Generator calculation

bighatnohorse wrote:

westend wrote:
Watts = Amps x volts.
Also, pay attention to the operational rating. Your generator may be able to handle 20 amps for a small instant but may fail if run continuously at that rate.

Yes, that simple.
Be aware that when a circular saw "binds" up or bogs down, it will demand a lot more power than for which it is rated.
The result will shut down the generator (or pop a breaker).
Best practice is to use a generator with more power than you expect to need.

"Yes, that simple."

Nope, it never is. :-) Your generator produces its rated power at a given elevation, temperature, air pressure, humidity, etc.

Just because it works perfectly at sea level and 70 degrees F doesn't mean it will provide adequate power at higher elevations and/or higher temperatures. I've beaten this to death in previous posts. The info is available in the archives.

"Best practice is to use a generator with more power than you expect to need."

100% agreed!

Chum lee

road-runner wrote:

swimmer_spe wrote:

Using your calculation, a simple 1200W generator that puts out 120v will give me 10A?

Yes the generator will give you 10 amps, but probably not 1200 watts. The generator will output 1200 watts only to resistive loads like incandescent bulbs, heaters, toasters, etc. It will not give you 1200 watts when powering a motor. How much less? It varies with each tool, if I were pressed for an answer I'd say derate the generator to 80%, or to 960 watts. But this doesn't cover the starting surge, a larger problem. Unless you have a significantly larger generator than needed for a tool's running power, it's almost impossible to answer if it will work short of just testing your tools with the generator. For example, some equal watt rating generators will start an run a typical circular saw just fine, while others won't get the saw started.

Worst case, thee generator shuts off or trips the breaker?

swimmer_spe wrote:

Using your calculation, a simple 1200W generator that puts out 120v will give me 10A?

Yes the generator will give you 10 amps, but probably not 1200 watts. The generator will output 1200 watts only to resistive loads like incandescent bulbs, heaters, toasters, etc. It will not give you 1200 watts when powering a motor. How much less? It varies with each tool, if I were pressed for an answer I'd say derate the generator to 80%, or to 960 watts. But this doesn't cover the starting surge, a larger problem. Unless you have a significantly larger generator than needed for a tool's running power, it's almost impossible to answer if it will work short of just testing your tools with the generator. For example, some equal watt rating generators will start an run a typical circular saw just fine, while others won't get the saw started.

there is little to no load starting a saw with a free wheeling blade

the real load comes when moving into the cut

the trick with cutting wood with a small generator is too 'MOVE SLOW'

make a slow even cut, take your time, don't plow thru it, like you have unlimited power

swimmer_spe wrote:

otrfun wrote:
Just like the compressors on RV a/c units, it's the "start" (inrush) current for various power tools that becomes the deal killer for many smaller generators---not their continuous current/amp/watt rating.

For example: Our 3100/2800 watt (25.8a/23.3a) Champion inverter generator will start/run a 15 amp Dewalt chop and miter saw, but it doesn't always "start" them gracefully--sometimes it struggles---especially with the Eco mode on.

When I used it to power my circular saw, I gently squeezed the trigger. The generator seemed to like that.

That's only useful if it's a variable speed motor (or if you're pulling people's legs). Snap switches in single speed tools switch on instantly, of course, regardless of how gently you pull the trigger. I don't think I've seen a variable speed circular saw...though it would not in the least surprise me to find that one exists.

otrfun wrote:
Just like the compressors on RV a/c units, it's the "start" (inrush) current for various power tools that becomes the deal killer for many smaller generators---not their continuous current/amp/watt rating.

For example: Our 3100/2800 watt (25.8a/23.3a) Champion inverter generator will start/run a 15 amp Dewalt chop and miter saw, but it doesn't always "start" them gracefully--sometimes it struggles---especially with the Eco mode on.

When I used it to power my circular saw, I gently squeezed the trigger. The generator seemed to like that.

westend wrote:
Watts = Amps x volts.
Also, pay attention to the operational rating. Your generator may be able to handle 20 amps for a small instant but may fail if run continuously at that rate.

Yes, that simple.
Be aware that when a circular saw "binds" up or bogs down, it will demand a lot more power than for which it is rated.
The result will shut down the generator (or pop a breaker).
Best practice is to use a generator with more power than you expect to need.

Refrigeration compressor motors are the hardest starting motors.

Just like the compressors on RV a/c units, it's the "start" (inrush) current for various power tools that becomes the deal killer for many smaller generators---not their continuous current/amp/watt rating.

For example: Our 3100/2800 watt (25.8a/23.3a) Champion inverter generator will start/run a 15 amp Dewalt chop and miter saw, but it doesn't always "start" them gracefully--sometimes it struggles---especially with the Eco mode on.

swimmer_spe wrote:

westend wrote:
Watts = Amps x volts.
Also, pay attention to the operational rating. Your generator may be able to handle 20 amps for a small instant but may fail if run continuously at that rate.

I know about the continuous rating.

Using your calculation, a simple 1200W generator that puts out 120v will give me 10A?

Yes, it's just that simple!