Are you ready to protect your home from a power outage and potential costly disaster, but aren’t sure what size generator you need?
Every home is different and every homeowner has different priorities about what they want to support when the power goes out. You can choose to just support your essential items, whole house, or something in between. The size generator you need will be contingent upon the items you choose to support.
Load Calculation:
To determine the appropriate size generator for your home, a load calculation must be performed. Load calculations take into account the wattage required for items to be supported by the generator.
Below is an example of a basic load calculation:
Based on this load calculation you would need a generator in the 10kW size range to support the items listed above.
Now, you may be thinking “I don’t need a 10kW generator because I am not going to be running all of these items at once . . . a 7kW would do just fine”.
Even if your intent is to use appliances one at a time, a properly sized generator assumes that everything connected to it will be running all at once.
In 2008, the National Electrical Code (NEC) changed the rules associated with installing fully automatic generators. Prior to the 2008 NEC it was acceptable to install a standby generator with the understanding that the homeowner wouldn’t use large wattage appliances simultaneously. The NEC now states that an automatic generator must be sized to support the entire load connected to the generator or automatic load management must be utilized.
Improper sizing of a standby generator could result in electrical inspection failure due to non-compliance with the NEC. Be sure to utilize a generator installer who is familiar with and adheres to the code to ensure your family’s safety and to avoid incurring unnecessary expenses associated with a failed inspection.
What are my options?
There are three categories of automatic standby generators:
Essential Loads:
You pick and choose the important electrical loads to be supported by the generator and only those loads are connected to the generator. The total wattage of these “essential” items must be less than or equal to the rated wattage of the generator.
A typical essential loads generator supports items such as: heating, refrigeration, well pump and/or sump pump, various lights & outlets, small cooking appliances.
RED – supported load
BLACK – unsupported load
Whole House:
The entire electrical panel is connected to and will be supported by
the generator.
The calculated load of the home must be less than or equal to the rated wattage of the generator.
RED – supported load
Whole House with Automatic Load Management:
The entire electrical panel is supported but large wattage appliances are only supported if there is adequate generator capacity.
As generator capacity becomes available, the load management system will allow the load to be supported by the generator. This allows for a smaller wattage generator to support an entire home, while still complying with the National Electrical Code and ensuring safe operation.
For example, if you have large wattage appliances in your home that you want to support (such as electric kitchen appliances, hot tubs, or central AC units) your load calculation could easily reach 40,000 Watts or more. This would mean you would need to install a liquid cooled generator of at least 40kW to comply with the NEC. However, if you decide to put those large wattage appliances on a load management system, you could potentially install a smaller air-cooled generator, which could save thousands of dollars. (The only catch is that the appliances on load management will only be available if your generator has enough capacity to support them).
RED – supported load
BLUE – supported load If the generator has enough capacity
Air vs. Liquid Cooled
What is the difference between an air-cooled generator and a liquid cooled generator? The difference is the engines.
AIR COOLED ENGINES
Fans force air across the engine for cooling.
Like lawn mower engines.
Usually in generators under 20kW.
LIQUID COOLED ENGINES
Use enclosed radiator systems for cooling.
Like car engines.
Usually in generators under 20+kW.
Most residential applications utilize an air-cooled generator due to the more economical price points vs the more substantial investment associated with a liquid cooled system!
Download this FREE worksheet to prioritize your needs!
There are also standard wattage requirements for common household items so you can get a general idea of what your load calculation might be.
A good exercise while you are filling out the chart is to remember what it was like during your last power outage and what your major concerns and inconveniences were.
