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Auxiliary electrical generator ready for (unsafe) use © D Friedman at InspectApedia.com Backup Electrical Generator Capacity or Size Determination
Tables of electrical generator capacity requirements

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Electric generator size tables:

Guide to determining the necessary size or wattage capacity for electrical generators for residential & light commercial use: this article discusses how to determine how much back up generator capacity or power in watts you'll require.

How big should my electrical generator be? How many things do I need to be able to operate if there is a power failure?

This article series discusses the use of emergency generators for electrical power backup at residential properties. We describe generator size requirements, where to buy an electrical generator or voltage converter, and how to hook up or wire a portable or backup electric generator. We also describe methods for obtaining temporary electrical power from a car, RV or other 12-V or 24-V vehicle or system.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

How Much Backup Generator Capacity do We Need?

Backup generator, commercial grade (C) Daniel FriedmanOur page top photo illustrates an EM portable generator mounted on a utility trailer. This electric generator was to be used to power a sound stage, outdoor lighting, and related equipment during the Guanajuato Film Festival activities in San Miguel de Allende, Mexico.

The commecial-grade backup generator by Katolight Corporation shown at left is installed at an adult care facility in New York.

Here we give backup generator size requirements tables for both backup electrical generators for home use and for smaller car or truck AC adapters.

How to Determine the Necessary Home or Office Backup Generator Size or Capacity in Watts

The minimum wattage capacity your standby or backup electrical generator needs to provide depends on your building, its occupants, their special needs, etc. Here we list the more critical electrical circuits and devices you should consider powering. Use this list, with additions or deletions to arrive at the total minimum wattage that your backup or standby generator system will need to provide.

Critical Home Electrical Circuits That May Need to be Powered
Electrical Circuit or Device Watts Required Comments
Air conditioning, portable or window unit? 1,500W - 2,500W

A/C may be critical in some climates and for some building occupants. Central A/C typically consumes at least 5000W, typically about 8,750W for 24,000 BTUH capacity.

Do not run flooded equipment before it has been dried, cleaned, inspected for safety; do not run warm air heating or air conditioning systems that use ductwork before assuring ducts are clean and safe to use
Emergency medical equipment used in the home ??

Check the individual equipmnent wattage tags.

Watch out: for emergency medical equipment or other electrical equipment that must be available full time to avoid personal injury or building damage be sure that your electric generator covers that capacity. Consider automatic transfer switch operation as well as additional battery backup.
Emergency lighting 400 W Sufficient for safe building occupancy, e.g. stairs, entrances
Heat , central boiler, furnace 500 W

Central heating, if functional.

Watch out: Do not run flooded equipment before it has been dried, cleaned, inspected for safety; do not run warm air heating or air conditioning systems that use ductwork before assuring ducts are clean and safe to use
Heat, portable heater (s) 1,300 W If central heat not functional, weather dependent. Multiply this figure by the number of electric heaters required.
Microwave 1,600 W - 2,800 W If you require preparing hot foods or certain other applications;
Refrigerator or Refrigerator / Freezer 600 W - 2,900 W  
Sump pump 600 W - 3,200 W Critical to avoid basement or crawl space flooding during storms, but will be overwhelmed by major disaster flooding in some areas. Multiply this figure by the number of sump pumps installed.
Well Pump 600-900W

Necessary to provide water supply;

Watch out: Do not presume that a private water well can provide safe potable water after area flooding; after floodwaters have subsided from ground surface, contaminants may still be in the aquifer as well as silt, mud, and contaminants in the well itself.

See DRINKING WATER - EMERGENCY PURIFICATION
 

Additional, non-critical electrical components desired for longer building occupancy in a disaster zone
Computer 25-250W  
Clothes dryer, Electric 5,000 W  
Clothes dryer, Gas 2,500 W [11]
Dehumidifier 1,450 W [11]
Electric cook stove 5,000 W  
Fan, window 1/8 HP
Fan, window 1/4 HP
Fan, window 1/2 HP
 1,000 W
1,850 W
3,500 W		  
Freezer 2,900 W [11]
Garage door operator 1,400 W Useful if the garage door is too heavy for safe manual operation when disconnected from the automatic operator track.
Saw, circular 81/4" 1,400 W  
Television 200 W  
Vacuum cleaner, wet-dry shop vac 900 W -1,300 W For shop vacs up to 2.5 hp.
Washing machine 1,200 W - 3,450 W  
Water heater, electric 5,000 W  

Notes to the Table Above

 

Car or Truck DC to AC Power Adapter Capacity Requirements

DC to AC Power Adapter Volts, Amps, Watts Data for Common Small Electrical Devices

Table of Electrical Power Requirements for Small Electrical Devices
Device 120 V Circuit Input 12-V Circuit
Input		 Device Adapter Output Typical Nominal Device Watts1 Comments
Cellphone charger 0.15A @ 120V AC 1A @ 12VDC 1 A @ 5V DC 5 Watts Most chargers at 100V - 240V, 50 or 60 cycle electrical input
Tablet Computer Charger 0.15A @ 120V AC 1A @ 12VDC 1 A @ 5V DC 5 Watts  
Laptop Computer 1.5 A @ 120V AC 15A @ VDC 1.56A @ 32V DC 25 - 100 Watts Varies by laptop model

Notes to the table above

1. Generally, without considering more accurate & technical power factors, we can use three trivial formulas to convert between amps, volts & watts

also

2. Details are at

DEFINITIONS of ELECTRICAL TERMS

AMPS VOLTS DETERMINATION

DMM DIGITAL MULTIMETER HOW TO USE

 

Backup Generator Capacity to Handle Demand Surges

05/05/2015, NHFireBear said:

It may bear repeating that the compiled list is the minimum total wattage, i.e., when everything on the list is happily running.

You also need to consider a potentially massive surge rating, if you're using an automatic transfer switch and have a bunch of motorized loads. Everything restarts simultaneously.\ Motor starters sometimes need several seconds at triple their rated operating power. The generator may run fine with all of them randomly cycling on and off over time (AC, fridge, sump pump, well, hydronic heat circulators0, but might be overloaded if they all restart at once. Some generators are designed to handle 110% of the load for up to a minute; others (less expensive, poorly designed) may simply burn out instantly.

Better transfer panels have load-shedding features to avoid overloads by sequencing cut-over, according to priority, when necessary. For instance, would you rather have the lighting on, air conditioning, or the elevators running ten seconds after a power outage?


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