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I have a potential customer who asked me to install a transfer switch for her. She already has a 5000 Watt generator and I told her she will not be able run her heat with that. She has a heat pump system. (I don't know the exact size of her heat at this moment but, let's say there are two breakers for her heating unit, one a 50A/240V and another a 40A/240V; this is probably pretty close to what it actually is. She also has a heat pump, if I am not mistaken; probably a 30A/240V breaker for that. Do you have to run that also? Or, can you power only the furnace? Can you use the emergency heat setting on the thermostat without damaging the system and leave the heat pump itself off the generator?) I am wondering how to properly size a generator that will handle her heat and maybe four of five more circuits. I really want to recommend something that she will be happy with. Any help will be greatly appreciated.
 

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Most likely, just one stage of emergency heat will be 5000 watts.

If she wants to heat her home to normal comfort temp, she will need some whee between a 15 to 25 KW generator.
 

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I'm no expert in this area, but I know a little about generators. Getting the right size can be difficult, even with experience.

Suppose you wanted to power a 50A/240 line. First you have to know the real continuous power required. The number on the brass plate is often only the MCA or MOC, while the actual current draw is considerably less. That works in your favor. On the other hand, the initial current surge is often much higher, and it sometimes last long enough to stall the generator. Related but uniquely different is the generator's ability to withstand a transient load. Some manufacturers may add a flywheel or other features to help, for example.

Bottom line: I would definitely call the manufacturer of any generator you are considering, and check with their applications engineer. Give them the whole scenario and get their input. You might be surprised how BIG the genny needs to be to really work well in that situation.

Lastly, you might consider renting a 25 kW dummy load and taking it to the generator seller. Hook it up and prove it out before buying. I've done it before for a 100 kW diesel. Sometimes its the only way to really know, as not all behaviors show up in the datasheet. A reputable seller wont mind a test. And the load is not big or heavy. One guy can move it.

--Upton O Good
 
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