the other hand, your HVAC system may be looked at as a necessity.

During an electrical outage, determine which appliances are critical and must remain powered. Some important appliances may include medical equipment, freezers, refrigerators, furnaces, sump pumps and water pumps for wells. These appliances prevent the spoilage of food, provide water, heat and sanitation, as well as prevent flooding of your home during an outage.

If you experience extended power outages in your area, you may need to consider keeping your lights, televisions and computers powered by supplying additional power. Keep in mind that this require a larger generator. Along with the larger KW output comes a bigger initial expense in the form of installation and equipment. However, carefully choosing to include a few select lighting and additional outlets will not necessarily result in increased expenses of a generator, which is intended for critical appliances.

Tip: Do not be fooled by physical size. By itself, hair dryers are rated anywhere from 1100 and 1800 watts or more. Contrast that to an LED light-bulb that may use only 8 watts.

Caluclating Requirements

To start, create itemize the appliances that you want your standby generator to power in the event of a power outage. Then read through your list and separate items into two categories, critical and convenience. Then go back through your list and anticipate different times of years, outside temperatures, lighting requirements, etc to validate your decision.

The power requirements for each device should be recorded in watts. Although the power requirements may appear to be confusing, they can be converted easily. A specification of volt-amps is the same as watts (1200 volt-amps = 1200 watts). If you are only supplied with the voltage and amps information, then the volts must be multiplied by the amps to obtain watts (120 volts x 10 amperes = 1200 watts). To convert kilowatts to watts, divide by 1000.

The initial starting of electric motors utilizes more power than when the motor is running. Because of this, you’ll need to figure out the initial power requirements of your appliances that have electric motors. It is important that the start-up power requirements are included when sizing the generator. Therefore, if the start-up current or requirements are not provided in the appliance documentation, then the manufacturer should be contacted.

Take your complete list of of all the circuits and total the power consumption requirements. Then take this number and divide by 1000. The resulting figure will give you your total required kilowatt output for the generator.

Home Generator Selection

Your standby home generator ought to be selected by the output KWs required as calculated above, keeping in mind that it is rarely recommended that a generator sized without an expansion factor be used. Increasing the capacity by 10%, 15% or as much as 25% is advisable. If your requirements are for 16 kilowatts, then buy a home standby generator system that can provide 18 or 20 kilowatts, to eliminate any worry about overloading, if you want to some additional lighting during a power outage.

* Please keep in mind that this article is just to give you an idea of what is involved in sizing a home generator. Have an authorized dealer like Heat Without Power come to your home and perform a free in-home sizing analysis and estimate.