Amps vs watts

I bought a small air compressor to use as a'traveling' compressor.Would like tto run it from an inverter if power is not available. It is a small 2 horse,12 amp. How many watts would I need to run it on a limited basis. Like for airing up tires in the field.Thanks. Steve.

1500 amp steady state, bet inrush will be many times that, so plan on 3500 watts

Also remember you will need conductors to carry 300 amps DC, so maybe 00.

Probably like this. It is a less expensive one. 3000 watts will probably start the compressor. If it shuts off it may be tough to restart. Jim
Untitled URL Link

Steady state? in rush? what is that? Please explain.

Bottom line, a 2HP compressor is a good sized compressor, and not really practical to run from an inverter. You are talking $300-$700 for the inverter. You will need good sized DC cables like 0-00 to supply DC to the inverter. You want something portable, get one with gas engine.

1) If it draws 12 amps at 120 Volts (I don't know for sure just using your numbers), that's 1440 Watts and to allow for inefficiency and heat losses etc. I sure wouldn't trust a 1500 watt Inverter to do the job long term, Id consider a bare minimum 2000 Watt OR MORE, which might possibly (subject to surge and duty cycle and actual capacity) work. As an example one Inverter I'm currently using has a rating of 2000 Watts Continuous and 4000 Watts Surge

ALSO a motors initial surge start up current may be 3 to 5 times its run current, so an Inverter would have to have sufficient short term surge capacity to get the motor started.

2) If an Inverter were to supply lets use that 12 amps of 120 VAC, that means it would draw something like 11 x 12 or 132 Amps from a 12 Volt DC battery, so you have to have enough battery capacity and/or it being charged via say the trucks alternator PLUS big enough cables from the battery to the Inverter.

Not having any specs or data Id venture a pure GUESS a 2000 Watt Continuous, 4000 Watt Surge rated Inverter just might??? possibly do the job (while Id feel better if it were bigger) and remember you have to have enough battery and/or it being charged so you don't discharge the battery

BOTTOM LINE 2000 Watt MAYBE??

NOTE have you looked into ViAir portable 12 VDC compressors ?? They are popular in the RV community NO INVERTER NEEDED !!!!!

Its electrical so you should get plenty of opinions

John T

John,

Good point there, it would quickly drain a battery, and most vehicles can charge 1000 watts tops unless heavy duty charging, like my 300 amp on truck, but that would barely cut it.

You got some good advice. I think a gas powered air compressor would work better. I spent some time trying to get my small air compressor to start and run on a 2500 watt generator. The numbers worked but the compressor would not. I was using a 3/4 horse motor with the smallest pulley that would run the compressor. Not an inverter but similar concept.

Power = amps times volts

Power = 12 amps x 120 volts = 1440 Watts

There may be some spike voltage for start so you may want to round up in an inverter.

Now working backwards, 1440 watts divided by 12volts is 120 amps draw on your vehicles circuit.

Fuhgetaboutit.

Get a cheap 12 volt compressor for those times when you're out in the sticks. It will be cheaper than an inverter big enough to run your 2 HP compressor.

If you need that much power the inverter needs to be mounted under the hood near the battery, connected with large cables and the vehicle must be running when using it.

DeltaRed,

Is this inverter and compressor to go in a truck or car? That would help determine what you need. By airing up tires in the field do you mean in a farming situation or traveling on the road?

I agree with everyone else. Two horsepower is not a small compressor to run with an inverter and would suck a battery dry in no time even with the engine running. I use 6 gallon pancake compressors for most things around the farm because my 3650 watt Champion generator will run them. It has enough output to air up most tractor tires and is easily lifted.

Like MarkB_MI said a twelve volt compressor would be better and you can get some that put out enough to do some good for $75-$125. That is a lot cheaper than a high watt inverter and would not require wiring high amp cables to your vehicle, Now they might not be powerful enough to air up a tractor tire from scratch, but might be enough to top one off.

David,
Electrical questions are asked almost weekly.
All the math experts demonstrating their ability to use Ohm's law and How to calculate power.

What is lacking is experience dealing with starting currents of a motor.

Let them blow their money on inverters even small generators.

My experience, A 3500 watt RV champion generator can't start a 2 hp capacitor start air compressor.

The starting amps of a 4 hp electric chainsaw is 75 amps. Running current is 15a. The chainsaw makes the generator shake, rattle, and grown when it starts. So I invented a soft start pack to cut starting current in half.

So they can do all the math they learned in a class room, won't change a thing.
geo.

Neighbor George, to your comment "So they can do all the math they learned in a class room, won't change a thing."

Actually math CAN ANSWER the "question asked" of how many watts are required to RUN a motor and actually math and engineering and physics CAN ANSWER the "question NOT asked" of what's required to START a motor, and I covered BOTH in my response. Having been an electrical engineer at the Century Electric Motor Company I can tell you from experience and education the math to RUN a motor may be less complicated then what it takes to START a motor. HOWEVER if you notice I told the poster the current required to START a motor may be 3 to 5 times what it takes to RUN a motor !!!!!!!!!!!!!

You're right, many lack the experience and/or education dealing with starting currents of a motor. I however do and try my best to explain it to the non electrical trained and experienced gents here.

You're also right, "the math wont change a thing" and that's true for the math to RUN a motor or the math to START a motor. Once calculated "it wont change" although physical wear and tear and friction and outside forces may change causing changes in the math.

Of course to each their own free choices, be it a Battery and Inverter orrrrrrrrrr a 12 VDC Compressor orrrrrrrrrrrr a Gas powered compressor. Each has advantages and disadvantages. Wouldn't it be a boring world if we all had to do things the same way or had to follow what one person chose.

Best wishes n God Bless neighbor, take care n stay safe

John T

David, EXACTLY if you start pulling 100 to 150 amps from a car/truck battery very long you better have sufficient re charge capacity or the battery will eventually discharge

John T

Why not just mount a compressor under the hood, that runs off the engine on a belt. Could just tip it put the belt on hook so it runs and let loose when done. No extra power needed when driving then. Or put a pto unit to do it. Will be more practical in the end just slide on shaft turn on PTO.

I'm also a fan of those 12 VDC portable air compressors like I mentioned down below. In the RV community ViAir are well accepted but there are a ton of other manufacturers. What I like about them is there's no need for an inefficient Inverter and less equipment to go bad and maintain. However, be it 12 VDC or 120 AC via an Inverter or a gas powered compressor each has advantages and disadvantages, so to each their own choices for whatever reasons.

Take care n be safe

John T

2HP on 120V and 12 amps is not going to happen . Maybe 2HP peak ?
Real continuous HP will be approx. 3/4HP.
You will be happier with this for less money and it will also power a sump pump, microwave and fridge during power outages .
https://www.championpowerequipment.com/product/100103-3800-watt-generator/

John,
You have to get a motor started before your theory can work.

Show me what equation you use to calculate starting current of a motor.

My experience starting currents are at least 5x the running current.

I throw out the challenge for anyone showing a a 12 inverter
Powering a 2 hp air compressor. Or for that matter any air compressor.

Let's put a stop to the pie in sky theory.
George

George, FYI there is no simple equation (way to may factors and variables and unknowns) I can give you for the starting current of a motor. Let me explain this for you.

Such requires complex physics, mechanical considerations, weights, inertia, starting load and torque, efficiencies on and on and on and YES a current motor design electrical engineer or when I was an engineer at Century Electric I could tell you for a certain motor based on the initial load BUT I CANT GIVE YOU ANY SIMPLE EQUATION THEN OR NOW. The equation for Watts is Volts X Amps but it takes moreeeeeeeeee then that to compute starting current...?.

I can tell you as an electrical engineer and a former engineer at Century Electric Motor Company the current to START a motor may be 3 to 5 times or even more then running current GET IT ?????????

Im not sure I understand your challenge??? A person can use a 12 VDC motor orrrrrrrrrrrr a 120 VAC Compressor and an Inverter orrrrrrrrrrrrr a Gas powered Compressor. Theres no "My Way or the Highway" when it comes to a persons free choices, you use what you like and others should do the same. Im not here to argue only to help educate you and others, but Im not perfect I cant explain in a sentence what takes years of study and training to comprehend so dont feel bad if you don't understand it

I will help you the best I can neighbor

Best wishes God Bless and protect you and your family George. As always its a pleasure chatting with you and trying my very best to help

John T

Hey George, I'll accept your challenge! "I throw out the challenge for anyone showing a a 12 inverter
Powering a 2 hp air compressor. Or for that matter any air compressor."

Only, I have a 1.6 H.P. Dewalt compressor(I'll post a pic and a link to it below). Will you accept that compressor for your challenge?

Please respond and let me know.


1.6 HP CONTINUOUS, 225 PSI, 15 GALLON WORKSHOP COMPRESSOR
Poke Here

Aw heck, I ain't got time to sit around and wait for you to reply George. So, here is my response/proof that your challenge is wrong.....

I just recorded this about 30 minutes ago for you.
Here you go George