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.

[Sarahps33]

Oh, just realized I forgot to add the diesel block heater to that 😕

[Sarahps33]

The camper I have was already ran with 120ac electric, nothing 12v to plug into except ciggertte lighter.

I've heard about the inverter powerless, I thought most of them were 80-90 % efficient,  so I'd lose that 10-20%.

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.

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

OldWizard said "when you want Amp-hours at 12V (instead of 120V) you need to multiply be 10, so about 31,000 Ah !"  I'm confused on why I would need to multiple that if I'm using an inverter and everything I'd being turned into AC?

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.

Also you mentioned "divide the watt-hours used in a day by 5 (example 2200 / 5 = 440 watts of panels)." Is the '5' in that formula the hours of sunlight a day? 2510/4.5 = 113
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...

To clarify no electric frying pans and no toasters lol. Just a propane stove top. We will be mobile and hope not to encounter weather any less than 40 degrees (at night.)

202ah sounds kinda like a lot to me considering what we plan on running, if anyone feels like reading a list, feel free if not I totally understand lol.

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.

[pianotuna]

theoldwizard1 wrote:
Is you inverter up to the task of running the frying pan, toaster and coffee pot at the same time ?

That would be a slam dunk for my system. Frying pan = 1200 watts max, toaster 800 watts, coffee pot 400 watts. Heck, I could add the fridge, too!

[pianotuna]

Use one or the other. I prefer watt-hours.

If planning on using an inverter for everything (which is not recommended), forget about 12 volt and jump to 48 volts. Why? Because the inverter may be a lot more efficient at 48 volts compared to 12 volts. Cabling between the battery bank will be far cheaper.

If you are stationary, and have a beer budget, use six volt golf cart batteries. Lead acid batteries do want to be fully charged as often as humanly possible. They needs must be equalized at least every 30 days, and once a week would be a better time frame.

To find the solar wattage needed go to a site that lists the solar insulation hours for your location. If that is too much trouble--then divide the watt-hours used in a day by 5 (example 2200 / 5 = 440 watts of panels).

For full time off grid the battery bank size ought to reflect at least 3 days of no sun.

If you are mobile and do not plan on cold weather use, go to 24 volt lifepo4 batteries with 2 in series. Or better yet, "roll your own". It will be needful to have a proper battery management system. Li are great, but they are UNFORGIVING. They can be seriously damaged or destroyed by just one seeming small error on your part (the human factor).

A back up generator will be needful. It needs to be large enough to meet the daily requirements AND recharge the battery bank.

[2oldman]

theoldwizard1 wrote:
Is you inverter up to the task of running the frying pan, toaster and coffee pot at the same time ?

My inverter might be, but my battery and the wiring wouldn't.

[theoldwizard1]

Re-reading what I just wrote and some of what others said in our other thread, none of this IS AN EXACT SCIENCE ! Too many variables !!

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.

Start by sizing your inverter. You will likely have to install a MicroAir EasyStart on you A/C to have a CHANCE at getting it to start. Wild Guess, you will need a 2000/3000W inverter. That should run most A/C units (with an EasyStart installed). How long depends on how big your battery bank is, but we will get to that.

When inverter shopping, pay the extra for an inverter/charger/auto transfer switch. Some even have a separate generator input. Get rid of your current converter. This will make installation and use easier. Also, you want a 24V inverter.

Prices on LiFePO4 batteries are dropping fast. Battle Born is still the Cadillac of batteries, but there are others out there that are a fraction of the price. (Watch DIY Solar on YouTube for reviews.) Another wild guess, you will need about 200Ah @ 24V.

[theoldwizard1]

Sarahps33 wrote:
Hi again!
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.

Good start, but you have to factor in the losses from the inverter. I will use 25% although many only waste shot 10%, so that is about 3100 watt-hours. Wh are Wh regardless of the voltage, but when you want Amp-hours at 12V (instead of 120V) you need to multiply be 10, so about 31,000 Ah !

Lead acid batteries (6V, 12V, flooded (wet), AGM) should only be discharged to about 80% of their State of Charge (SOC). That would lead you to believe that you need 155,000 Ah at 12v! Cutting it by 4 for 48V Battery is Bank is still 38,750.

You need to re-do your load calculation. What you really need us a 30A/50A Kill-a-Watt. I have never seen one. With it you can measure your usage directly.

[wanderingaimles]

In looking at it for a "bang for your buck" type of application look at the 6 volt GC2 batteries, Sams club and others will price the around $100 and 2 will give you a start on a system, with a pretty easy upgrade to four later.
Again, if looking at the best value, a member in here KD4UPL, lives down in Staunton, he is a solar contractor who installs larger systems for homes. If you have room for the 36 volt residential panels, he may have some leftover from larger jobs, he was great for assisting me with 325 watt panels. Size of these panels is around 66"x 39" so make sure you can fit them on your roof. You will have to install a MPPT controller to reduce the voltage to your battery setup.
Price on these larger panels is down to about 65 cents a watt, and you may find some for even less. Because of size, shipping on these is high, that's why I suggest contacting the guy in Staunton, you can save on the shipping and just pick up if he has any.
If your location is to be permanent there in Winchester you could even mount them off to the side of the camper.

[Itinerant1]

Pianotuna gave a good link in your other thread to read and learn from.

Read this a couple times till you understand the 12v side of life. There is a couple parts.

http://www.marxrv.com/12volt/12volt.htm

[Itinerant1]

With little time and little money. Conservation of power will be needed. 200ah daily is a tall order and you will be carrying a lot of dead lead if you have a reserve of a couple days. No sense beating up that horse again it's been beaten in your other thread.

12v x 100ah= 1,200wh
24v x 50ah= 1,200wh
48v x 25ah= 1,200wh

You are confusing yourself now and will most likely using 12v that's in the trailer already.

[Ed_Gee]

You were making a big mistake in your AmpHour calculation -- assuming all your appliances run off 120VAC. Your Inverter will draw approximately 100 Amps from the battery in order to provide 10 Amps of 120 Volts AC. Rule of thumb is a ratio of 10 to 1.

[Lwiddis]

“We plan on getting a few solar panels and possibly a small generator if needed.”

Generally you’ll need one solar watt for each wet battery amp as a minimum. Lithium is more IMO. Unsuccessful solar systems are always too small. Generators are costly, loud and require maintenance. Adequate solar eliminates their use most days.

[Lwiddis]

Reconsider using an inverter for pretty much everything because there is power loss in going from 12 to 120 volt. Do what you can with 12 volts items. Using 202 amps per day is a bunch of power and would necessitate a wet battery bank of at least 400 amps. A 300 amp Lithium battery would be smaller and weigh less while offering 3000 to 5000 cycles vs. 300 to 500 on wet or AMGs. About $1000.

[2oldman]

The chances you'll be going to 24 or 48 volts is probably about zero, so amp-hours is fine. I think we've already been around the block on this.