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How to Size a Home Generator

Find Out What Size Generator to Get with this Calculator

home generator calculator
Summary: This home generator sizing calculator provides an interactive online service which will enable you to accurately estimate the size of a generator based upon the selected electrical devices and equipment that will be connected while on generator power.

Quick Start Guide for the Generator Sizing Calculator
Let's get started. Advanced
Experienced Electricians or Engineers,
No explanation needed.
Review the basics. Intermediate
Inspectors and Teachers
Short Explanations to help you get started.
Please explain the process. Novice
Students and Homeowners
Detailed explanations to help you get it right.

Please Note: This online calculating tool requires javascript to be turned on in your browser.

Detailed Instructions for the Generator Sizing Calculator

Introduction to the Generator Sizing calculator

The purpose of the generator sizing calculator is to determine the estimated size of the generator which is based upon the electrical equipment that will be used while on power that is supplied by the generator.

This home generator sizing calculator focuses on identifying the electrical items and equipment that you would like to use while on generator power.

Special consideration should be given to the all of the options in the form drop down choices within the form to identify your specific items for best results.

The Starting Watts is mainly for electrical devices that have one or more electric motors. The Starting Watts will be filled in automatically by the calculator when a [Update] or [Calculate] button has been pressed.

Step 1 - Home Essential Equipment

The General Electrical Load Requirements are based on the inside square feet area of the home which is then used to calculate the basic lighting load and required appliance circuits. The Residential Electrical Load Calculator is Pre-Loaded with electrical information for you to chose from. Click on the Question Mark Buttons for more specific information relating to each section and entry.

Step 2 - Heating and Cooling

Heating and Air Conditioning Equipment that will be used while on generator power requires proper identification because the equipment typically consumes large amounts of electricity.

Step 3 - Home Appliances for the Kitchen, Laundry Room, Family Room and Office.

Electrical Appliances and Motor Type Loads are typically larger equipment found in the Kitchen, Laundry Room, Family Room and Home Office. Be sure to check the nameplate information for accurate watt calculations.

IMPORTANT: If electronic equipment will be connected to the generator its would be best if the generator has built in electronic voltage stabilizer features.

Step 4 - Other Equipment

Other equipment and devices focuses on bathroom counter top devices as well as other essential access equipment, safety and security systems.

Step 5 - Perform the Calculations

Perform the Calculations: The Calculate button will perform calculations from the information you have provided in this form. The Calculate Button Must Be Pressed when any changes are made to the form.

The Basics of How the Size of a Generator is Calculated

Electrical Power Measurement in Watts

A watt is a measure of true power which is required to perform work at the rate of 1 joule per second. The required Watts of a device is calculated by multiplying Volts times Amps.
Example: Watts = Voltage x Amps.

Frequently Asked Questions about Home Generators

How many watts does it take to power the basic items in an average size house?

A typical home using only essential items will average between 5000 to 7000 watts of generator power.

What does Running Watts and Starting Watts mean?

Running Watts: All electrical devices need a constant flow of electricity to keep them operating which is measured and described as the Running Watts.

Starting Watts: Some electrical devices and equipment such as motors, require extra watts when the device is first started. This extra requirement for starting watts can be two or more times the running watts. This is an important factor when estimating the size of an electrical power system because if the system is not sized large enough when the additional starting watts are required it will cause the whole power system to experience a drop in voltage which can cause some electrical equipment to overheat and may even cause damage.

Clarification of Terms:
Running Watts and Rated Watts are the same.
Starting Watts and Surge Watts are the same.

Why is only one starting watt used to calculate the total starting watts?

Starting watts are only required when the item is started and the duration for the additional starting watts lasts only a few seconds. Typically this calculation is accurate when only one item will start at the same time, however if more than one item is required to start at the same time then the generator size will need to be increased. NOTE: Optional equipment may be installed to prevent more than one item from starting at the same time.

How can I find the running and starting watts of a tool or appliance?

Most electrical devices and equipment will have a nameplate which is located on the device that will provide information such as the Brand, Model Number, Voltage, Amps, Phase, and Watts. As described above, Watts may be calculated by multiplying Volts times Amps.

Brief Instructions for the Generator Sizing Calculator

This Generator Sizing Calculator is divided into five sections which are Pre-Loaded with electrical information that reflects most of the electrical device options for the home which may be essential to operate while on generator power. The calculator focuses on identifying the electrical items and equipment that you would like to use while on generator power. Special consideration should be given to the all of the options in the form drop down choices within the form to identify your specific items for best results.

The Starting Watts column is for electrical devices and equipment that have one or more electric motors. The Starting Watts will be filled in automatically by the calculator when a [Update] or [Calculate] button has been pressed

Please use additional [?] Buttons for more specific information.
Please Note: For your convenience the [Update] and [Calculate] Buttons will recalculate the entire form. The [Reset] Buttons are located at the top and bottom of the form which will Clear the form and reload the default values.

Home Generator Sizing Calculator

Step #1 - Home Essential Equipment Running Watts Starting Watts
Watts of Lighting
Sump Pump
Well Pump
Electric Water Heater

Section #1 - Sub-Total =

Step #2 - Heating and Cooling Running Watts Starting Watts
Space Heater
Portable Fans
Ceiling Fans
Furnace Fan Blower
Window AC
Central AC
Heat Pump

Section #2 - Sub-Total =

Step #3 - Home Appliances Running Watts Starting Watts
Kitchen Microwave
Coffee Maker
Electric Stove
Dish Washer
Can Opener
  Kitchen - Sub-Total =

Laundry Clothes Washing Machine
  Laundry Room - Sub-Total =

Family Television
Video Movie Player
Audio Sound System
  Family Room - Sub-Total =

Office PC Computer
Printer Copier
  Office Equipment - Sub-Total =

Section #3 - Sub-Total =

Step #4 - Other Equipment Running Watts Starting Watts
Security System
Garage Door Opener
Curling Iron
Hair Blow Dryer

Section #4 - Sub-Total =

Step #5 - Perform the Calculations  
Total Estimated Running Watts =
Highest Estimated Starting Watts =
Total Estimated Starting Watts =

Calculated Estimated Generator Size =
Calculated Generator Size at 125% Capacity =
Reset the Form

© Copyright Dave Rongey Ask-The-Electrician.com

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Home Generator Sizing Frequently Asked Questions

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IMPORTANT: We provide this calculating tool as a guide to assist you to understand the process of accurately sizing a home generator based upon estimated proposed connected loads. The results of the calculations are based upon the accuracy of the information that you provide. The [Calculate] Button must be pressed when any changes are made to the form. Accurate generator sizing is best performed with exact wattage requirements which is provided by the equipment manufacturer, and is best calculated by a professional electrical engineer which is reviewed by the building department in your jurisdiction.

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