Battery Bank Sizing

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butchiiii

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Hello, we are looking into solar for our next upgrade. Question what size of battery bank would
I need if we have approx 2000 watts of panels? Not sure what the rule of thumb is. I'm currently
trying to figure out how much power we use over the course of a day. Thanks for the input.
 
butchiiii said:
Hello, we are looking into solar for our next upgrade. Question what size of battery bank would
I need if we have approx 2000 watts of panels? Not sure what the rule of thumb is. I'm currently
trying to figure out how much power we use over the course of a day. Thanks for the input.



2000 Watts on an RV.. that's going to take a lot of room...!

anyway, before you even think you need 2000 watts, first get a good estimate of your daily power needs.

I suggest you setup a spreadsheet and list all the devices you intend to power and for how long.

daily use ( over 24 hours ) is measured in watt hours,  so if you need to say power 5 lights for 6 hrs and they are rated at 5 W
each then the total is 5*6*5 = 150 Whr

a gas fridge is typically 18 W and could be 75% duty cycle so 18 W * 24hrs * 0.75 = 324 Whr
a fantastic fan on #2 setting uses about 24 W.. so if you run it overnight say 10 hrs then 24*10 = 240 Whr

do this for each appliance and total it up. once you have that then I can help you with panel sizing and battery selection.

rules of thumb are just guesswork and give you a compromised system that will be too small or too large and cost more
than a correctly designed system.

I do this for a living, so I know.  :)



 
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.
 
Your plan to know how much power you typically consume in a 24 hour period is a good one. However, I doubt 2000 watts of panels would physically fit on a 33 foot RV's roof, unless they were installed above the AC units on some type of rack. Trying to wedge that many panels in between AC unit(s), antennas, roof vents etc. would likely result in shading, which would render a panel, and any others connected in series, useless. Do you have a real need for that much power, or are you just "going big?"

If you scour the web, you'll find some who suggest 100 watts of solar for every 100 amp hours of battery capacity. After nearly two decades of relying heavily on solar, I don't find that recommendation to be very realistic. It's just not enough solar to fully recharge lead/acid batteries that get used like most RVers use them. It may look doable on paper, because the output of solar panels is measured in lab tests under ideal conditions, but in my experience, it's not enough solar.

I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do. Do they want to replenish some, or all of the power they use? Clearly, your goal is to replenish all of the power you use. That was our goal too, and we achieve it with a lot less than 2000 watts of solar, even though our coach is one of the more power hungry RVs.

As a frame of reference, we've got a 960 watt array and eight Group 31 AGM house batteries, with a total capacity of 840 amp hours, and we almost never have to run our generator. A lead/acid battery bank with 2000 amp hours of capacity (to match a 2000 watt array) would require 19 similar batteries. At nearly $300.00 a piece, that's gonna be a pricey (and heavy) battery bank. Do you even have enough room in your RV for that many batteries?

Of course you don't "need" to have a battery bank with enough capacity to store all the power your panels generate, but it's kind of an expensive waste to buy solar panels and then not use the power they produce. Having said that, there's something to be said for having extra solar power - for a stretch of cloudy days, but a 2000 watt array would be serious overkill for most RVs... IMO. If you installed a lithium battery bank, you could get away with having a lot less battery capacity, and it would take up a lot less space, but the cost would be astronomical.

Kev
 
The battery bank size has little to do with the panel wattage. Instead, you need to determine your daily energy consumption and size the battery bank to hold enough energy (amp-hours) to meet your needs when the solar is not actively generating more power, or at least not as much as you are using.  If the sun shone directly overhead 24/7, you would only need a tiny battery to even out the varying power from the panels, but in most places you need to operate off battery for 12-20 hours/day.

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days. You have to estimate the average charging rate from the panels to determine if they will produce enough power for the batteries to store and later dispense as needed.
 
Isaac-1 said:
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Wondering the same thing.... Unless your finger stuck on the keyboard and you really mean 200W....  This is 2000 Watts on a 40' Newmar Coach with tag axle...
 

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Isaac-1 said:
I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Yup, I put the max I could on mine but with kitchen and bath skylight/vents, shower skylight, fridge vent, kitchen hood vent, satellite and batwing the max was 800. As it was the installer removed the rain cover from the kitchen Fantastic Fan due to shading. I do kinda wish I'd looked harder at the higher density residential panels. Mighta been able to approach 1000.
 
Gary RV_Wizard said:
The battery bank size has little to do with the panel wattage. Instead, you need to determine your daily energy consumption and size the battery bank to hold enough energy (amp-hours) to meet your needs when the solar is not actively generating more power, or at least not as much as you are using.  If the sun shone directly overhead 24/7, you would only need a tiny battery to even out the varying power from the panels, but in most places you need to operate off battery for 12-20 hours/day.

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days. You have to estimate the average charging rate from the panels to determine if they will produce enough power for the batteries to store and later dispense as needed.
In essence, that's correct, but unfortunately, it's one of those, "The devil's in the details" things. A battery bank that wil get you through a 24 hour period could end up being at a 50% or 90% SOC, depending on several factors. If one were only going to boondock occasionally, a lead/acid battery bank that only occasionally ended up at 50% SOC wouldn't suffer too much, but as I learned the hard way, frequent boondocking, and repeatedly discharging the batteries to such a low SOC will kill them prematurely.

When sizing a lead/acid battery bank for frequent boondocking, I'd recommend one with significantly more AH capacity than one that's only used for occasional boondocking. Yes, both may meet your needs and get you through the night, but the larger capacity battery bank will live longer and give you a little padding. We're frequent boondockers and found that we had to increase our AH capacity by about 30% to keep our batteries from being at about 55% each morning.

If you're a frequent boondocker, and you can keep your lead/acid batteries from discharging below 70% (I use 75%) and they're fully recharged soon thereafter, your batteries will live a lot longer. Once you've determined how much AH capacity you'll need to achieve that, you can size the array you'll need to fully recharge them IF that's what you want your solar array to do.

Kev
 
Kevin Means said:
I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do. Do they want to replenish some, or all of the power they use? Clearly, your goal is to replenish all of the power you use. That was our goal too, and we achieve it with a lot less than 2000 watts of solar, even though our coach is one of the more power hungry RVs.

guesswork, pure and simple


As a frame of reference, we've got a 960 watt array and eight Group 31 AGM house batteries, with a total capacity of 840 amp hours, and we almost never have to run our generator. A lead/acid battery bank with 2000 amp hours of capacity (to match a 2000 watt array) would require 19 similar batteries. At nearly $300.00 a piece, that's gonna be a pricey (and heavy) battery bank. Do you even have enough room in your RV for that many batteries?

this is the very mistake i'm trying to help the OP avoid, eight 31 agm's ? no,no,no.. after 40 years experience as an MSEE and 130 solar installs, some over 20KW, i've never seen such a bad solution. I not trying to be a dick here, but I am qualified to make these statements.
 
Gary RV_Wizard said:
The battery bank size has little to do with the panel wattage.

hogwash, battery size and panel watts are dependent on each other

You also size the solar panel wattage to produce the power you expect to use, plus some margin for cloudy days.

correct, also where do you think the energy is stored for cloudy days ?
 
solarman said:
this is the very mistake i'm trying to help the OP avoid, eight 31 agm's ? no,no,no.. after 40 years experience as an MSEE and 130 solar installs, some over 20KW, i've never seen such a bad solution. I not trying to be a dick here, but I am qualified to make these statements.
Please explain this statement.
 
Isaac-1 said:
I have to ask where are you going to put 2,000 watts worth of panels?  I have 400 watts on the roof of my 28 ft motorhome, and there is not all that much empty space, and still have room to access the roof.  I might be able to fit 600 - 700 watts worth of panels If I were to give up room to walk on the roof, and still have roof top vents, air conditioner, TV antenna, etc.

Hi Issac,
Next upgrade will be a 40ft 5th wheel. I was looking at 335 watt panels that are 77"x 39".
I think that there would probably be enough space to put 6 panels. This is all in the planning
stages. I just trying to get all of my home work done 1st.
 
Hi everyone,
Just got back from Sin city. Give me some time to digest your answers. I appreciate
them and will probably have more questions.
Thanks
 
Hi Kev,

Kevin Means said:
I've installed several solar systems and I usually recommend 125 to 150 watts of solar for every 100 amp hours of battery capacity, but it really depends on what the RVer wants to do.

Great advice. I was working with 100Ah/100W but will be sure to take your adjusted rule of thumb in consideration.

Thanks for posting it.

Cheers,
--
  Vall.
 
Hello Solarman,
solarman said:
guesswork, pure and simple

I do not agree. I think rules of thumb, like the one Kev posted, are very useful in the early stages of designing a system, for example before one is able to measure his exact power requirements and is more interested in getting a "ballpark figure" for budget/planning purposes (exactly my case right now).

Of course, as soon as one *can* measure power needs (and before investing in an expensive system), he/she should do it and adjust the initial planning/design accordingly.

Cheers,
--
  Vall.
 
Solarman, initially, there was indeed a fair amount of "guesswork" as I tried to size our battery bank and solar array to meet our needs. So much so that I hired a guy named Bob Sheerer (AKA HandyBob) to help me. A Killowatt meter was used to determine our A/C usage, and a Trimetric RV 2030 was installed to measure our typical DC consumption. In the end, our system works very well.

As long as we have good sunlight, we go to bed with about a 90% SOC and our house batteries are never below 75% SOC in the morning. During the winter months, our batteries are fully charged by about 1:00 PM if we tilt the panels, 3:30 PM if we don't.

We've got a 2800 watt inverter (85% efficient) that runs 24 hours a day to power our 22 CF res fridge, and those two devices alone consume between 180 and 200 amps per day (depending on how often the compressor runs.) The coach has two macerator toilets, a 40" LED TV that runs maybe two hours a night, LED lighting throughout and we charge our phones overnight. The microwave oven and TVs have A/C cutoff switches to eliminate parasitic draws when they're not being used, and we typically use only our catalytic LP heater to heat the coach vs. the Aqua Hot furnace.

I think we were pretty thorough in determining our actual usage, but I'm open to constuctive criticism. Our goals were to prevent our batteries from discharging below 75% SOC overnight, and to reduce generator run time as much as possible. In the end, we had to increase our battery bank's AH capacity by about 30%, and install 960 watts of solar to achieve that. With those goals in mind, what would you have done differently to determine actual usage when sizing a solar array and battery bank?

It's been my experience that most RVers don't monitor their battery's SOC very well, usually because they have no way of doing so. They just want their solar panels to recharge their batteries, and they don't want to have to think about it. Getting them to measure their actual usage, for the purpose of determining how much solar and AH capacity they'll need, can be a lesson in futility.

Kev
 
I can tell you the minimum battery recommendations for an INVERTER but for solar panels???

Well the maximum recommended charge rate is 30% C/20 that is for every 100 amp hours (At teh 20 hour rate) of battery.. 30 amps. 2,000 watts is what. 166 amps.

Of course you'd never get that much but that means you need around 600 amp hours of battery. 3 pair of GC-2 should do it

FOr the inverter one pair per 1,000 watts is recommended.

Now, as to finding room on an RV roof fore 2KW of panels. Not there yet but panel effiency is improving what used to take a humungous panel now takes one that is only muntug and soon will take one that is only mung (Relative word length applies here) as they improve the effiency of the cells.

But I don't think 2KW will fit on my house.....yet....
 
Panel efficiency really has not improved that much in the last 10-15 years, though there has been some improvement in closer cell spacing on the panels.  I forget the exact efficiency improvement, though i did some research on it last year and found since the year 2000 cell efficiency has only went up 4 or 5 percent on commonly available commercial panels.
 
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