Introduction: How to Add a Backup Generator to Your Home

This is a great addition to any home where power is likely to be interrupted for an extended period. Care must be taken to make sure the addition is safe for you and for the grid.

Step 1: Introduction

Losing power for a few hours is one thing. Inconvenient. Boring. Eye opening. But when an accidental telephone pole topples or an ice storm hits, especially in winter, it is another thing.

About 10 years ago I was away for several days in mid February and while I was basking in the 70s, my wife was home when a 10-year ice storm hit, taking down power lines, trees and branches by the thousands. Power was off for 2-5 days depending on where you were on the list and on the severity of the damage. Power crews were called in from many nearby states to help restore thousands of power outages. It was in the 20s. As I heard her describe the sole kerosene heater struggling to keep the house in the 50s, I wondered: wouldn't it be nice to have a back up generator?

As it turned out, the opportunity to add such a generator occurred when our house burned down and had to be rebuilt. This Instructable is about what I learned and how I added a backup generator to the house electrical system.

Here are the “musts” for a backup generator:

Must have the necessary capacity

Must be correctly installed

Must be simple to operate

Must be maintained

Let's go through them one at a time.

Step 2: Capacity:

Power is measured in watts which is simply voltage x current. Your house will be equipped with either a 100 amp or 200 amp electric panel. That means that the panel can supply a maximum of 100 amps (current) x 240 volts (24,000 watts or 48,000 watts if a 200 amp panel). If you were to match the power supplied by the city, a 24 or 48 kilowatt generator would theoretically be needed. That would be in the $15,000 range, just for the generator. The reality is that the maximum is never used and most of us can and do live quite comfortably on much less. I purchased a six kilowatt generator for around $500. With that I can run most of the house. When on the backup generator we purposely do not use electric heat, the oven or the dryer. The system is adequate to run everything else, although not everything at the same time!

Keep in mind that smaller generators (< 1 kilowatt) typically do not provide 240 volts but only 110 volts. Even if you can get along without running 240 volt equipment during an outage, if you are going to wire the generator into the house wiring through the breaker panel, it will need to provide 240 volts. The breaker box splits the 240 volt supply into two 120 volt sides. Half the load is on one side and half on the other. Appliances such as a dryer, range, well pump and electric heat use both sides with a special gang breaker.

(The alternate uses of 110 v / 120 v and 220 v / 240 v have no significance. Grid (city) power is delivered to residential homes at single phase 240 volts with two 120 volt out-of-phase lines. Each measures 120 volts to neutral and because they are out of phase, 240 volts between the two legs, but this is more than you need to know).

Ideally there should be enough power to supply heat, light, water and refrigeration. The capacity of a generator is measured in kilowatts and is the maximum power the generator can continuously supply. To figure out what you need, make a list of what you want to keep running and how much power each requires.

The power required by light sources and electric appliances is measured in watts (1/1000 of a kilowatt). A 100 watt light bulb needs 100 watts to run (0.1 kilowatts). Multiply the number of bulbs by the wattage of each to get the power required to run your lights. Computers typically need a maximum of 200 watts, 100 for the monitor and 100 for the PC. Chargers are insignificant. Most appliances have the power requirements on the manufactures label.

There are many types of heating systems. Unfortunately, if you have electric heat, this will be the largest load on your generator and may not be practical. A typical electric space heater requires about 1,500 watts (as does a toaster, hair dryer, iron and electric fry pan). Unless you actually know how much power your heating system uses, it can be estimated by multiplying the number of rooms you are heating by 2,000 watts. Five rooms would require 10,000 watts or 10 kilowatts.

If you do not have electric heat, the requirements are much lower. For instance, an oil-fired or gas furnace only require enough power to run the blower and circulation fan. I have in-floor heat so the requirements are for five water pumps for circulation and one for the boiler (the boiler is wood fired).

You will need to decide what you can live without or if you want to have it all. You may not actually need to use the dryer or oven for a few days. These are both big power consumers. There are numerous standby generator sizing calculators on line. Pick one and enter the devices you want to keep going during the outage. Remember, the more power required, the larger will be the initial investment: the larger the generator, the higher the cost. Running cost is not really important since it will be seldom used and only for relatively short periods.

Here is a sample list:

Lights: 10 x 50 watt light bulbs = 500 watts (½ kilowatt)

Computers: 2 x 200 watts = 400 watts

Heat: 5 x 50 watt pumps = 250 watts

Water: city water, nothing. Well water will require 240 VAC and can use 800 watts

Refrigerator: 200 watts

Assuming everything is running at the same time, that would be 2,150 watts or 2.1 kilowatts. If these were your needs, a 3 KW (kilowatt) generator would suffice.

Step 3: Installation:

This is the important part. Do this wrong and someone's life may be in danger.

Now that I've got your attention you should know that is not an exaggeration. Correct installation ensures that both the city power (the grid) and the generator cannot be connected at the same time, even though the city power may be off.

Imagine that the city power is out and your generator is providing power to your house but the power company wiring is still connected. Your generator would be putting power back into the grid. What's the problem? The problem is that the power company lineman will be out there at some point trying to repair the problem believing that the wires he is working on are dead. From his side they are. He has disconnected the source from the lines he is working on so that he doesn't get electrocuted while working on the lines. But your generator is making the lines hot. This is one way to get electrocuted: working on lines that are not supposed to have power but in fact do.

There is a special mechanical lockout device that makes it physically impossible for the main breaker and the breaker that connects the backup generator to be on at the same time. This device must be installed. If you don't know what you are doing here, it's time to get a qualified electrician.

Be certain that the wire leading from the generator to the plug and the wire leading from the outdoor outlet to the generator circuit breaker are correctly sized. For instance, I have a 6 kw generator connected to a ganged 30 amp circuit breaker. The correct size wire can be found by checking your local code. Where I live, 30 amps requires 10 AWG wire.

There needs to be some way to know when the grid is back up. One way is to connect a night light to the hot side of the mains breaker. Consult an electrician before wiring this directly. As long as there is grid power the night light is on. When the grid loses power the night light turns off. It does not light when the generator is on because the main breaker has been turned off.

The generator should be mounted outside in a well ventilated area. The exhaust should not be near any air intake for the house. It will be running for an extended time so there should be enough ventilation to keep it from overheating. The generator should be protected from the weather but open enough so as not be become a home for mice or worse. The control panel, gas tank, pull rope and gas turn-off should be accessible. Since the generator will be vibrating it should be securely mounted.

Step 4: Operation:

Under normal operation the breaker that feeds the generator power into the circuit breaker box is off and the main breaker is on. When the grid loses power the main breaker is turned off. The interlock now allows the generator breaker to be turned on. Turn off any breakers that would allow the generator to be overloaded (or the generator circuit breaker to trip), such as the dryer, oven or electric heat. As long as the generator is running and its breaker is on there will be power to all of the house circuits which have their breaker in the on position. When the grid comes back up, the generator is stopped, its breaker turned off and the main breaker turned back on.

Below is the instruction sheet I have posted outside near the generator and next to the circuit breaker panel.

Emergency Generator

Switching the house to generator power

Start generator:

(located under the east deck)

Turn on gas (white lever on north side of generator below gas tank)

Hold choke on (north-east side of generator on top)

Turn key to start

Release choke after it starts

Connect the Generator:

Connect the cable laying on top of the generator to the house outlet

(mounted west of the generator on a beam on the bottom of the deck)

Connect the other end of the cable to the generator outlet marked “240”

Switch house power to generator:

(at the circuit breaker box, south wall in the utility room – you will need a flashlight)

Switch the “Main” breaker off (top of box)

Switch on two 30 amp breakers in slot 1 and 3 by lifting the metal interlock

(this interlock cannot be lifted until the Main breaker is off)

Switching the house back to city power:

The night light mounted on the west side of the circuit breaker box at the top indicates whether or not the city power has come back on. If it is out, there is no city power. If it is on, the city power is ON. The house can be switched back to city power.

Switch house power source to the city

Switch circuit breakers 1 and 3 off.

Switch “Main” circuit breaker on. (top circuit breaker)

The house is now on city power.

Stop the generator:

Turn the gas to the “off” position

Wait for the generator to run out of gas and stop

Turn the key off at the west side of the generator (under deck)

Disconnect the Generator:

Unplug the cable between the generator and house outlet and place it on top of the generator


Step 5: ​Maintenance:

When you need the generator, it should work. The way to be certain it will work is to make sure it works, regularly. I run mine for 15 minutes every three months. Be sure to turn off the stop cock and run the carburetor dry to avoid internal buildup. Use a gas stabilizer to keep the gas from going bad. Have enough gas on hand to last a couple days. You may not be able to get to the gas station... or THEIR pumps may not be working!