? re sizing generator

Old 07-16-18, 09:29 AM
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Question ? re sizing generator

2 numbers needed to size gen, running & surge. running is total of just that. surge is total of all the running and the highest surge. my example:

item: surge / running watts

frig 1: 2160 / 240
frig 2: 1560 / 150
furnace: 1080 / 480
misc: 0 / 77

total running is: 947
total surge is then: 3107 (947 & 2160)

1st question: if frig 1 is surging @ 2160 why am I also adding in the running wattage to the calculation? it's not using 2160 + 240 - it's just using the 2160. so then would not my total surge be 2867 (3107). What am I missing?

2nd question: on the other hand, could not all three items surge at once? pulling 2160 + 1560 + 1080 + 77 or 4877 surge? sure not likely, but possible? is this why the convention is to add the top surge to all the running watts - as a built in precaution?

what happens if all 3 surge at once and tries to pull more than what the gen can provide?

of course i could buy a much larger portable generator and not worry about any of it, but i'm in need of a propane feed with 5% or less THD, which pretty much leaves me with an inverter....


Old 07-16-18, 11:21 AM
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Question 1: I don't know why you are adding the two together. I just look a the starting wattage or the running and don't combine them.

Question 2: Yes, it is possible to have more than one thing start/surge at the same time. It can trip a circuit breaker on the generator, drag the generator down until the engine quits or can trip an overload protection circuit on the generator. If you overload an inverter generator the overload protection will kick in. The engine will continue running but no power will be output. With many models you have to shut down then re-start the generator to reset it.

Obviously if you add up all your starting wattages you'll need a pretty big generator. If the generator will only be used during emergencies I would size it for your largest single load. You can always alternate unplugging one or both fridges so you don't have multiple items starting at the same time.
Old 07-16-18, 11:26 AM
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The starting surge with refrigerators is usually a second or less in length. For the furnace it could be as long as a minute under adverse conditions because with the furnace it is heating up the "hot surface igniter" and that takes some time.

I have a Yamaha 3000 watt (peak) inverter generator and I have never had a problem keeping my (single) refrigerator going along with either a microwave oven or a toaster oven and also having the furnace light. Although generally not advertised inverter generators have a very high short term output, sometimes double the nominal output, for a second or two. This should be sufficient to start the refrigerators if you size the generator as you have.

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