Power Invertor question

[Anonymous-0]

How many watts worth of power invertor (12 volt to 120) do I need to operate a power tool such as a weedeater, 5 inch angle grinder or reciprocating saw (7.5 amp I think)?

 

[Cummins 1955]

As large as you can afford. You will never be let down. Motors are hard to start, and will need alot of capacity. I have a 1200 watt (10 Amp)I don't know if it would run a large grinder. It will barely start a 110 V 1/4 hp fuel pump. It was only 4.6 Amp. The first one I bought I gave away as it was pretty much useless.

 

[LP GAS ONLY]

For the same money you can buy a cub cadet 4stoke weedeater and get the job DONE.
Gary

 

[buickanddeere]

No less than 1500W and 1750 VA continious.
Modified square wave is ok for motors but devices with electronics may balk.
Good news invertors are now dirt cheap. They used to be a dollar a watt.

 

[Anonymous-0]

I don't think the four stroke weedeater will do suvh a good job of cutting or grinding metal out in the middle of nowhere.

 

[Anonymous-0]

Thanks. I probably would have bought an inadequate one. May consider a generator instead.

 

[souNdguy]

Remember.. calculate your wattage. Standard household circuit is 15a.. though you can find 20a circuits.. adn things that 'request' them.. etc.

15a = 1800w
20a = 2400w

Get a 2000w continous /4000w peak inverter, and you should be able to run most appliances that plug in to your house. I've seen them in harbor freight and other tool and discount catalogs. The price of the 2000/4000 unit is approacting a low end generator of the same specs... You might actually want a 3kw genny, vs a 2k/4k inverter.. but that's your call. An inverter can be run inside a tent.. etc..

Keep in mind your current source.. etc. If you are drawing 15a at 120v.. your 12v source could be discharging at nearly 200a.. If you get that monster 2k/4k inverter.. i'd get a big hause of a battery.. something with 800-1200 cca.. like a 4DLT.. ( again.. may be cheaper once you figure the battery price to get a genny.. )

If you are running electronics and they have a triac based power supply ( switching power supply ).. they won't care about square wave or stepped sine, or pseudo sine wave output.. ( ferro-ressonant supplies will though ).. Plain xformer type supplies will notice the difference.. but the effect will depend on what they do with the power.. if it is being dumped to filterd DC.. probably not to bad.. if it is dropped to lower votl ac ( or stepped up to higher AC ).. then you can get a few weird problems.. etc..

Soundguy

 

[Tim B from MA]

Soundguy gets at a useful tid bit but does not come out and say it ....

Watts = Volts x Amps

So if the starting draw on the 120 volt grinder is 10 amps, you need a surge wattage of 1200 watts.

Of course, also as Soundguy indicated, just because the label says it can do 1200 surge watts doesn't mean it can.

Look for a generator on craigslist (stay away from Coleman generators).

 

[buickanddeere]

Watts = DC volts X DC amps

or

Watts = AC volts X AC amps X power factor

A 1500W/120V load with a P.F of 0.8 is drawing 16.25 amps which smoking range for a 12.5 amp 1500 watt invertor.
That 0.8 PF load would require a 1875W or 1875 VA invertor.
This is where persons who purchase a generator or invertor that is "just enough" find it isn"t "just enough" plus some smoke.
Even with a deep cycle group 31 battery. That grinder is going to flatten the battery in 5-10 minutes. Deep cycling the battery(s) shortens their life and capacity.

 

[JDemaris]

You will need a minimum of a 2000 full-time rated inverter. An exception can be some better built inverters that come under-rated - e.g. like some sold by Xantrex/Trace.

You can buy an inverter for $140 that will work just fine. Rated 2000 watts continuous and 4000 watts surge. I bought two from Harbor Freight (one is a spare) for $139 each. I'm on my third summer of working at an off-grid cabin site using power tools all day long. If the inverter burnt out tomorrow - it's wouldn't owe me a dime. And - the Harbor Freight version has a thermal activated fan which is a nice feature - since it has never come on and is therefore quiet.

At the camp, at night we watch a 13" TV hooked to a DVD player. The inverter we used previously had a fan that ran all the time and the noise can drive you nuts after awhile. So, we started using the HF inveter instead and it works great.

The only power tool problem I had was with my 13 amp Makika circular saw. It would somtimes trip the inverter breaker. So, I downsized to a cheap Home Depot Ryobi 10 amp saw and all works fine now.

I run, off and on, a circular saw - and other tools - all day long without killing my batteries.

You could plug in your 7.5 amp saw - and hook the inverter to a Walmart 12 volt combo deep-cycle starting and deep cycle battery - and run it for 10 hours,continuously. That is taking into account the excess surge current for starting along with the 10% inverter loss.

First two years working on my cabin - all I used was the single Walmart 12 volt battery I just mentioned - 12 volts, 110 amp hours. This year I started using a pair of Trojan 6 volt, true deep cycle batteries hooked in series - which gives me around 230 amp hours at 12 volts. I did that since we keep using more stuff. Power tools during the day, TV, DVD, fans when it's hot, and a large 12 volt Sundanzer chest refrigerator.

If you're wondering how long you can run anything - just figure a 10% loss with any inverter. Some have less loss for certain uses - but it's not worth discussing. The cheaper modified wave inverters are actually more efficient that the expensive full-wave inverters.

So - figure a 7.5 amp tool is going to actually draw an average of 10 amps - since starting it can surge to 12 - 15 amps. Add the 10% factor and you've got 11 amps. Cheap Walmart battery is rated at 110 amp hours when drawn down to safe voltage level. That gives you 10 hours of approximate use.

 

[ScottP]

I have been running off a modified sine wave inverter for over 3 years. There is only one thing that wont run off it, the washing machine!

Most UPS systems are modified sine wave.

Scott

 

[IH2444]

Umm 10 amps at 120 volts pretty much equals 100 amps at 12 volts. roughly speaking. A watt is a watt.
Plus the 10% loss.

a 100 amp deep cycle battery will run a 10 amp 120v ac device thru an inverter for a bit over an hour.

 

[buickanddeere]

That isn't how batteries are rated. It's CCA and minutes reserve capacity at 25amp draw.
If the battery is rated at 120 minutes it means a machine can limp along for 120 minutes without a working charging system if the current drain is 25amps.
That same battery at 150amp draw would last at best 20 minutes and in the real world about 10-15 minutes.

 

[Brian Jasper co. Ia]

I have a 600 watt continuous 1000 surge inverter that will run a 4 inch grinder. It doesn't have the full power as plugged into the wall, but it works. Just all depends on what you want to run at the same time. Add everything up you plan to run at once then add 20% to that. That should get you in the ball park.