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[begreen]

No, I agree there is a startup surge but I do not agree that it is 6X. That’s BS generator salesman talk.

It wouldn't be. Not every refrigerator has an operating power of 100 watts, that's quite low. A 25 cu ft fridge can draw 300-600w. Older fridges (pre-1993) with less insulation and less efficient compressors could draw even more.

 

[gthomas785]

Just tested starting my fridge. 742 watts peak, 121 watts running. It's a 10 year old Whirlpool.

 

[Highbeam]

Just tested starting my fridge. 742 watts peak, 121 watts running. It's a 10 year old Whirlpool.
View attachment 339065

Almost exactly 6X for a millisecond. Would you suggest sizing a generator based on that millisecond demand?

 

[gthomas785]

Almost exactly 6X for a millisecond. Would you suggest sizing a generator based on that millisecond demand?

Yes, for the peak rating. Did you read my previous posts?

Also it's not a millisecond it's more like 1 second.

 

[Whitenuckler]

Almost exactly 6X for a millisecond. Would you suggest sizing a generator based on that millisecond demand?

Maybe for an inverter type as it would likely trip out. It would also depend if the motor was attached to a high inertial load
like maybe a loaded cement mixer. Until the motor get up to speed it is going to draw more juice.

 

[Highbeam]

Then the next question becomes whether you assume these tiny surges stack. Do you assume they all hit at once? For example four refrigerators from the chart earlier in this thread. Each run near 100 watts ( part time) so only need a 400 watt continuous generator or do you assume that you need to handle the simultaneous startup surge for all of them and run a 2400 watt peak genset? That would waste a lot of fuel.

I don’t believe these little startup surges actually matter that much. They are extremely short and the flywheel of a generator like in the OPs post is ripping along at 3600 rpm. The little blip you hear is the fridge popping on.

A few hundred watts here or there is no big deal to overestimate for. Fueling a 13000 watt gas hog would be a big chore.

 

[gthomas785]

Then the next question becomes whether you assume these tiny surges stack. Do you assume they all hit at once? For example four refrigerators from the chart earlier in this thread. Each run near 100 watts ( part time) so only need a 400 watt continuous generator or do you assume that you need to handle the simultaneous startup surge for all of them and run a 2400 watt peak genset? That would waste a lot of fuel.

I don’t believe these little startup surges actually matter that much. They are extremely short and the flywheel of a generator like in the OPs post is ripping along at 3600 rpm. The little blip you hear is the fridge popping on.

A few hundred watts here or there is no big deal to overestimate for. Fueling a 13000 watt gas hog would be a big chore.

There are no questions this is all figured out already. You use the peak draw of the largest motor and 125% of the continuous loads to size the generator. This is exactly why generators have a peak rating.

 

[Highbeam]

There are no questions this is all figured out already. You use the peak draw of the largest motor and 125% of the continuous loads to size the generator. This is exactly why generators have a peak rating.

I do not agree with your alleged rule of thumb but if it works for you then great.

 

[gthomas785]

I do not agree with your alleged rule of thumb but if it works for you then great.

I don't get what you disagree with. Electrical engineering is not a matter of opinion. There's a limit to what any generator can handle and motor inrush is a real thing. I have stalled my portable generator a few times trying to start large motors with it.

I agree a fridge is not going to give any trouble to pretty much any portable generator on the market today, but that's because they are usually rated well above 600 watts peak. Try starting a larger motor like a portable air compressor or water pump and you could very easily run into issues.

 

[Whitenuckler]

I don't get what you disagree with. Electrical engineering is not a matter of opinion. There's a limit to what any generator can handle and motor inrush is a real thing. I have stalled my portable generator a few times trying to start large motors with it.

I agree a fridge is not going to give any trouble to pretty much any portable generator on the market today, but that's because they are usually rated well above 600 watts peak. Try starting a larger motor like a portable air compressor or water pump and you could very easily run into issues.

I can tell you these big portable air compressors could be the worst. A few years back I was selling a like new Husky 20 gallon. The type that is upright. It was 120V and probably a full 15A. I used it for awhile and it always worked good. I thought I'd use it for air tools but that did not work out. I sold it to someone after running it for them in my yard. They brought it home and a few hours later they said it was broke and would not start. To make a long story shorter, it was because they plugged it into an extension cord. With the high inrush on those you can have a big voltage drop and it won't even try to start.

 

[gthomas785]

Then the next question becomes whether you assume these tiny surges stack. Do you assume they all hit at once?

I feel like I didn't answer this adequately. There are certain cases where you'd need to consider load syncing. For example if you have 2 HVAC systems on programmable thermostats and they're both set to turn down setpoint at 4 pm, there's a good chance they will come on together especially if they are on astronomical or internet time. But if they are on old school thermostats, then the chances of that are practically zero. So, this will affect how your generator should be sized.

There was a paper that came out a few years ago documenting how smart thermostats have actually created a huge strain on the grid (both electric and gas) due to this load syncing problem as many thermostats get left on their default settings or at least are set to a round time (e.g. 6 am) to kick on. I'll see if I can find it.

 

[Whitenuckler]

I feel like I didn't answer this adequately. There are certain cases where you'd need to consider load syncing. For example if you have 2 HVAC systems on programmable thermostats and they're both set to turn down setpoint at 4 pm, there's a good chance they will come on together especially if they are on astronomical or internet time. But if they are on old school thermostats, then the chances of that are practically zero. So, this will affect how your generator should be sized.

There was a paper that came out a few years ago documenting how smart thermostats have actually created a huge strain on the grid (both electric and gas) due to this load syncing problem as many thermostats get left on their default settings or at least are set to a round time (e.g. 6 am) to kick on. I'll see if I can find it.

That makes sense to me. In one place I lived they had smart meters and did off peak billing rates. Those are based on time, so guess what? People set their AC systems to set back at a certain time. As most of these are wifi devices, they use internet time.

 

[Highbeam]

How about after a night of zero power and you go to power up the home with genset power. All the fridges have warmed up some and perhaps all will immediately try to energize the compressors when power is restored via genset in the morning.

The concept of a hybrid battery generator system is really showing it's benefits. Charge the battery with the right sized genset running at peak efficiency and then use the battery and inverter to power your home. The inverter would be oversized since there really is no penalty for that and so better able to handle these surge loads whether real or exaggerated.

 

[begreen]

How about after a night of zero power and you go to power up the home with genset power. All the fridges have warmed up some and perhaps all will immediately try to energize the compressors when power is restored via genset in the morning.

That's why I power up the garage first with the extra refer and fridge, wait a minute, and then connect the house breaker.

 

[gthomas785]

How about after a night of zero power and you go to power up the home with genset power. All the fridges have warmed up some and perhaps all will immediately try to energize the compressors when power is restored via genset in the morning.

Yeah, that's another common scenario during an outage. You could be practically connecting the generator to a dead short initially. So if possible it's a good idea to manually connect the large loads one by one similar to how begreen suggested. Utilities also deal with this problem when restoring power. They have various tricks up their sleeves to get people connected without blowing too many fuses.

 

[RustyShackleford]

My well pump died recently, and the new one is "soft start". The sucker does not have ANY startup surge current. Part of me had been hoping this would happen (if you forget the $1800 bill), because the well pump is the one big fly in my home backup ointment.