electrical question

[benburnett]

I have an electrical question for anyone who knows enough to help. I have a customer that has an electrical requirement that we need to meet for him. He needs to run a machine that draws 2.5 amps using 115 Volt.

He has a 400 watt inverter hooked up to a 12 volt battery for the machine.

According to my research, the following are the formulas to figure out the amp draw by the inverter when being used.

Watts = volts x amps 2.5 amps x 115 volts = 287.5 watts

Amp = watts / volts x 1.11 287.5 watts / 12 x 1.11 = 26.5 amp draw per hour.

Can anyone confirm this for me or disprove it for me. Would appreciate any help.

 

[craig333]

Thats what I get.

http://www.angelfire.com/pa/baconbacon/page2.html

 

[HERR42]

whats the 1.11 for RMS approximation? efficiency loss?

either way your pretty close.....

Power in watts = I amps X E volts

 

[pvstoy]

Here is two links that could help you. you are correct how you'r getting Watts. Little rusty on the 1.11 number.
http://www.1st-optima-batteries.com/optima_inverter_applications.asp

http://www.whistlergroup.com/faq-inverters-answers.asp

 

[pvstoy]

Right from the Optima Battery page that I linked above:

After determining the size of inverter needed, you need to determine the size of battery (or batteries) required for the running time you want. The storage capacity of batteries is measured in "amp hours" (abbreviated "AH".) The definition of "amp hour" is the ability to provide 1 amp of power for 1 hour. To convert from total watts of load to AH, first use the formula Watts ÷ Volts = Amps. That will tell you the continuous amperage rating of your load. Then multiply that times the number of hours you want to run, and you have the required "amp hours" required from your batteries.

Example: Looking at the table above, you'll see that a laptop computer might require about 70 watts of power (at 120 volts.) So we first multiply 70 watts x 1.15 (to add 15% for the consumption of the inverter) and get 80 watts. Dividing 80 watts by 120 volts = .67 amps of constant load. A mid-size Optima deep cycle battery like the model D34 is rated 55 AH (amp hours) @ 12 volts. Because the inverter will be converting the battery's 12V current to 120V (which is greater by a factor of 10), we divide the battery's 55AH rating by that same factor of 10, and the result is that the D34 battery has a rating of 5.5AH @ 120V. If we then divide that 5.5AH battery rating by the laptop's consumption of .67 amps, we see that the Optima D34 deep cycle battery will run the laptop for 8.2 hours through an inverter.

 

[benburnett]

pvstoy,

Thank you. That is the info that I was looking for. Too tired right now to run the math. Looks like the math works out about the same though. Will run the figures in the morning.

Thanks again.

 

[benburnett]

amp draw

Thanks again for the links. Did the math this morning and it comes out about the same. I really appreciate your help with this.

 

[Nosmo]

All the calculations look fine for steady supply of the wattage.

There may be one other consideration - that is whether on intial startup there is any surge that might exceed the rating for the inverter....

Sorry to be pain, but that might be worth checking with the unit's detailed specs....

Nosmo

 

[benburnett]

electrical

your not a pain at all. I am trying to find out that information now. We know that the machine he is using will not exceed the ability of the inverter at all. Just a matter of what the start surge is and then the constant use is of the machine.

Thanks again to everyone for all your help.

 

[flinchlock]

I know this is an old post...

My wife is a high school science teacher and she brought home a What's Up? power meter for me to mess with over the summer.

This would be a fairly cheap way for a business to make sure they don't do a costly mistake. For "only" about $96, you can have the following info.

Mike

 

[benburnett]

power meter

Thanks for the heads up on that. I click on your link though and it starts to bring up the page with info and the switches to a page that will not allow me to look at it. Could you repost the entire link please.