Amp Hour Calculation

[tcrenshaw]

Okay let me see if I have this right. I have added up that if I were to run several of my home electronic devices all at the same time they would pull around 2300 watts from 120v AC or around 19amps. If I had a 12v source, say a Rolls battery and I was using an inverter, lets leave the drain the inverter will have out of the picture for now; I pull this 2300 watts or 19 amps for 6 hours from my battery, so that's 19a x 6h = 114AH, right? So something like a Rolls 275AH battery should be able to supply me with the power I need for the full six hours? Or am I not even in the right ball park on this one? Don't even think about recharging the battery. I'm just wondering if this calculation is correct or not.

[TomW]

tcrenshaw;

First lets say you draw 20 amps at 120 volts for ease of the numbers. If you draw the same power from a battery bank at 12 volts that will be approximately 200 amps depending on exact battery voltage. for 6 hours time you will consume 1200 amp hours. This all is using round numbers and assumes no losses so real world use will be more along the lines of 1200 amp hours plus 15% or so. Now we are up in the 1400 AH range or thereabouts

T

[tcrenshaw]

Okay, that's what I was thinking, that my numbers were ten times off. So in fact for the draw I would need say five 275AH battery's. Then the fun starts, making sure they are fully charged the next day for the next evenings use. Thanks for the information. I'm trying to figure out what I'm going to need, what I can afford and of course what I can do without. Finding I can do without a lot! Thanks again,

Todd

[JW]

Hey Tom, is it safe to say a heavy-duty automotive battery is only worth 275AH? If so, does that mean five auto batteries in parallel has 1400AH capacity?, when fully charged. which could run a 200 amp load for 6 hours? considering 15%percent losses. Also what is the projected state of charge in the batteries experiencing this type of load at the end of six hours?

JW

[old55olds]

No no no

Welcome to the world of salesmanship, Just bend the truth and make it look impressive. You are thinking of CCA or cold Cranking Amps. In truth this battery will put out this amperage for about 20 seconds tops. Real batteries are rated in amp hour rate. My forklift battery (approx 13" X 30 " X 22 " high will put out 250 amps for 8 hours Max

Ken

[tcrenshaw]

Okay so going on to the next step. I have to recharge my battery bank of five batteries each day. Now still leaving out the 15% loss and I'll toss in the fact that I'm calculating on 5 hours of 100% output capacity of my PV panels which of course is also not true (no tracking devices ..etc), but anyway so my PV panels output is around 18V at 40~42 watts in direct sunlight. Those will go through a charger controller and into my battery bank. Figuring to get the approximate 14,400 watts that I used the night before I have taken the 14,400w / 40w = 360 panels to produce in one hour the power used. Now I take the 360 / 5, again assuming 5 hours of direct full sunlight on the panels and I come up with 72 panels. So basically in 5 hours with 72 PV panels I can recharge the battery bank of what was used the night before if there were no line losses, lack of having a full five hours of direct sunlight and a multitude of other things to account for.  Am I getting this or am I still out there in left field like my original post? I am finding out one thing for sure - we humans burn up a ton of energy because it's readily available.

Thanks,

Todd

[tcrenshaw]

Yep I found that one out while looking at batteries and finding most car batteries are measured in CCA while true deep cycles use the AH measurement over 20 hours and where deep cycles can go as low as 20% of their charge with no damage, while if you do that to an auto battery, it's bye bye or at least the life of the battery is shortened. I think I'm starting to understand some of this stuff, I hope!

Todd

[windcruiser]

We human are just inefficient in using the energy supplies we have. If you try your math backwards minus the inverter you will realise you have a power station in your hands.

Simon

[bob g]

better do alot more research on this plan

all of these efficiency losses add up quick,

i wouldnt run a set of batteries below 50% SOC, to maximize life cycle

charging to approx 80% state of charge can be done fairly quickly if you have the charging capacity to do so, but we are talking perhaps 300 amps + to do so.

if you can get up to this level of charging then you can get back to 80% in an hour or two.  but.....

getting from 80% to full charge will take much longer and at a much reduced rate...

there is alot to consider before you lock in on a plan.

i myself wouldnt consider a 1400 plus amp hour system that cycles to any significant depth of charge and has to be brought back up to full in such a short amount of time, without a broad knowlege of the system and some level of sophistication in the charging routine.

i like your choice of battery manufactures tho'  

bob g

[tcrenshaw]

Hi Bob and yes I agree the figures are very un-realistic. This really isn't so much a plan because the equipment being run are all my computers, my TV, DVD, sub woofer, receiver, a few 60 watt lights, satellite telephone, satellite Internet, laser printer, network switches, wireless router, page scanner, tape backup, USB hub, okay you get the picture - more equipment running all at the same time for six hours than I would even think about running even here at my home in Texas, but especially while in the back country.

I'm just trying to get an idea on the math to make sure I'm doing that correctly and the information/help I've found on this forum has been great. I like asking questions/reading here because I don't see the "salesman" approach to answers. I like the real world stuff, kind of like what you posted. Anyway lighting will most likely be cold cathode, computer equipment will be optimized most likely down to a laptop at most and I'm sure I can greatly reduce power consumption. Audio equipment can also be greatly reduced along with a host of other power saving measures, reductions including discontinued use of some items. I mean how much TV am I really going to watch? I don't watch but a few hours a week now. So the goal is just to try and see if the math is basically right.

In fact when I start building I'm sure I'll be starting out very small. Just enough lighting to not have to use gas lanterns much and maybe enough power generation for some sort of communications device (radio, satellite phone ..etc). I'm sure I'll be busy enough with heating, water and all the other alternative methods of living of a back country off the grid lifestyle and home require.

So I haven't seen any objections to the math so far so I'm taking that as I got it right? Of course DC is very inefficient so I know there's a lot of loss there, along with loss with the inverter, lack of truly having 5 hours of direct perfect sunlight on all panels ..etc and much more.  

Thanks everyone for the comments and help.

[nothing to lose]

Maybe if your trying to get the math right you should try to use more realistic figures also so you can find were you truely stand as you go.

Like add up what your really going to want to use. Like one real computer and monitor, or 2? As I recall looking at a chart, 300watts for Pc and 17" monitor. That would be about 25amps @ 12vdc per hour. How many you want to run at once and for how long? I do 3 for various times, that adds up to like 16hrs a day for 3 people, that's 400dc amps a day for computers here. Often they all run at once so my inverter will have to handle the 1200watt load, but most the time only one is running.

 When I kill the grid I will be dropping back to around 4hrs a day for computers, 100amps supposedly. So that's how I am figuring mine.

[tcrenshaw]

Well I see what you're getting at but to be honest, using general figures or real world figures doesn't make a difference in weather I'm doing the math right or not. The formula if you can call it that is still the same and that's what I'm verifying right now. Along with that I really have little idea of what I'll be using in say 10 to 20 years when I'm ready to retire. There's two ways to look at this. One is that in the future, electrical devices will be more efficient so we'll use less power. The other is, because the devices are more efficient we'll use more devices and thus use more power. I think the latter is more accurate. Trying to come up with real world figures that far in the future is not really possible. You don't need real world figures to verify that the math is correct, only to know what you are really going to need when you're ready to build. Right now I'm just in the experimentation stages of my project, playing around and learning. Once I get some real data such as what real losses I'm going to see from my testing then I'll start moving into actual planning of what I'm going to need. I've been really surprised by some of the simplest ways to conserve energy, and equally shocked at how much I waste! I seriously doubt I'll even use 1/4 to 1/2 of the power from the numbers I provided for checking the math. After all I think instead of being inside my house in the middle of the back country, watching a DVD on a big screen TV with full surround sound while my computer is running full tilt printing whatever from the scanner while the tape backup runs with music blaring out the computer speakers while I'm on the phone would be pretty dumb when I could be out on the front porch with my telescope watching the night sky

Thanks for your input. The more the better.

[nothing to lose]

Ok, you got point about 10-20 years away not Knowing anything now.

Heck by then we will be creating our own power from garbage as we need it anyway and not need batteries or inverters anyway

Since oil prices are not likely to come down and most likely go far far higher most likely, I beleave the next 10-20 years we will see many new things pop up. They exist today but compete with oil and gasolene so we don't here much about them. Probably the oil and power compaines will get a monopoly on the market as people demand alternatives and once that happens the monopolies have protected there profits and will let us have RE and a price that suits them.

For now conserve, and build your own before they get control of it.

So why wait, figure up some real needs and numbers, check em out a few times and put together part of the system and see how it works.

Conserving is good, making your own is better

[tcrenshaw]

Hey we can make power from our own garbage now, it's just as you point out though, competing sources are cheaper at this time. A friend of mine who lives in Colorado is looking into BioDiesel for his truck and a waste oil furnace for his home. I was really surprised at how much heat a simple waste oil furnace can put out. And of course the fuel is pretty much free for the taking.

I've always been a conserver of electricity and gas. I have all gas appliances and low power lighting. That's not far enough though. I'm looking at CCFL lighting as a possible alternative. From what I've been reading they put out much more light than the more common fluorescent lights for the same or even less power consumed.

You're right on about why wait, and I'm not. I have two PV panels now and have ordered the 5Lbs box from eBay to see what I can do with those. Hopefully I'll get six more panels. If that goes well I'll go ahead and order another box and hopefully have about 12 panels here soon. I know what I need for batteries (type that is, not AH, but I have a pretty good idea, testing will help figure that one out). I also have figured in the inverters and found some rather nice ones that will work well for testing and may move into actual use. I'm still learning how and what I need to tie the power I'll make into my house and the AC from the power company.

[troy]

You're on the right track now.  Just remember that watts are watts.  If your device uses 1000 watts for one hour, that's one kilowatt hour.  Your battery (at a different voltage) still has to come up with 1000 watts for one hour (plus inefficiencies.)

Volts x amps = watts

watts x hours = watt hours

Flooded lead acid batteries are about 70-75% efficient.  Maybe 80% if you're a saint and you're lucky too.

Inverters may do a bit better on efficiency, but I'd count on 80% and be ecstatic if I did better than that.  So for overall efficiency of the battery inverter portion of your system, count on:

.80 x .75 = 60% overall efficiency.

Good luck and have fun!

troy

[tcrenshaw]

Thanks Troy! Yep, it took a bit for me to find out watts is watts. 60% efficiency? Ouch, I don't think that percentage can be called efficient. Well I'm going to start testing more soon. I received my solar cells from eBay today and out of 232 cells I've found that 150 are un-damaged. The other 82 are broken in some way but are still usable in some way. I'm going to try for four panels out of the un-damaged cells. Along with the two panels I already have that should give me about 240 watts under ideal conditions.