Best way to calculate load for portable generator?

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joemly

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Does anyone have any advice as to the best way to size a portable generator for a residence? I just need to know what is the best size portable generator to cover critical loads in a particular residence.

The loads are well pump, furnace, fridge, outlet for computer modem, a couple of lights and living room outlets.

The problem is that I don't have any precise specs on the well pump and furnace. The best guess on the well pump (which is 120 volts) from very general documentation off the web is about 13 amps; and the furnace tag says "less than 12 amps." Do I need to take actual readings when they cycle on? Or can I just multiply amps x volts? Do I need to use a multiplier for start up current -- if so, what is it?

And is there a formula to calculate lights and receptacles?

Sorry that my questions are so basic -- this is the first time I've had to actually recommend a particular size of generator.

Many thanks for any help.
 
100920-1201 EST

First, some general comments.

I have a Honda, roughly 5000 W, nice pretty red color and overpriced. Starts well and has good voltage regulation. To connect to a grounded system it is necessary to remove a jumper related to the internal GFCI. Honda carburetors are ones that you do not want to let gas sit in when not used. Thus, you close the valve from the gas tank and run the generator until it runs all the gas from the carburetor before storage.

To your question.

Most electrical motors and generators can tolerate substantial overload for a short time without damage. There can be electrical or electronics controls that may not allow overload. Internal combustion engines just quit, stall, when you hit the overload point.

Thus, starting inrush to a motor load could be a greater problem to an under-sized generator from the gas engine point of view than its electrical capability. But if the inrush is short enough relative to the mechanical inertia, then you maybe OK.

It is highly unlikely that two or more high inrush loads will come on at precisely the same time. Thus, figuring one peak inrush is probably adequate.

Your computer modem is insignificant. The computer and peripheral will consume many times the modem. Maybe 5 W for the modem. Maybe 200 to 300 W for a computer without a laser printer. A laptop is less.

Lights -- add up the wattages, but change to CFLs to reduce consumption.

Each of my freezers runs about 300 W when running, and double that for less than one second for start-up. Peak inrush current for 8 to 16 MS might be 6 to 8 times running current.

If the furnace motor is simply a hot air fan motor, then maybe 1/3 HP. There is inertia here so start up current is an initial high peak, followed by current to get the squirrel cage blower rotating, and last steady state current.

The water pump is likely the largest peak inrush and highest steady state load.

I have not run experiments on furnace and water pumps to have any personal experience on values to expect. On my 1.5 HP DeWalt radial arm saw with about 120 ft of #12 copper from the main panel and on 120 V I have a steady inrush of about 70 A for maybe 3 to 5 seconds. Large voltage drop causes slow start up.

Do some current measurements. Without the proper equipment and multiple tries you won't know the very short initial peak inrush, but you will get an idea of 1 second inrush with an analog Amprobe.

Off hand my guess is that my 5000 W Honda could do the job. If it didn't, then I would stagger turn on times of the loads.

One question to ask is how much real need is there for this backup? That will in turn define the economic strategy.

Also be concerned about the gas engine noise.

Others will talk to you about code and safety concerns.

.
 
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