# Help understanding Watts and Amp hrs..

Posts: 23Registered Users
I've been googling around and can't seem to find some answers I need to learn. also ordered a kil-o-watt the calculate my wattage later too.

I'm trying to figure out a few things here. and here they are.

Can't seem to grasp this watts to amp hrs thing to easy. say I have a 200 ah battery thats (dod just for example). I have one 200 watt panel that puts out say, 19 volts and 11 amps. also say I have 5 hours of good clear sun on a given day, knowing I'll need a charge controller for the battery.

So in a perfect world it would pump 1000 watts out to the battery in 5 hrs? Also say there is no draw at all, just charging the battery bank.

Am I thinking right that since this battery is putting out 11 amps, that It would take it around 4 days just to charge the (Dod) 200ah battery @ 5 hrs per day?

Also , does It make more since to buy 2 100 watt panels than one 200?

thanks..

• Posts: 205Solar Expert ✭✭
Re: Help understanding Watts and Amp hrs..

What voltage is the battery?

Watts = Volts x Amps
• Posts: 23Registered Users
Re: Help understanding Watts and Amp hrs..
jeffkruse wrote: »
What voltage is the battery?

Watts = Volts x Amps

Sorry, I knew I left something out....It will be a 12 volt.
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..

the answers are here on the forum and here they are again in bold just for you.
Sparky wrote: »
I've been googling around and can't seem to find some answers I need to learn. also ordered a kil-o-watt the calculate my wattage later too.

I'm trying to figure out a few things here. and here they are.

Can't seem to grasp this watts to amp hrs thing to easy. say I have a 200 ah battery thats (dod just for example). I have one 200 watt panel that puts out say, 19 volts and 11 amps. also say I have 5 hours of good clear sun on a given day, knowing I'll need a charge controller for the battery.

watts is volts x amps
watt hours (wh) is volts x amp hours (ah)
amp hours is amps in reference to 1 hour of time.
examples; 200 amps for 1 hr = 200ah,
1 amp for 200hrs = 200ah,
50 amps for 4 hrs = 200ah
dod is depth of discharge and is a percentage rating of how far discharged the battery is.
0% dod is full and 100% dod is dead. if your battery has a capacity of say 220ah and is 50% dod then .5 x 220ah = 110ah
state of charge (soc) is the opposite of dod and indicates the % of capacity in the battery.
100% soc is full and 0% soc is dead.

So in a perfect world it would pump 1000 watts out to the battery in 5 hrs? Also say there is no draw at all, just charging the battery bank.

yes, in a perfect world a 200w pv could give 1000w in 5hrs if under a solar intensity of 1000w/^2, but pv ratings are at 25 degrees c and pvs get much hotter and they get derated, plus many other losses and efficiency factors so forget perfect.

Am I thinking right that since this battery is putting out 11 amps, that It would take it around 4 days just to charge the (Dod) 200ah battery @ 5 hrs per day?

you are confused in using the terminology as the pv would be outputting the charge current in amps to the battery and i already explained what dod is. now if you are charging a dead 200ah battery, which would be 100% dod, then 4 days at 5hrs per day would be 20hrs of charge time at your specified delivered current rate of 11a and would be 220ah and is 20ah over the charge point in your perfect world. in reality, it would taper the charge current some near the end for a longer timeperiod.

Also , does It make more since to buy 2 100 watt panels than one 200?

it soesn't matter unless the 2 100w pvs would cost more than the 200w pv or that the 200w pv is larger in size and the space to mount it in isn't configurable somehow to the 200w pv. an wxample may be for a boat or an rv.

thanks..
• Posts: 23Registered Users
Re: Help understanding Watts and Amp hrs..
niel wrote: »
the answers are here on the forum and here they are again in bold just for you.

Thank you. I'm not trying to be lazy and have everyone do my work for me. I really do appreciate all of everyone's input.

I did a search on it here and couldn't find what I thought I wanted.

After I made my first thread, I've realized I have a ton to learn and don't want to keep making threads day after day....;)

And I was going to use a 975 watt coffee pot too.. :roll:
• Posts: 10,311Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..

no problem as i didn't make you search for it all, but it was only a reminder to all that it is out there. i admit that it is allot for somebody new to it to grasp.
• Posts: 23Registered Users
Re: Help understanding Watts and Amp hrs..
niel wrote: »
no problem as i didn't make you search for it all, but it was only a reminder to all that it is out there. i admit that it is allot for somebody new to it to grasp.

I've copied your post to notepad. It will be of a big help to me. I had two choices on my thread, either make one a get the answer or my have wife strangle me for sitting at the PC for countless hours. I got the answer and saved my Marriage too in one thread.

EDIT......Also, sorry I meant 11 amps that the PV panel was putting out, not the battery...
Re: Help understanding Watts and Amp hrs..

Just to be clear in your first post... you typed about 1,000 watts (5 hours * 200 watts). That should be 1,000 Watt*Hours instead.

Watts is like Miles per hour or Gallons per hour--it is a rate. When you run the load/generator for 5 hours... then it becomes an "amount" (Watt*Hours, Gallons, etc.).

Watt*Hours=200 watts * 5 hours = 1,000 Watt*Hours = 1.0 kWatt*Hours

Note that for many battery system designs, they talk about Amps and Amp*Hours instead of Watts and Watt*Hours.

The conversion is Amp*Voltage=Watts; Amp*Hours*Voltage=Watt*Hours; etc.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Help understanding Watts and Amp hrs..

Hello all newbie here!!!

So let me see if I got this right, if I have a battery at 12v 51ah which should = 612watts and if I want to run 250 watts of power basically I have about 2 hours worth give or take?

Also Sears has a battery I'm looking at it say Amp hours at 20hr rate is 115, does this mean its 115ah?
Re: Help understanding Watts and Amp hrs..

To be clear:

12v * 51 Amp*Hours = 612 Watt*Hours (0.6 kWHrs)

612 WH / 250 W = 2.448 Hours run time

However, there are some limits to that calculations.

Do not run the battery to dead. It will not last very long at all (a few weeks at most). Typically, discharging to 50% capacity will give a longer battery life.

Batteries are not perfect energy storage devices. They have internal resistance and limits to their chemical processes. Normally, battery capacity is rated at a "20 Hour Rate". So--if a battery is listed as having a capacity of 51 Amp*Hours at a 20 hour rate, it may have a capacity of 35 Amp*Hours at a 2 Hour discharge rate.

So, putting the above into an equation:

12 volts * 35 Amp*Hours * 50% (for longer battery life) = 262.5 Watt*Hours of usable capacity

262.5 Watt*Hours / 250 Watts = 1.05 Hours of useful load

There are other limitations too... Such as batteries are temperature sensitive.
• A very cold battery will have much less capacity vs a 77F battery.
• A very hot (100F) battery will "age" faster than a very cold battery.
• As a battery discharges, the electrolyte specific gravity falls (Acid+water becomes mostly water). A fully charged battery will not freeze until -90 F. A 40% state of charge battery will freeze around 16F. A 0% state of charge battery will freeze at ~32F.
• Hot batteries (100F+) self discharge very quickly. Cold batteries (32F and below) will self discharge much slower.
A couple of battery FAQs to read:

www.batteryfaq.org
Deep Cycle Battery FAQ

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Help understanding Watts and Amp hrs..

Regarding the Sears Battery... Yes, 115 should be 115 Amp*Hours.

But, you also have to look at the type of battery. If this was a DieHard car starting battery (a very good car battery)--you would not want to discharge it by more than 15% or so--the plates are very thin (to provide high starting current in a "light weight" battery--plates are not thick enough to take operate in a "deep cycle" operation).

Marine/RV batteries are typical a trade-off between a starting battery and a true deep cycle battery.

A true "Deep Cycle" storage battery will give you the best cycle life (but are not designed to start a car engine).

-Bill

PS: There are other ways of rating batteries too (especially for cars). Reserve Capacity and Cold Cranking Amps are a couple of others. You should confirm that it is 115 is Amp*Hours at a 20 Hour discharge rate.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: Help understanding Watts and Amp hrs..

Thanks BB, and yes the Sears battery I mentioned was a sealed cell marine battery. What I'm trying to do is set up solar system for my work shop to run my fluorescent(240 watts) and an air filter fan(6watts). I use my workshop 2-4 days a week from 1-4 hours at a time. So it sounds like to me a couple of batteries in series, 400 watt inverter , Sunsaver ss20 charge controller and 2@ 15 amp solar panels should do it. So thanks to all for providing this helpful information!!!!
• Posts: 71Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..

BB - I'm rank beginner. Can you comment on significant differences in the AGM /UPS batteries....I have 12volt 100 amphr types.....but I'm hearing that 2 volt with much higher amp hrs. are becoming a standard.

Also I am confused how "load" or ohms is used at all when considering battery bank calculations.

Thanks,
Steven
Re: Help understanding Watts and Amp hrs..

Steven,

First, you can read here for Electricity 101 for Boaters.

Regarding the differences between AGM, Sealed, and standard storage batteries. You can read the battery FAQ here.

Short answer is that AGM batteries are a much nicer battery (don't need to add water, can sustain deeper discharge cycles, withstand freezing better, not as messy with acid fumes + hydrogen gas, better than 90% energy efficient, etc.).

However, AGM's do have downsides too; ~2x the cost, limited lifetime if over charged leading to easy to damage for new users, etc.

Sealed Gel Batteries have not been to recommended here (issues that gelled electrolyte may not withstand high currents as well.

Plain old Storage/Wet Cell Batteries are certainly cheaper and pretty rugged (except if left less than 75% state of charge for more than a day or so--then sulfates and looses capacity). Cells require maintenance (refilling with distilled water, cleaning terminals, venting to prevent hydrogen gas buildup, around 80%-90% efficient, etc.). Generally, recommended for "newbies" and their first battery system. As long as kept reasonably charged and filled with clean distilled water--they will last pretty well. And if "mistakes where made"--less money lost on the educational event.

Simple loads (like resistors) behave according to the formula:

V=I*R (voltage in Volts = current in Amps * resistance in Ohms)
I=V/R
R=V/I

And the rate of power consumed is called Watts:

P in Watts = V^2 / R = I^2 * R = V * I (example "v" squared divided by "r")

Now you know the "rate" (like gallons per hour for water flow) of energy usage--you need to know how much energy is used per unit time (in physics, that would have been Seconds--but for homes and our needs, we use Hours so that we don't get really big numbers).

So, if you have a load that takes 2 amps at 120 volts for 3 hours:

P=I*V= 2 a * 120 v = 240 watts

And the Work or Energy used is:

E=Watts * Time (in hours) = 240 Watts * 3 Hours = 720 Watt*Hours

Note that Watt*Hours is just the same as your home electric bill in kWatt*Hours times a factor of 1,000.

3kWhrs = 3,000 Watt*Hours
720 Watt*Hours = 0.72 kWhrs

For battery systems--many times you will see their ratings in Amp*Hours instead of Watt*Hours... The results are pretty much the same, but Amp*Hours needs to know the battery bank voltage. So, for example, assuming 5 amps at 12 volts for 4 hours:

E=W*T=I*V*T=5 a * 12 v * 4 h=240 Watts*Hours
AH=Amps*Time=5 a * 4 h=20 Amp*Hours (at 12 volts)

Notice that the only difference is the 12 volts is "assumed" in this particular AH equation. Voltages of 12, 24, 48 volts are common ones used here for Battery Banks.

Now we get into Battery Ratings... The common one used for Storage Batteries is the 20 Hour Rate... Basically, if a battery supplied current for 20 Hours before it went dead, what would that current be.

For example a 100 AH Battery rated at 20 Hour rate would supply:

100 AH / 20 H = 5 amps for 20 hours

Note that there are other Battery Hour ratings (1 hour, 10 hour, 20 hour, 100 hour, etc.)... Basically, the faster the current is withdrawn for the battery, the less apparent storage it has (battery gets hot if discharged/charged with very heavy currents).

For our needs, the 20 Hour Rate works out pretty well for our basic, back of the envelope, calculations.

Back to your question about Ohms and loads... Remembering the equation V=I*R---Using basic algebra, I=V/R and you can see that for a fixed voltage, low Ohms lets more current through the load vs higher Ohms (resistance).

However, with today's equipment--just knowing the Ohms of the load is not actually enough. Loads (current) are not simple linear equations anymore. Many are "constant power" type loads (motors, electronics, inverters, MPPT charge controllers electronic lamps, etc.). They are typically governed by:

P=V*I (power = voltage * current)

Assuming P=constant--you can see if you raise the voltage, the current will drop, and vise versa.

Batteries are not constant voltage devices... When charging their voltage may be ~14.2 volts (for a 12 volt battery). And when discharging they may be 12.6-11.0 volts as they discharge.

So, many times we will convert to Watts (instead of Amp*Hours) to account for these losses.

Please feel free to ask more questions--I am not sure if I answered what you needed to know or not.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 6,869Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..
StevenB wrote: »
I'm hearing that 2 volt with much higher amp hrs. are becoming a standard.

If you have a standard size battery case "Group 24" (that's a standard size that's easy to lift and move) it can only hold so much power. Say 100A @ 12V

If you need more power, it used to be common to bolt another 12v battery in parallel and call it done. (12V @ 200A) That lead to "current hogging" problems, so to solve it, folks put 2, 6V 200A batteries in SERIES (to make 12V @200A)

Then the 42" TV's came out, dual screen laptops, and keg-er-rators. People needed more power, and knowing about the parallel/hogging problem, the cell count in batteries went down to 2 cells, providing 4V @300A . Now string 3 of them in series,
and you get 12V @300A, and no parallel wiring problems. Batteries are available in 2V cells also, if you are really needing the 7.1 sound system to go with the dish tv .

So the ez to handle case size of Group 24 stays the same (about 50 lbs) but by going to lower cell count, and lower voltages, you can still move them about, and not get a single honking monster 12V 300A battery that weighs 180#.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Posts: 71Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..

oh boy. terrific stuff. I have to print this out.
I have 16 12v AGMs in parallel and series to make a 48 volt system...A guy wanting to sell me big amp 2 volt batteries said that lots of 12 volts batteries together had troubles charging up evenly.............??
Re: Help understanding Watts and Amp hrs..

I assume you have 4 parallel strings of 4 batteries each 12 volt batteries in series.

Generally, it gets more difficult to balance large numbers of parallel batteries... The little differences in resistance between each bank (longer wire here, dirty connection there, a little corrosion, etc.) can lead to battery imbalance (even a 1/10 of a volt with a 12 volt battery can be a 10% state of charge difference).

And trying to parallel batteries of different ages, brands, types (AGM/flooded) is very difficult to achieve proper bank operation (small differences in cell age/chemistry make for small voltage differences which will change state of charge between strings).

If you decide to go for the "good deal" -- keep the "new" string separate from your other bank(s). Either completely separate Off-Grid systems or use an isolation switch to allow you to switch between banks.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 71Solar Expert ✭✭✭✭
Re: Help understanding Watts and Amp hrs..

Yes hooked up like you stated. The batteries were purchased from a guy who take them out of renovating corporate ups setups...so they are the same lot. I'm trying to understand the principals of charge, keep them healthy...........and until I go to "phase2" (adding more pv and turbines) ....learn to compensate in healthy ways for the low power production. Thanks!
(I'm printing out all these amazing replies I've been getting...I'll try to follow up with questions based on the reading).
• Posts: 1,164Solar Expert ✭✭
Re: Help understanding Watts and Amp hrs..

See also "No such thing as watts per hour" on the Homepower site.