Generator question.

We are buying a portable generator to with power outages. The one I am thinking of is rated at 4400 watts starting and 3500 running watts. A question has arisen about when two appliances, a new 10,000 BTU window ac and a new refrigerator try to start at the same time, in which case each would try to draw its maximum inrush current which would slightly exceed the rated starting wattage of the generator. The missus has talked to some electricians and is under the impression that this is a serious problem. I, on the other hand, think that the peak inrush power is probably measured in tenths of seconds and because of that, this type of simultaneous starting situation would be relatively rare. Furthermore, the consequences of the motors/compressors NOT having immediate access to peak wattage would not be catastrophic and would probably mean the motor would start slightly slower, again measured in tenths of seconds.

Somehow, sizing a generator by adding up the peak current draw of all motors or compressors which could be running seems like a it would result in a generator twice the size which I always thought necessary.

Your thinking?

I also have a 4000 W generator. I use it to power my whole house during a power outage.

I don’t worry too much about the situation you’re describing. The generator has a circuit breaker, and I figure it will trip in the event of a sustained overcurrent. The generator is also able to accommodate a certain amount of surge current, up to a certain point. Above that, and the voltage momentarily droops or the breaker trips.

However, I should mention I do turn off high loads I deem “non-critical” such as air conditioning, range, and clothes dryer when powering the house from the generator.

I hope you’re installing a transfer switch to connect the generator to your homes electric panel rather than a “suicide cord”. Using a suicide cord is dangerous for both you and any workers trying to restore power.

How do you secure the generator?

I had a 16k Generac installed in my mom’s house Aug, 2018.

The electrician demonstrated a special feature that controls when high loads start. It’s monitoring the load on the generator and won’t let something start that will overload it.

So, if I’m using the range, washing machine, and clothes dryer. The central air won’t start until one of those devices turns off.

The Generac module keeps checking the load, every few minutes, until the AC can start.

The 16k generator can handle nearly everything in my mom’s house. It’s rare that the generator gets too much load.

We wouldn’t run any of those things.

It would be portable, not a permanent install

I have seen those setups described on YouTube. Dangerous stuff. We have installed a manual transfer switch.

We are not thinking of such an installation.

Posted in another thread:

I originally started with a 1500W Honda generator.

It is surprising how much even a small generator can run. I always plugged in the Fridge, tv, and a couple lamps at night. We used the coffee pot and had a hot plate to cook. I never used the hot plate and coffee pot at the same time.

I used that Honda for 6 years. I finally got tired of dragging out extension cords and setting up the generator. Got a whole house installed.

My biggest complaint with the Honda was the 1.5 gallon tank. It ran 4 to 5 hours. I’d fill it at 9PM and woke up every morning with no power. That let the food in the freezer slightly defrost.

I flatly refused to get up at 1 AM to refuel the thing.

I have an emergency generator like that, and the simple solution is to avoid power surges by not plugging in everything all at once.

In the OP’s case, let’s say plug in the refrigerator first (because you don’t want the food to spoil), and after it runs for a few minutes (when its power draw has dropped back to normal), plug in the A/C, then plug in the other things with successively less power draw until you reach the load you’re comfortable with Things will still cycle on and off, but it’s highly unlikely that the refrigerator and A/C will both pick the exact same moment to click on.

Green Acres played this for laughs, but this scene actually explains the idea pretty well.

My grandparents lived ‘off the grid’ in Oregon from the '60s through the '90s. The house was wired with 12 v lights and 110 v as well. Grandpa had a generator that he’d start up around dusk. When it ran out of gas, it was close to time to go to bed. The 12 v lights allowed for extended evenings. (As for appliances, there were two or three Servel gas refrigerators, a gas water heater, and a gas stove fed by a huge propane tank, and a wood-burning stove.)

My thinking is like this.

When connected to utility power and a motor starts, it will draw multiples of its running amps/watts. This is more or less an instantaneous spike, but lasts only on the order tenths of a second, possibly milliseconds, before it drops down to the rated wattage of the motor.

Is this true?

When connected to a generator which is essentially coasting with no load, there is no instantaneous power available. The governor has to open to increase the fuel supply to crank out the amperage. Meanwhile the motor has begin to turn before being able to draw the high amperage normally available from utility power. Once this happens, it will no longer draw its maximum starting power as the armature is already in motion.

Is this true?

So question. Typically, for say a refrigerator, how much total time ,T(t), are we talking about from peak draw, T(0), to the time when the draw drops to normal running amps,T®.


Hope this makes sense.


I listened to the generator. It’s loud when it’s surging and running hard.

The fridge made the generator surge. But only until the compressor shut off.

I knew when the gen ran quietly that I could make coffee or cook on the hot plate.

The hot plate ALWAYS made my 1500W gen surge. That isn’t a problem. I knew not to start another appliance like the toaster.

When a generator is running at a idle and a motor is connected to it and trys to start the voltage will drop and the motor will start to spin, But with lower voltage it will have to draw excess amps the governor will open and the engine will try to pick up speed so it can produce more amps and restore the voltage. But for a short period of time the motor will be trying to draw too many amps for its windings and the generator’s windings. As the voltage returns the amps will drop back to normal. But for a few seconds if the breaker does not open the system will be over loaded and the insulation will be weakened.

When I studied instantaneous current was many years ago. Generators were constant speed with voltage regulated by relays and resistors to excitement current. On over load could cause regulator failure. But todays generators are variable speed and voltage. The out put cycles and voltage is controlled by electronics so thing have changed.

So does this mean that there is a serious possibility of frying either the motor or the generator if the motor tries to draw too much amperage? It seems to me that the breaker is there to prevent that. You would think there would be an adequate safety margin built into the breaker. Is that wrong? Also, how much of a voltage drop would invite disaster?

You mention that generators were different back in the day and that now they are electronically controlled. Do you mean inverter generators or generators in general. My impression has been that regular generators run at the correct 60 Hz speed all the time, with the governor making sure the engine keeps pace. Is this correct?