Amp draw vs battery bank & solar array = totally confused!

[Sarahps33]

Hey there, so I'm hoping some of yall have already dealt with this and could shed a little light.

Things aren't making since to me. I have checked each appliance's output power that I plan on bringing on the road.

For example my laptop says 20v/2.25a on the charger.
For each device like this I went to this website for a quick calculation: https://www.batterystuff.com/kb/tools/dc-to-ac-amperage-conversion-run-through-an-inverter.html

And I'd turn it into 12v DC - giving me the amp draw.

In the case of my laptop it was converted to 4.14a
I took those amps and multiplied it by how many hours a day I would use it.
The most I'd ever use it in one day would likely be 7hrs.
4.14a x 7hrs = 28.98a

So that tells me how many amps I'd use from that laptop a day, right? Am I doing it right so far? Lol.

I did this for my coffee maker, phone charger, flashlights, printer, and so on and came up with 348 amps that we could "potentially" use in a day if we used and charged everything we owned in a single day (I think that's overkill, but just in case right.)

And that's without a fridge. We want a fridge with a freezer so we can be off grid for longer periods of time. I have one now and if it ran 12hrs outta 24 it would be an additional 192 amps.

So the total with a fridge is 539

That seems like an awful lot to me. I dont understand how people have whole gaming systems, fridges, and microwaves and can run off a small batt bank.

Did I mess up somewhere?

I'm reading that AGM's have a depth of discharge rate of 80-50%. If we used 80% of each batt then we'd need seven 100ah batts just to run for one day! (539a/80a=6.7 batts at 100ah). If we wanted enough back up power for 2 days (say it was cloudy out) than we need 7x3 = twenty one 100ah batteries. That's what I dont understand....

But! I also understand that this is where solar charging rates come in. I've only seen 60a being one of the highest charge controllers. So does that mean I can charge with 60 amps an hour with my solar panels? (Assuming I have full sun light?) And say I have 4 hrs of full sunlight a day 4hrs x 60 amps = 240 amps charged per day.

With an amp draw of 539 it would take roughly 2 days and 1 hour to put that back into the battery bank. And that would still only be replenishing our daily draw but not as fast as we use it and wouldnt provide those 2 days of cloudy weather cushion.

I'm so in need of some help here, lol. I cant wrap my head around it.

Thank you so much in advance for any info you can give me.

[Itinerant1]

The OP has mentioned they want to go fulltime and mostly away from hookups. Obviously they are very new to understanding battery power with different draws from items in the trailer, then add solar into the mix and it might be a little more confusing to them during this time of the year with caviar power wants.

Having 1,280w of solar might be considered moderate amount by some. Unless you don't mind dragging out the generator and listening to it for long periods of time then there shouldn't be no issues at all.

I personally don't like listening to them even mine and it's only eu2200i so it gets run only when necessary and try to use it for the least amount of time. Over the past 40 days boondocking traveling out of CO, UT, NV and AZ now. Shorter daylight hours, overcast and rainy days and one location was no option than pointing the trailer west which is just horrible for my solar production potential. Batteries living in the PSOC (bouncing between 25%-95%) using whatever is needed at anytime during this period of time with the daily use of 200-250ah. Decided yesterday to do a full charge using the generator, it was another overcast/ drizzle day and at 1:45pm (47% SOC) with solar production of only 50ah total for the entire day. The batteries needed 265ah (53%) taking 2 3/4 hours to fill the batteries completely charging to 14.1v at 100a.

If the OP can afford them LiFePO4 batteries can have alot of benefits over dead lead. Weight is major factor when living fulltime and traveling. My entire system solar panels, inverter/ charger, solar controller, batteries and misc wiring was 500# total. Just food for thought.

[valhalla360]

Itinerant1 wrote:
Prepare for long generator run times to care for the batteries if longevity might be a concern. Solar will help but won't do much on cloudy/ rainy days or the shoulder seasons.

A moderately sized solar system with generator can address this.

Lead Acid batteries can take a fairly large bulk charge up to around 80-90%...it's that last 10-20% that are slow...so...

Run the generator in the morning to bulk charge quickly to 80-90%, then let the solar top up the batteries slowly over the rest of the day.

Yes, occasionally, the solar won't do the top up but as long as it's not all the time, the batteries should remain happy.

[Gdetrailer]

For some reason the anti robot anti spam filter keeps flagging my posts with more than one link :h

So, I will add..

For laptops, you can buy a "universal" laptop charger which can operate directly for 12V battery and some offer both 120V and 12V capability. The ouptut is universal meaning by just changing the tip to one that fits your laptop the voltage and amperage is set correctly for your laptop. Looks like this..



That is the one my DD has been using on a Dell two in one laptop with a i7 processor, has worked well on 120v and 12V.

Can be found HERE

The more low power items you can run directly from the battery the better off you are, running those items through any 120V inverter then back to 5V or 18-19V for laptop wastes a lot of your RV battery capacity.

Some items, just cannot avoid using an inverter, most TVs the 120V power supplies are built into the device as well as sat receivers.

As far as trying to figure out your power needs from the wattage listed on devices goes, you most likely will way over estimate your power needs. Electrical devices in order to get certifications from UL or CSA they must list the absolute max power draw rating. Most devices will draw considerably less than what the name plate says.

Because of this, to get a more accurate reading one must monitor the actual power draw. One such device is called a "Kill-O-watt"..

[Gdetrailer]

Running everything through an inverter can waste a lot of battery power depending on just how big or small the load is.

For devices like cellphones, tablets, laptops and any other low power device you are better off buying power supplies/chargers which can take the 12V directly from the battery and convert that to 5V (USB items) or 18V-19V for laptops.

Much more efficient than charging cellphones, tablets and laptops from 120V inverter.

For 5V USB charging, you simply use a cig lighter plug in USB charger, like this.



If you do not have a 12V cig lighter power port in your RV, they can be added fairly easily.

[pianotuna]

Sarahps33 wrote:


We dont plan on being plugged into shore power hardly ever, so I'm looking for something "off grid."

I talked to a friend yesterday who dabbles in electric and he pointed out one huge mistake he thinks I made. I plan on running everything through an inverter.

Running everything off the inverter immediately causes about 12% "waste" from the overhead of the inverter.

Golf cart batteries are not smaller.

I suggest reading the link I sent earlier in the thread. Your understanding of batteries, solar, and electricity are flawed.

[Itinerant1]

Prepare for long generator run times to care for the batteries if longevity might be a concern. Solar will help but won't do much on cloudy/ rainy days or the shoulder seasons.

I fulltime and boondock continously. From my experience living like I'm on hookups using microwave whenever I want, make a couple pots of coffee daily, charge phone/ tablets/ laptops, watch tv/ dvds for hours if we want, have the furnace going and any other appliances in the 5th wheel and inverter on 24/7. With my system (see sig) during spring, summer,late fall we'll average 35% +/- DOD or 175ah daily. Late fall, winter with shorter day light hours for solar DOD goes up to 45% +/- or 225ah daily. Just to give an idea, if you're more of a minimalist or conserve more you can certainly get by on less. 😉

[Sarahps33]

Thank you everyone for the replies and happy holidays!

My husband and I plan on living in the camper full time and travel for at least a year.

We dont plan on being plugged into shore power hardly ever, so I'm looking for something "off grid."

I talked to a friend yesterday who dabbles in electric and he pointed out one huge mistake he thinks I made. I plan on running everything through an inverter. So he said I wouldnt need to multiply the amps by 12v because I'm running everything off AC via the inverter. So, the amps would not increase, does this sound right?

Thanks for pointing out that the chargers and etc list the maximum volts & amps, that's a good point.

I have also recently stumbled upon people using 6v batteries which increases amp hours quite a bit. I've seen for example say they have six 6v batteries, they'll wire pairs in series to create 12v then run them in parallel to up the amp hours. They sell relatively small 6v batts that start at 225ah.

After learning all this, things seem much more possibly without having to use a huge bank and or huge array.

When I finish my new list of appliances/watts/wh etc - I'll share it here to see if it makes sense and for future folks with the same question.

[2oldman]

and now totally gone

[Ivylog]

Welcome to the forums and I’d say you’re worried about some items that I do not worry about… charging electronic as their usage is minor. Even a residential refrigerator is not as much as most think…1.2 to 1.4 Kw/day or 8 amps DC for 10 hours…80AH or one extra battery.
Now the big items like a coffee maker or the micro is real easy, you crank the generator when using them so you are charging them instead of heavily depleting your batteries.
As for solar, you want at least as many watts as you have AH of batteries and double if the batteries are Lithium.

[CA_Traveler]

Good input and lots of questions on the OPs usage. 7 hours on a laptop might suggest it's work related or games or ???

[SJ-Chris]

wanderingaimlessly wrote:

A more modest solar and battery package that meets most of your needs on a "most of the time" basis, with a portable generator to supplement charging during bad weather, or high use periods as a backup is very common.

^^^This

My opinion: The BEST way to see how much power each of your items will ACTUALLY drain from your batteries is to purchase a Battery Monitor. They have real fancy ones available. But I have used the $45 Aili (from Amazon...or try google) battery monitors and have been VERY happy with them. Pretty easy to set up and once you do it will take ALL guesswork out of your equations.

I'm not sure if you are planning on living in your RV full time and having the ability to run 100% off batteries for multiple (even cloudy) days is something that is critical for you or not. If you're just someone who uses their RV for occasional trips for a few days or a couple weeks, and you have a generator as a backup charging, or perhaps you drive (which charges your batteries) a bit every couple of days to a new destination.

Most casual (not-fulltimers) RV users can probably get by with 200-400AH of cheap lead acid batteries and 300-500w of solar power (assuming you are boondocking).

It would help to know HOW you plan on using your RV. How long will your trips be? Will you be boondocking for extended periods of time? I'm sure you are aware that every time you plug in at a campsite for a day, or drive for 4-5hrs, your batteries will generally get a good charging.

Before you overdue it on adding way too much batteries, and/or very expensive Lithium batteries, and/or a huge solar system....Start small and go on a trip and see how your battery needs work out.

Happy Camping!
Chris

[wanderingaimles]

Another consideration is how you will be using/consuming the power. The fridge with freezer, are you saying you want a large residential type unit for a cabin? Or is a 10-14 cuft Gas/elec rv type unit sufficient?
Big difference in electrical consumption between the two.
Also trying to scale the solar to worst case scenario is forcing you to an extreme sized unit in both panels and batteries.
Attacking the issue from multiple fronts is usually the best.
A more modest solar and battery package that meets most of your needs on a "most of the time" basis, with a portable generator to supplement charging during bad weather, or high use periods as a backup is very common.
Also off grid folks often tend to eliminate things like electric coffeemakers, and use stovetop methods to save electricity.
Please give a little more info on what you are actually trying to do. Is this to be a full time proposition? In a fixed or mobile setup, and at least a region, so amount of sun available can be figured into the calculations.

[valhalla360]

Make sure to distinguish between amps & amp-hours (also watts & watt-hours).

Amps used per day is a nonsense term. So when you say you came up with 347 amps per day it makes no sense (I have a pretty good idea of what you mean but it confuses the discussion)

Amps and watts are an instantaneous measurement. Your truck engine may take 600amps @ 12v to turn over but since it only takes a few seconds to start, it uses almost no amp-hours.

On the other hand your TV may only draw 4amps @ 12v but if it's on for 10hours, it will burn thru 40 amp-hours.

Second, issue is your laptop has two separate things going on.
- Your average laptop will draw somewhere around 60-70watts when running.
- The battery charger may or may not charge at the same time or the same rate as it's feeding the battery...ie: if you shut down the computer and the battery isn't full, the charger will keep charging.
- Then you have parasitic losses. If the battery is full, don't leave it plugged in as keeping the brick powered can eat up 5-15watts. Not a big issue when plugged into shore power but when operating on the RV's battery bank, it adds to the amp-hour consumption.
- In the end, I would find the wattage draw of the computer and add 20%, then multiply by the assumed hours per day to get an estimate of the watt-hours per day. Divide by 12 to get an idea of the amp-hours.

[dieseltruckdriv]

I agree with the other posters here. You seem to be taking the loads that are listed as maximum and trying to correlate that to a 24 hour draw. Almost nothing works that way. Your laptop might use that much power for two hours a day while you are recharging the battery.

When you are doing your calculations, convert everything to watts. Then understand that in my experience my wattage charge numbers come in slightly lower than what 2oldman posted because when the sun is out, you are getting 13.6 volts or higher and that drops the amps needed even more. The 12.5 amps is a safe voltage number with no solar input. I am assuming some solar input during the day based on my experience.

On top of that you have to remember that almost everything has a duty cycle. That means maybe 2 hours a day to recharge your laptop. It might mean (I have no idea) 45% of the time your fridge needs peak power but not 100% of the time. Taking that into consideration you have to do some more homework before you get some true useable numbers.

Now that I have put that out there our personal experience is we can get by on probably 300 watts of solar. We know from experience that 200 is the bare minimum, and the 420 we have now can last us a long time, unless we get several cloudy days. That's why I am adding at least a couple hundred more watts. That gets us as much tv as we want to watch with the fridge, coffee maker and water heater on propane.

For some background, when we bought our current fifth wheel my wife told me she wants to be able to live a normal life without having to worry about power. I bought what I thought would work and over time found out I undersized the inverter which is 375 watts. Last summer she thought she would like to be able to use the microwave once in a while without the generator also. I had a 1200 watt inverter picked out but I might upgrade to an even 2000 watts after she said that. I reminded her that we would also need some more battery capacity to do that. We have two group 31 AGMs currently. If I buy more batteries I am getting lithium batteries for the duty cycle.

[CA_Traveler]

The calculator adds about 15% due to loss of efficiency so 14.1A is reasonable for the calculator. However you need average input not maximum. AC appliances draw a lot of Ah (amp hours). The high efficiency Samsung 18 cu ft refer uses 100+ Ah per day.

Basically if you are going to use significant AC usage for devices then you'll need a very large battery bank, a large solar array AND a generator for those cloudy days. This is the reason most RVs to use propane for HW, MW, stove, refer and NO ACs for boondocking. Even propane for coffee!


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