Sizing a battery bank. Ah VS wh?

[Sarahps33]

Hi again!

So, I'm trying to size out the battery bank I will need for my husband and I to live off grid in a camper. We don't have tons of time or money, so that's a bit of a factor. We plan on using an inverter for pretty much everything. And using either standard lead acid batts or agm. We plan on getting a few solar panels and possibly a small generator if needed.

I've read tons of forums and watched hundreds of youtube videos and it seems like everyone I look into tells me something vastly different, from calculations and formulas to bank size & voltage and so on.

I believe we need roughly 2510 watt hours.
I calculated this by getting the watts of each appliance and multiplying it by the number of hours we'd use it each day.

I tried to do the same thing with amps. I divided the watts by volts to get amps then multiplied it by how many hours we'd use it. I came up with 202 amp hours.

I've read that some folks use 6v batts and wire them both in series and parallel to equal 12v with higher amps, while others just use 12 in parallel, or wire in series for 24v or even 48v.

What really confuses me here is that from what I understand the concern should be amp hour capacity? Or should it be watts hour capacity?

I've seen people ramp up their watt hours by using 24v and 48v batteries while the amps stay quite low. While others ramp up the amp hours by using 6v golf cart batts and get tons of amp hours.

Which one should I be concerned about and why? Do they equal the same thing essentially?

And how in the world do I figure out a batt bank that will fit our needs with all this contradicting info. I'm stumped, lol.

Thanks for reading, looking forward to your responses.

[StirCrazy]

BFL13 wrote:
Surprisingly, Steve's above post has many errors wrt PWM and MPPT. It would be easier to delete it than try to correct it all. I am not feeling like trying to.

good you think so but it matches how my two systems work perfectly , in low light there is even more of a difference.

[StirCrazy]

2oldman wrote:

BFL13 wrote:
Steve's above post has many errors...

He could use a good spell checker too.

I could

[PerryB67]

I'm dyslexic and have enough problems reading and writing.

So, why would I take seriously a person who: won't capitalize the first word in a sentence, put a period at the end of many sentences, writes a run-on sentence with 85 words and no commas, and/or doesn't bother to spell check. And then I'm supposed to rely on that person for accuracy/knowledge?

I will correct that person if he's quoting me, otherwise, like others here, just ignore his posts.

Perry

[2oldman]

BFL13 wrote:
Steve's above post has many errors...

He could use a good spell checker too.

[BFL13]

Surprisingly, Steve's above post has many errors wrt PWM and MPPT. It would be easier to delete it than try to correct it all. I am not feeling like trying to.

[StirCrazy]

Lwiddis wrote:
“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”

if you looking at it as 25AH then no, they will never put out full power. a 12V panel on a PWM controler will put out 17ish v all day long which is 17v to the controler at 17.6 amps. since a PWM doesnt convert it only used a percentage of that and is faily inefficent so lets say 14.4 at about 15 amps.

this should be fairly close as 480 watts of 12V panels on a PWM in my 5th wheel will only give me about 23 amps.

this changes when you go to something like MPPT controler and say what I have on my camper which is a 72 cell 24V panel. this puts out aproximatly 32 to 36v depending on the panel so with a MPPT controler it is actualy a built in dc to dc charger so it takes that input and converts it directly to the output voltage and you only lose the effeciency of the controler. mine is 98.6% so lets pick 34V at 300 watts that is 8.8 amps but since the battery needs 14.4 it converts it to that output - the effeciency loss so you end up with 295.8 watts of 14.4v and 20.54 amps.

so the same output panel in different configurations with different controlers in ideal conditions can varry widely with out having to get into increased preformance in low light of a split cell 24V panel.

I realy started looking into this when I found I got more charging out of a 325 watt system than my 480 watt system and wondered why.

Steve

[valhalla360]

Lwiddis wrote:
“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”

Rarely do 300 watt solar panels produce 300 watts. The sun must be directly overhead, the temperature must be moderate and very importantly the batteries must be willing to accept 300 watts. During bulk charging they may but during absorption they won’t. The majority of the time recharging wet batteries are in the absorption phase. Lithium is different.

True they don't put out an instantaneous output of 300w often.

But I believe the point was if you take the nominal rating...it will generate total watt-hr per day of about 4 to 5 times the rating (ie: 300w of panels generate 1200-1500w-hr per day). They will actually output for far longer but at lower wattage for much of the day.

Solar also largely solves the issue of acceptance by default. Sometime after 2pm, as the angle of the sun gradually drops, so does the panel output as it gradually tails off for the next 3-4hrs (depends a bit on latitude, time of year and shading). Come 4-5pm, those 300w panels may only be throwing off 20-50w, so battery acceptance is less of an issue.

[Lwiddis]

“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”

Rarely do 300 watt solar panels produce 300 watts. The sun must be directly overhead, the temperature must be moderate and very importantly the batteries must be willing to accept 300 watts. During bulk charging they may but during absorption they won’t. The majority of the time recharging wet batteries are in the absorption phase. Lithium is different.

[valhalla360]

theoldwizard1 wrote:
For example, you want bacon and eggs with toast and coffee for breakfast. Easiest thing to use is an electric frying pan. Is you inverter up to the task of running the frying pan, toaster and coffee pot at the same time ? Throw in a hair dryer and few inverters could handle that.

Your average 30amp trailer on shore power isn't up to that...so it's a bit unrealistic. Even with a 50amp rig, you are going to have to be careful to keep those on separate circuits or you will be popping the breakers.

As far as amp-hr vs watt-hr...watt-hr is the more correct way to do it but as long as you keep the voltage the same, you can shortcut to using amp-hr (ie: you are generating, storing and using 12v all the way through, amp-hr works fine).

The problem with amp-hr is once you add an inverter, the voltage is changing (also DC vs AC). By going to watt-hr, it really doesn't matter (at least not enough to worry about).

Then it's a good idea to oversize by 10-30% because the system isn't going to be 100% efficient. Also, technically, 12v is not not exactly 12v. Depending on battery type and state of charge, it will vary a little from 12v but if you oversize a bit, that shouldn't be an issue.

[pianotuna]

Sarahps33,

Are you in the RV now? If so get a kill-a-watt meter and measure the consumption over a six day period.

From that determine the watt-hours of storage needed.

By extension that will allow you to size the solar wattage to fit your needs.

I suggest looking at the predator 3500 inverter generator from Harbor Freight.

I don't mean to be harsh but 2500 watt-hours per day is pretty low.

[KD4UPL]

Low budget and running an entire RV off grid just don't go together. Batteries are quite expensive right now and getting worse. Inverters, generators, and gas are also expensive.
You can figure amp hours or watt hours, which ever is easier. I tend to use watt hours. Either way that's a big heavy expensive battery bank. 2510 WH per day is a lot. You would need at least twice that in battery capacity bare minimum if you want your batteries to survive so that's 5020 AH. With 12 volt batteries that's 418 AH. You could use 4 6v golf cart batteries wired in series/parallel to get that capacity. I would recommend going larger because draining your batteries to 50% every day is still rather hard on them. Further, if you have a couple cloudy days in a row and the solar isn't producing much you either have to run the generator or have a battery bank big enough to last thru the cloudy days. Most people figure on a battery bank big enough for 3 days of no sun. If you do this you will need over 1,200 AH. Now you're getting into a size that really needs to be done with 2v batteries in series. Something like a Rolls battery.
In order to put back roughly 5 kWh a day with solar panels in the winter in VA you will need about 1,500 watts of perfectly oriented panels; that is set to a tilt angle of 45 degrees and pointed due south. Again, this is bare minimum, more solar would be better.
A $275 generator is a piece of junk. It might be fine for someone who only needs it once a year to power their refrigerator during an 8 hour power outage. You'll need to run a generator for at least 4 or 5 hours a day every day to keep up with your demand if it's cloudy or snow is covering your panels. With the price of gas what it is it won't take long for a higher quality more fuel efficient generator to pay for itself.

[ajriding]

Everything off an inverter screams newbie, knows nothing (no offense) and future regrets in short order...

You haven't watched or read nearly enough it sounds like. I have said many times. if youre going to camp. learn to camp. there are a whole world of camper items, and kitchen camper items that are made to work when camping because 120 volt AC current is not always available and will come at a HUGE premium when you have to make it yourself.

On batteries. 6 volt GC batts are talked about a lot bc they are true deep cycle batts. Marine batts are not true deep cycle. there are true deep cycle batteries other than what Autoparts store sells, but you pay more.
General consensus is AGM is a terrible waste of money for an RV.
Two 100 amp hour 6 volt batts run to make 12 volt will equal 100 amp hours.
Two 12 volt 100a/h batts run together to make 12 will equal 200 a/h.

The one watt for each amp hour on a battery sounds about right. You could go two even to be safe as solar panels really are going to to be the cheapest part in the long-term.

Watt hours is a better way to think than amp hours, but amp hours terminology is hard to make go away, learn both.

What are you running or plan to electrically?

Yes, 24 volt can be an advantage under certain circumstances, but will make things more complicated. I have a compressor fridge that runs off either 12 or 24 volts. It is the main electrical draw. I would go 24 volts if I were to do it over again just for the fridge and then make 12 volts for the other RV items off of whatever converter is needed. Unless you have a specific 24 or 48 volt need then stay 12.

[theoldwizard1]

Sarahps33 wrote:

Not sure what you mean by the 10:1 ratio. From what I've learned so far I've seen that different inverters draw different amount of amps depending on the wattage inverted.

12V in, 120v out !

Sarahps33 wrote:
Generators I've seen locally are pretty cheap, like $275 for 1500w. I'm thinking when its cloudy it may be worth having.

TOO SMALL. At least 2000W

Sarahps33 wrote:

Thanks Piano, I didnt think about the leap the inverter would have to take from 12 as opposed to 48v. I would have liked to run a 12v system but I dont have but a month left before my lease is up. So maybe something for the future.

Can't change that down the road.

Sarahps33 wrote:

Laptop x2
Mp3player charger x2
E-cig charger x2
Phone charger x2
Flashlight charger
Coleman lantern charger
Mini dehumidifier 22.50w
Small fan
Overhead lights
Coffee maker 700w (10mins daily)
4.5 cubic foot fridge (not 12v)
Printer (100-200v/.6a)
Heating pad
And a couple other odds and ends
With that I ended up with 2510wh / 202ah

That's without the coleman mach 3 AC on top.

If you leave the A/C out, a good quality 1500W continuous inverter and 200-225Ah of 12V batteries (two 6V golf cart batteries) would easily cover you for a couple of days.

[wanderingaimles]

If you are on a budget, in a hurry, and are really planning on being mobile. Start with a pair of GC2 6volt batteries, four would be better. If you don't want to get the residential panel, get at least 2, 100 watt panels, a 40 amp mppt controller, and if you foresee yourself wanting to run a rooftop AC, add a generator, inverter units are more expensive, but much quieter.
GC2 batteries Sams club $90 each
mppt controller Renogy/amazon $140
solar panels typically $1 per watt Amazon or others if you dont want the residential panel
3500 inverter generator predator/harbor freight $899

The oversized mppt will allow for adding panels up to 400 watts without having to replace the controller.
along with these items add for a cut out switch for the panels, wire, connectors etc.

When you are travelling, the batteries should be charging from the tow vehicle. As well as the solar.
The items I listed are among the cheaper versions while still getting decent quality.
As to an inverter to power other items You likely will want 2000 watt continious with a 4000 watt surge, you can rewire one or two of the existing circuits to run from the inverter instead of shore power. The limitation at that size will be that items such as a microwave, hairdryer, etc, will have to be used , one at a time, but multiple small items can work while a large one is in use.
Hope this helps.