Rational Preparedness

Portable Generator Sizing: What Size is Right For you?

Goldilocks3 - Generator Sizing

Portable Generator Sizing: In order to know if the size of generator that is best for you, you need to know two pieces of information, how much power will you need to run at one time and what is the max output of the generator.

What do the numbers mean?

Generators are rated with two wattage ratings.  One is for constant power and the other is for starting power.  Appliances often require more power when starting than when running.  Generators supply a boost of power, while that happens, that is above what they can sustain long term.  Usually these wattage ratings are right on the box or title of the generator if purchasing online.

How much energy do my appliances use?

Appliance Power Label

Every appliance uses a different amount of energy.  There is a label on nearly everything that consumes electricity, telling you the maximum power consumption of the appliance.  Sometimes you will get a range but more often, it is just the maximum that is given.  The label can give you this information in one of 3 ways.  It can tell you the watts, the amps, or the kilowatts (thousands of watts).  If it tells watts, you are good to go.  If it tells you the amps, multiply by 115 volts, to get watts.  If the label tells you kilowatts, divide by 1000, to get watts.

Find the watts for all of the appliances you plan to run at the same time and simply add them up.  This will tell you the maximum power consumption of all of your appliances together.  Now see how that number compares to the wattage ratings of the generators you are considering buying.  You want to have some wiggle room for plugging in extra things but the more wiggle room you have, the more fuel you will waste with extra capacity and the more money you will spend on capacity you don’t need.

For example: I found the label for my refrigerator inside the appliance, on the side wall.  It tells me that the maximum power consumption of the appliance is 7.2 amps.  7.2 Amps x 115v = 828 watts.  My stand alone freezer has a label, in the same place that tells me it consumes a maximum of 5 amps.  5 amps x 115 volts = 575 watts.  828 watts + 575 watts = 1403 watts.  Note: I’ve used 115 volts in this example because US home voltage is usually between 110 volts and 120 volts.  Labels usually have a voltage, in that range, on them.

So I want to be prepared for a power outage.  I know that if I purchase a Generac iQ, a Honda EU2000I, or a Yamaha EF2000iS, which all have a 1600/2000 watt rating, I can run both, my refrigerator and deep freezer at the same time.  I know that I won’t lose my food and that I have some extra room to run other devices, such as a lamp, a laptop, and/or maybe a TV.

On the surface it appears like I only have 200 watt of power left over to run these devices.  The reality is, I have a bit more.  Remember that these labels usually tell you the maximum power consumption of the appliance.  Most of the time, the refrigerator and freezer are going to be consuming less.  If they both happen to be starting at the same time, they are going to be consuming 1403 watts.  During that start up, I have an extra 400 watts of startup power to use.  After they appliance start, they will be using less power.

An Example: You Can be Over Capacity and Not Know It.

One time, I was running 4 fish tanks (I was selling coral online) on one circuit.  This happened because the circuits were run in a crazy way in my home, causing non-shared walls in two rooms to have the same circuit.  Together the tanks had, 4 heaters, 11 light circuits, and 6 pumps.  My peak consumption was over the peak capacity for this circuit.  At different times the different heaters would be on or off and the different lights would be on or off.  The system could run for weeks at a time without popping the breaker.  In fact, it was set up for months before my first pop and my realization that I was on one circuit.

I was able to quickly solve this problem by running one of the tank systems on a different circuit.  You won’t have this option if you are over your generator’s capacity….Unless you have a 2nd generator.  So make sure you get one that does what you need it to do.

Getting technical but it’s easy.

Picture of a Clamp-On Amp Meter

If you really want to know how much power the appliances are practically using, to get your generator sizing just right, you’d need to measure their consumption.  The Generac iQ mentioned above, has a meter, built in to tell you how much power you are using.  You’d need to get both appliances to start at the same time to see this maximum usage measure on the meter.  That could be difficult to do.  There is another way.

For a more accurate reading and for the other generators, you can purchase a clamp on meter that can measure current flow in amps.  The meters are safe and easy to use and don’t require any contact with electricity.   They simply clamp on your wire and tell you the amps flowing through it.  Though you will have to remove the outer insulation from your extension cord, exposing the three insulated wires inside.  So, you may want to have an extra cord you use, just for this, as this will make the cord less durable.

You can then calculate watts as explained above.  This will give you one or two readings for all of your appliances at once.  (If you are using both outlets on the generator, you’d need two readings, one for each outlet).  Make sure you measure while appliances are starting and while they are running, to get both the running and starting watts.  Try playing a streaming video on your laptop, for maximum usage.

Sizing for Large Backup Generators

You are thinking of getting a generator to plug into your house (Amazon link to these kind of generators) and you want to have a life that is basically seamless when the power goes down.The first thing you may consider is what is the total service for your home?  You can see this by looking at your breaker panel.  At the top of the panel will be your main breaker.  It may be 70 amps, 100 amps or something else.  You may have more than one panel with main breakers that need to be added together.  Whatever your total service is for your home, if you have a generator that size, your home will run the same with the generator as it does from the power company.

Remember that multiplying amps by 115 gives you the approximate watts your home is serviced for.   So if you have 100 amp service, you have 11,500 W or 11.5 kW Service.  If you have 200 amp services, you have 23,000 W or 23 kW Service

Do you need the home to run exactly the same as it could from the power company when there is a storm?  If you have enough money, the answer could be yes.  If you want to save some money, the answer is probably no.  You probably don’t run your home at full capacity in your daily life and would not need a generator the same capacity as your service from the power company, to live almost as if the power never went down.  Some of the information that I gave in the sections above Can be of help, especially when thinking of running large appliances like electric stoves or air conditioners.

You can add up your max potential power consumption and buy a generator that meets those needs.  For example, what if you have lights on in 5 rooms, you are running your stove, electric water heater, and air conditioner, at the same time?  What if the TV is on, you are running a computer and you have fish tanks too?

You can consider taking steps to reduce your maximum power consumption.   Do you need to run your electric stove and the air conditioning at the same time? Could you replace your electric stove and water heater with a gas stove and water heater?  Could you install LED lights throughout your home? Do you need to run that extra fridge in the basement, when the power goes down?  More on this in the subsection below.

Base Load vs, Controlable Peak Usage

Chances are you are going to have a kind of base load or even a base peak load of power consumption in your home.  You have items that you either leave on for a long time, or that are self running.  These include things like the fridge, the TV, the lights, etc.  You have some items that are so small they are not worth worrying about, like cell phone chargers.  These are items that are either not consolable or are just inconvenient to control.  Do you want to have to unplug the fridge to cook?  Probably not.  The cord is buried back there and that’s a real pain.

Some appliances are very controlable and consume huge amounts of power for short periods of time.  These include things like the garbage disposal, curling irons, hair driers vacuum cleaners.  Basically, these are anything that produce heat, have a big motor, or some combination of the two.  It might be very easy to not run the garbage disposal, hair dryer, and vacuum cleaner at the same time.   Your home is built to do this but do you need to be able to do this during an emergency?  Do you need to run the supplementary electric heat wall panels, in the back room?  Could you shut those panels off for a couple weeks and let the room be 65 degrees while the rest of your house is 72?


Another Source of Information About Your Power Consumption

If you have a smart meter on your house, sometimes your electric bill with come with extra data.  It may include things like peak usage.  Your peak usage will tell you your highest level of power consumption for the month.  That’s a good indicator of how much power you actually use all at once.


Sizing Inverter Vs Conventional Generators

Inverter Generator Vs Conventional

Inverter generators tend to have variable speed, where conventional generators need to maintain a constant RPM to keep the voltage and hertz regulated.  This means that if you run an inverter generator on a light load, it can slow down to save fuel.  If you put a small load on a standard generator, it has to run at the same speed as if you ran it at a full load, consuming more fuel.  This means getting the right size is probably a bit more important for a standard generator than an inverter.

With both types of generators, having a bit of extra capacity could improve the longevity of the machine.  Having a bit of extra room means you are putting less stress on it.  If you drive your car at 130 mph, every time you get in it, it probably won’t last as long as if you drive it 65 mph, every time.  Usually, this won’t be a problem because your load will vary and most of the time, you won’t be running everything at once that you want to be able to run at once.  Chances are, your fridge and freezer won’t be starting at the same moment.  They also won’t be just starting all day long.  So having a generator that can handle that will mean having one that is generally running at less than full capacity.

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