Sizing Your Portable Generator To Your House

We commonly install what we call Portable Generator Connections.  We have a lot of customers calling us who are interested in this service, but one thing that a lot of people simply can’t believe is that a small portable generator can actually power their entire house. 

Now one of the reasons why this is possible is because most appliances in the house are powered by natural gas, such as the heat, water heater, dryer, and stove.  Very few large appliances in the house use electricity as a main power source.  The one thing that uses electricity is air conditioning.  You are not going to be able to power central air conditioning from a portable generator, however, you can get one or two window units.  That’s a good option, even if you have central air, just keep a window unit handy so you can throw it into a bedroom or living room window in case a power failure occurs.  For the most part, everything else you do doesn’t use that much current.  Televisions, computers, modems, routers, lights, phones, etc.  They use very little electricity. 

Some smaller appliances with heating elements use a decent amount of power, such as toasters, coffee makers, microwaves, hair dryers, curling irons, etc.  As long as you aren’t using all of those at once, you will be ok.  You can use 3 or 4 of those while still powering the rest of your house.  As far as lighting, you can light your entire house with less than 1,000 watts, and even a small 5,500 watt generator can easily power that load along with everything else you want to run.  Most people can live normally with a 5,500 watt generator.  Bigger houses may want a 6,500 watt generator.  If you have a lot of people living in the house, I will sometimes recommend a 7,500 watt generator.  However, that is almost always much bigger than necessary. 

People always tell me “I only need to power the heat and a few small lights”.  But the truth is that they can go about life like normal, using everything that they normally would.  Many of my customers e-mail or text me saying how great it was when the power was out because they continued on with life as if the power never went out.  They had all their lights and TV’s and everything else they wanted to use.  That’s the beauty of an interlock system.  By using an interlock we turn the entire panel into a transfer switch, so you are not limited to just 6 or 10 circuits, you can power the entire panel.  The worst cause scenario is that the breaker on the generator trips.  That tells you that you are using too much power, so you shut one or two things off. 

One thing that customers typically ask me is why they were told that they would require a really big automatic standby generator, such as a 20,000 watt unit.  The reason is because electrical code changes.  In 2008 the electrical code started requiring an automatic standby generator (one that switches itself on) to be able to power the entire calculated load of the house.  The calculated load of a house is pretty extreme ,this is the number that is used to size the electrical service as well as the transformers on the street that power your house.  The calculated load is essentially the most power your house will ever require, but it’s not realistic. 

The problem is that you need a 20,000 watt generator to cover the calculated load of your house, even though you will never actually pull that much power.  That’s why an automatic standby system is so expense.  A portable generator connected to your house is done via a manual transfer switch, so code doesn’t have any size requirements.  You can use any size portable generator that you want.  The only requirement is that it is large enough to power what you choose to power, which is usually a very small load.

I do offer a special service to customers for free during the in-home estimate.  Often times customers don’t believe that a portable generator can power their entire house, and I understand that.  It was hard for me to believe that such a small piece of equipment can power an entire house before I started connecting them for a living.  So what I do is I use an amp meter on the customer’s electrical panel.  The customer turns everything in their house on, everything they may ever use during a power outage.  Then I show them the reading on the meter, which shows exactly how much current they are pulling.  The generator connection that I typically install is a 30Amp system, that can handle up to a 7,500 watt generator.  A 6,500 watt generator provides 27 amps and a 5,500 watt generator provides about 23 amps.  Generally the amp meter will show the customer somewhere around 15-20 amps being pulled.  Sometimes, with larger houses that have more appliances and pumps or fans for the heat, it may be around 25 amps.  Seeing that, they know that they are fine with a 6,500 watt generator.  This makes them feel much better because now they can go about business as normal. 

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