# Trying to understand things....watts/amps/voltage

Registered Users Posts: 17
Okay, I'm tyring to get an idea how long I can run my battery before I drop it down to around 60%, but I'm getting a bit confused.

I'm running a couple of lights at a total of 31 watts from the inverter at 115 volts.

Now to convert everything to amps it would be 31 watts/115 volts = .29 amps.

This works great for 115 volt system, but how does it relate to how much power it is taking from the 12 volt battery? Does it work differently for 12 volt system? Should I be figuring the amps considering the 12 volts versus the 115 volts. The inverter pulls <.4 amps just sitting there on.

Any help or information given that can help me wrap my head around this would be appreciated.

• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Trying to understand things....watts/amps/voltage

Volts X Amps = Watts

Watts/Volts=Amps

That equations hold no matter what the volts, or amp or watts.

In simple numbers if something draws 1 amp @120volts that would be 120 watts
The same 1 amp at 12 volts would yield only 12 watts.

In your case your .29 amps of 120 vac= ~2.9 amps @ 12 vdc

Watts is watts is watts.

To figure out your battery number, convert the watts to amps/volt and compare them to the capacity of your battery set.

Say you have a 200 amp/hour battery, 50% discharge would be 100 ah. 12volts X 100 amps would yield (in simple numbers!) 1200 watt hours of capacity.

In the simple world of 12 volts and 120 volts the numbers can easily be confused since they are 10 times one another,, so the products are either 10 times or .1 times.

20 amps @ 120 volts would be 2400 watts
200 amps @ 12 volts would be the same 2400 watts

2400/12=200
2400/120=20

See the relationships?

Clear as mud?

Tony

PS The .4 amps that inverter draws just doing it's thing is probably measured on 12 vdc,,, 12X.4=4.8 watts. If it were measured at 120vac it would be 48 watts,,, big difference.

31 watts= .25 amps @ 120 vac,,, or 2.5 amps @12vdc.

To really figure this stuff to the gnats ass you have to figure battery efficiency, charger efficiency and inverter efficiencies to get it completely right.
• Registered Users Posts: 17
Re: Trying to understand things....watts/amps/voltage

Well, I guess that kind of answers the question. I understand how to configure the watts to amps; that is pretty straight forward math...pretty basic and it would all be simple if I were figuring out how many amps I was using of 120v for the items and if I were using 12 volt items, but where the confusion come in is I am using 120v items on a system the is converting power from a 12 volt system.

Even if amps were equal to amps and I were using .29 amps of 120v, I'm thinking that logically it would use more than .29 amps of 12 volts, if for no other reason that some power will be lost during the conversion from 12 volt to 120 volts.

I have seen everyone talk about using a Killawatt meter to determine wattage usage and such, but this really only will tell how many watts of 120 volt AC power is being used, it is not telling how much is being pulled or will be pulled from a battery.

Again, I'm having a bit of a time wrapping my mind around this, but I guess it is a close guestimate of how much power is being drained from the battery by figuring it to be the same for the 12 volt system as it is for the 120 volt system.

On another note, is there a particular recomendation as to a decent meter I can attach to my battery that will let me know how far I have drained it down? My charger tells me when I first turn it on whether the battery is at 90% , 80 % and such, I was just wondering if there were such an animal outside the charger.

Thanks again for the input and trying to get me to understand all of this.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Trying to understand things....watts/amps/voltage

medic29,
i think you did not grasp what was said by icarus. with the conversion aside from dc to ac what basically changed was the relationship of the voltage and current to still come up with the same watts. .29a at 120v = 34.8w. from a 12v point of view this is 34.8w/12v=2.9a and not .29a. it's a case of raising the voltage and lower the current and visa-versa. the twist is that in any conversion it is not 100% so this would represent a higher current draw on the 12vdc to achieve that same 34.8w out on the ac side.
simplified(i hope) watts out = watts in plus losses.
• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Trying to understand things....watts/amps/voltage

Re read what I wrote,, and play with some numbers and a calculator, and perhaps it will become clearer.

3 amps @ 120 volts = 360 watts

3 amp @ 12 volt= 36 watts

100 watts @ 120vac=.83 amps

100 watts @ 12volts= 8.3 amps,, see how 1/10 the voltage=10X the amperage.

As for a meter,,, consider a Bogart Tri-metric meter. It will give you all the basic information you will need, including, battery voltage, percentage of charge, amp hours used, amp hours from full etc.

http://store.solar-electric.com/tm2020.html

Shout if you have other questions,

T
• Solar Expert Posts: 1,164 ✭✭
Re: Trying to understand things....watts/amps/voltage
medic29 wrote: »
Even if amps were equal to amps and I were using .29 amps of 120v, I'm thinking that logically it would use more than .29 amps of 12 volts, if for no other reason that some power will be lost during the conversion from 12 volt to 120 volts.

Actually it is very simple. Multiply the amps you use at 120 volts by 13 and that will give you the amps you are using with a 12 volt inverter. That will nearly always be accurate within 10% or so under 1500 watts, which is close enough to figure battery loads and sizing.

.29 amps @120 volts = 2.9 amps @ 12 volts. Figure 10-20% losses from inverter etc, which comes out to around 2.9/.85 = 3.41 amps.

.29 x 13 = 3.77 amps.
• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Trying to understand things....watts/amps/voltage

What WindSun is adding here,,, and I didn't mention, and perhaps was the gist of your question was the fundamental efficiency of the inverter. This is a very relevant number in doing calcs about system capacity etc. but the efficiency of inverters will vary with load percentage as well as design. A large inverter with small loads will be much less efficient than a smaller inverter loaded closer to it's capacity.

Having said that,,I use a simple calc for overall system efficiency. In rough numbers, I take 50% of rated panel capacity. Using that number gives me a pretty easy, down and dirty number as to how much I can put in/take out over a given time period.

So for example a 100 watt panel, into a charge controller, figuring battery charging efficiency, inverter efficiency all added in gives me pretty close to 50 watts of useable power at the outlet. Not perfect in every case, but close enough to get what I need.

Tony

PS. I hope I didn't confuse the issue any further
• Registered Users Posts: 17
Re: Trying to understand things....watts/amps/voltage

Okay...I think I'm starting to understand this stuff a little bit. Let me try to put into words what I think I am understanding.

1) I'm more concerned about watts and how many watts an item is using, regardless which system it is plugged into. If it uses 13 watts plugged into household system or 13 watts plugged into the converter.

2) To figure out how many amps is being used on a particular system you base it on the system (ie: 120v household system, or 12v system using an inverter)

#2 will give me an idea of how many amps I'm using from the 12 volt system and thus I can figure how far I'm taking the battery down when I'm using it.

I like the idea of the multiple of 13 times the 120v amperage to give me a guesstimate of how many amps I would be using from the 12 volt system.

Anyway, am I on the right track here?
• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Trying to understand things....watts/amps/voltage

You're getting it,,,

Watts, is watts, is watts. Watts are the measure of energy use(d) and it is the product of multiplying amps times volts. The amount of watts doesn't vary, even if you use it from 12 volts, or 120 volts, or 480 volts. (Not including inverter efficiencies!)

In the real world,, even if you are using your watts at 120vac, IF you are powering that load from a battery (12vdc) what you are interested in knowing is the amount of energy being drawn out of (or put into) the battery.

As Windsun suggests, if you use the "rule of 13" with regards to 12 volt conversion,, you will be pretty close. In fact if all your loads are 120 vac, you can add up all the 120vac loads, (in watts) convert to amps, multiply by 13 and you should have a pretty good idea.

On the other hand,, if you get a Tri-metric,, it will take all the guess work out of it. It wil give you net amp in or out of the battery in real time, state of charge, amp/hours from "full" etc,,, all in real time. You would never have to do any conversions if you didn't want to.

Tony
• Registered Users Posts: 17
Re: Trying to understand things....watts/amps/voltage
icarus wrote: »
On the other hand,, if you get a Tri-metric,, it will take all the guess work out of it. It wil give you net amp in or out of the battery in real time, state of charge, amp/hours from "full" etc,,, all in real time. You would never have to do any conversions if you didn't want to.

Tony

Yeah, I'm looking at picking one of those up. It sounds like it would really help everyone involved, including my son, know the status of the batteries. But...I like to know how to figure things out manually as well; kind of like knowing how to do math manually, but still using a calculator to make things faster. LOL 