Figuring watts for generator use

[okie]

I think I know how to do this but just checking to make sure. I am looking at purchasing a generator to run some tools off of in an area that is without power. If I'm thinking right you take the voltage of the tool multiplied by the amps to get wattage. Is that correct?? So my 110 circular saw that says 12.5 amps would draw 1375 watts while being used ??? . I'm just looking for a generator to run a saw and or drill and a pancake compressor off of. I reckon 4500 watt will be enough but not 100% sure.

[ErikC]

  Right. watts=amps x volts. That generator should do it, it might be a little loaded down if they both run at the same time, depending on the compressor motor.

[Larry]

My generator is 5,550 watts.  Runs my 1 1/2 HP DeVibiss pancake compressor just fine.  For a while I had it running a refrigerator in the camper along with the compressor.  It really struggled when both were running together so I unplugged the fridge while I was working.

You might see if you can try the generator with your compressor.  Some generators don't put out all what there supposed to.  Also some compressors suck more than there supposed to...I have an old Thomas pancake compressor that uses way more juice than what it's supposed to.

[Woodwalker]

This will take the guess work out of it.
http://www.dieselgenerators.com/power.htm
It ain't so much what the generator will run, it's what it will start.

[scsmith42]

According to my generator guy, to calculate the size that you'll need take your highest wattage motor FLA (full load amps) load, x 1.2, and then x 2, and this is the size generator that you will need.

So, if you have an appliance with a motor that pulls 1,500 watts FLA, then the formula would be 1500 x 1.2 (1,800) x 2 (3,600), and that's the minimum size generator that you'll need.

Increase the minimum amount by the total wattage that you need to operate all of the equipment, but calculate your minimum based upon the largest motor size with the formula above.

[sawguy21]

The 4500 should be considered a minimum for what you want to do. I tried running a 120VAC pancake compressor off a Honda 3800 and the engine did not have the power to recover when the unit kicked in. Once the comp got up to speed no problem. Scott's formula should work in most cases.

[okie]

Thank you all very much, Y'all have been very helpfull.

Morgan

[snowman]

Iv'e done alot of research on gens lately trying to figure out what I need for my woodmaster. Honda site has a great link, very helpful. Starting a motor is the big energy user. It can take 2 or 3 times as much as running it. Theres a code on all motors that tells you what your motor needs. Honda site goes into detail.

[chevytaHOE5674]

A 12.5 amp motor can draw upwards of 30 amps at startup. I have a 13 amp sawzall that draws around 38 amps at startup...

[dolittle]

All good info in this thread and I will one more thing.  You should be using 120v in your calculations instead of 110v.  It also changes your watts to 1500 for that saw.  You won't be sorry for buying 5000 watts or higher.

[sbishop]

i ended up buying a 6500watt because I need a generator to help me build my cabin in the summer time and one for a backup for the house during the winter months.

I need 6500 because of my sump pump at the house and for my compressor at the cabin.

the start up wattage is the key, be careful some generators says 5000 watts but it's actually 4000 watt runnning and 5000watts startup...meaning it will run at 5000 watts for a few seconds but not all day long!

In this case, bigger is better i think!  ;D

Sbishop

[nsmike]

scsmith42's formula is close to the rules of thumb I used in the Army. One hp=760 watts but for load calculation we would use 1000 w. So it figures out to about 1.3 the rule of thumb was that no one inductive load would exceed 1/2 the rated capacity and total load sould not exceed 3/4 rated capacity.  To reinforce what sbishop said, Military generators are all rated so that they can substain a peak load of 125%. which means you need to pay attention to operating capacity not peak in a civilian generator.
Mike