Calculator  Solar
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Joe I love this webpage and have learned a lot reading through the comments and responses. My questions is regarding my dual battery bank on my boat. Battery 1 (12v) is a 1000 cranking amp for starting the engine only. Battery 2 is a DieHard deep cycle 565 CCA for the house (the bilge is on this battery and is set to automatic since the boat sits in the water when not in use). I just want to install a solar panel for each of the batteries to keep them topped off in between runs with the boat. I can go weeks to several months without using the boat at times. I am assuming a 7 to 10 watt panel per battery without a regulator should do the trick. Would you agree? Also, is it okay to keep the panels connected to the battery while the engine is in use? I have read on other websites different opinions about this. Any advice would be greatly appreciated. Thank you.

BatteryStuff Tech When it comes to solar, it is basically a math problem. You have to know what your pulling in amperage out of a battery in order to put it back in. In regards to your battery ratings you need to be looking for the AH rating, or you can at least estimate based off the group size. We do have a BCI Group Size Chart with average AH ratings. A 10 Watt panel put about 2.54 Amps a day back to the battery dependent on sunlight condition. If you don’t know how much your pump draws in a day, then try to average how long your battery lasts with charging. Your battery holds only so much Amp/Hrs, so you should be able to come up with some average usage.


Erick I am building a small weather station 12V power system, and was trying to solve for hand calcs for number of panels, I saw there was a mention of line 29 was the total wattage PV over the size of panel. So is that the equivelent of taking line (10 or 11) and dividing it over the product of line (24 and 25). i.e. If my total daily wattage was 4.2 WH or 4.284 WH adjusted do I divide that with the product of the 5 watt panel and the 2.903 Hours of sunlight adjusted. The populated form line 29 has an answer of .393, but my answer was .2951. Just trying to wrap my head around it, could you please help with explaining how to set up to solve for line 29.

BatteryStuff Tech I’m not sure of all the fields you had filled out, but this is how line 29 is determined:
Line 25 / Line 26 = Result A
Result A * Line 24 = Result B
Line 13 / Result B = Line 29


Ka8jmw Whats the recovery time in days after the battery bank has been deplete? After 7 days of no sun how many days before the battery bank has been recharged?

BatteryStuff Tech It is a math problem in order to figure out… Your battery bank has an amp/hr rating, and your solar panels put so many amps an hour to the controller. Most controllers have about a 15% loss of energy, so the remaining energy goes to the batteries.


Ratty Hi, this is really good, but I am still totally confused, apologies for my lack of experience. I have a hot tub and want to use a solar water panel to add supplementary heat to the water, this requires a small 12v pump to circulate the water, but only when the sun is out, therefore I don’t need any permanent or backup power, only on when sunny. The pump I have purchased is classed as 2060 watt 12v. I plan to connect this directly to the panel. What size panel will this require? Sorry but I think the calculator only works when sizing systems with batteries, but this is not how I want the system to work, thanks.

BatteryStuff Tech It would be best to check with the pump manufacturer to make sure the Solar panel can be connected directly to the pump. A 12v Solar panel will output around 1618.5 volts when connected depending on the panel. You need to ensure the pump can take that voltage. As far as what is required you said it is a 2060 watt pump, so any panel in that range in full sun will turn the pump as long as the pump is designed to run at the higher voltage the panel puts out. Personally I would get a panel towards the higher end of the range as it will most likely make the water flow quicker.


Baldbemused This was a great find – thanks for being there!

Dieter I can install my 2 300W solar panels 30 feet from the inverter/battery with about a 30% loss of sunlight hours (5.5 yearly avg.), or 120 feet away at 100%. Any suggestion on cabling size differences, potential power loss and any other consequences? Thanks

BatteryStuff Tech When getting into this range of solar I would suggest talking to a home solar expert.


Harish i want to know the size of a solar panel for supplying a 48V,50A battery. this battery is to be used for a kart.pls do reply soon

BatteryStuff Tech Solar systems can be quite detailed so I would suggest reading our article: Solar Systems The Right Way, or contact our TECH Department for more detailed information.


Tyler The batteries in parallel (field 19) and the batteries in series (field 20) should be multiplied to get the value for total number of batteries (field 21). Right now batteries in parallel is divided by batteries in series to get the total number of batteries. Also I would add the battery voltage as an input and calculate the batteries in series based on that input, right now it assumes 12 volt batteries but your website has 6 volt and even 2 volt available too. Just some suggestions to improve the output.

BatteryStuff Tech A minimum of 12 volts is assumed as that is in general the lowest system voltage one uses for solar. Yes we offer 2 volt and 6 volt batteries, however they would be put in parallel to achieve 12 volts at a minimum. I will however see if we can maybe expand the calculator to give people options to dictate which type of batteries they would be putting in parallel or series. Thank You for your suggestion.


Gray 50 watts solar panel + 40 amps battery ist good for running a 32” led and charge some gadgets…
We would require more information about your situation. Please contact one of our techs at tech@batterystuff.com.

Frawsen sorry for my bad english! what I called “Earth Socket” in my inquiry is actually “cesspits”. I sincerely apologize for confusing you! :)
I hope you will be able to respond to my email if you do not think my inquiry fits to the comments in here.
Thank you very much in advance! and I’d like to ask for a quotation for such a solution.
Kindly, FrawsenWe would require more information about your situation. Please contact one of our techs at tech@batterystuff.com.

Frawsen Hi,
Thanks for this calculation tool! It is fantastic!
However, since I’m filling in it for the first time, I would appreciate to compare and assure that I’m doing wright. So please give the end answer (i.e. number of batteries as well as solar panels) if the need is an earth closet with an electrical rotor (25 WAC with 230 VAC that goes ca 1.5 minutes after each use. Assume 3 uses/day) and an electrical heating motor (of either 255 or 80 WAC with 230 VAC depending on the temp. The ambient temp shall be 80 degrees Celsius. The use of the earth closet is only summer time – i.e. May to September in southern Sweden, Scandinavia). There is a fan as well, which consumes 25 WAC and a light bulb with 5 Watts. I guess these two go on when the earth closet is in use.Due to the fact that we don’t have all the information for your system we would need you to contact us at tech@batterystuff.com so that we can ask the appropriate questions, to get you the answer that you may need.

Barry Thank you for ths this great calculator. I have 4 × 12v 100 amp hour batteries wired in series to 48v. I also have 2.5 kw of solar panels and a 48v 60amp max charger. Total max charge current is 36amps at 48v. Does this mean I have too much charging current going to my batteries? At 12v they have a max recommended charging current of 10amps each. Thanks in advance if you can help me.
We would require more information about your situation. Please contact one of our techs at tech@batterystuff.com.

Zaks Am having a challenge with line 29: to determine the number of solar panels in parallel. Please advise. Thanks.
Zaks

BatteryStuff Tech Zaks, That is taking the total wattage (pv) required to run the system, and dividing it by the size of panel you input, which gives you the number of panels of that size that would be necessary to run your system.


David B I want a small standalone 12 volt DC system to power 2 × 14 watt DC L.E.D outdoor light for 12 hrs/day. Only 1 day of back up needed as I will retain the existing AC light. I entered “20” in field #18 [Battery Amp Rating (20)hr]; 5.6 hrs of sun/day (field #22); 70 watt panel and ended up with a total requirement of 1 battery. Was the Field #18 entry correct?

BatteryStuff Tech With that info, and with a 20AH battery, it should recommend 3 of them.


Steve J First line says total watts per hour is DC Amps x 12 but if I’m running a 24VDC Nemo well pump then wouldn’t it be DC Amps x 24 ? The pump needs 44.2 amps at its current depth.

Jeremy Steve, Yes that is correct. The calculator was set up for 12 volt systems originally. Thanks for catching that :)


David Brown great posting, thanks

Peter G What size panel do I need to trickle charge a 12volt battery for a John Deere riding lawnmower?

Jeremy Fear I would use a 5 watt panel.


Dewale Thanks for a very useful tool.
What is the limit of total solar watts that can be connected to a given ah battery? I have a 800 watts, 12V, 200AH inverter system. I have a 500 watts AC load to be powered daily for at least 8 hours. The calculation showed I need 15 nos of 120 watts panel in parallel connection to charge the 200ah battery. At what point do I reach the maximum no of panels (in watts or amp) that can be connected on a given battery?
Jeremy Fear Great question. We would approach it a different way though. By starting with the load, and making sure you have the appropritate amount of backup power to sustain the load for at least 2 days of ‘blackout’ (no solar input), and then sizing the solar panels to meet the demads of the load based upon the minimum average sunlight in your area, you end up with a system that is perfectly porportioned.
If you undersize the battery backup it would be possible to mess up the ratio, but as a rule of thumb, never have more than 30% of the battery capacity inputting into the battery.


Kevin hi I have a 60 watt solar panel – i need to run a laptop for 4 hours a day and 3 lightbulbs for 4 hours per day – i have a 88ah/640amp battery and recieve roughly 89 hours per day. I have 1000 watt sine inverter i have a new xundel controller which always shows that the battery is fully charged however the controller beeps that battery power is low after only using the laptop for 2 hours what else do i need.

BatteryStuff Tech If your battery runs low after running your laptop for 2 hours, then I recommend using a battery with more capacity (higher AH rating).


Waseem Dear Tech, its nice to come to know about your such a good and infomative site, great work.
my quesition is
I can get calculation about Solar Panel and Battery bank but I can not see any Information about charge controller to find the exact value? so how to calculate the amperage of charge controller that to be used in the system to charge the battery?thanks

BatteryStuff Tech I recommend you use OHM‘s Law. Watts = Volts x Amps.
Say you use a 200 Watt solar panel. If your system is 12 volts (nominal) this is what I recommend you do:
200 Watts = 12 Volts x ? Amps
200 Watts / 12 Volts = ? Amps
16 2/3 = Amps.This is a simple version for estimates only. But clearly a 200 Watt solar panel can charge at a rate of roughly 1517 amps per hour. Therefore, the controller should be rated for more to compensate for this. I recommend a 12 volt 20 amp controller for this example.


VYAPU how much amp battery is required for 40 watt solar panel where sunlight
is available for 9 hour. is 5 am battery is suitable for 40 watt solar panel
BatteryStuff Tech It‘s always best to start with the purpose of the battery first, instead of your strength of charge. What is your battery being used for?


Chris What is meant by batteries in parallel and is this absolutey necessary

BatteryStuff Tech Batteries in parallel is inportant. For example, If you put in that you were going to use 10 AH batteries, but based on your load you need 100 AH available, then in order to run your system you will need 10 batteries in parallel. 10 × 10 AH = 100 AH.


Catprog On figures of 3,24,12,1,40 I get <0.5 battery required. Then when rounded it gets rounded down to 0.

BatteryStuff Tech Instead of inserting a 40 AH battery, if you use a 13 AH battery the field #29 & 21 will indicate a total of ~1 battery needed. Because a 40 AH battery is being inputted into the formula, your power consumption indicates that you will only be using ~1/3 of the battery capacity. Rounded to the nearest whole number, that explains why it says 0. Hopefully common sense will tell you otherwise. :)


Willie Hi, most interesting. What is the maximum DOD ( degree of Depletion) would you allow your batteries to drop to on a daily basis, taking into consideration the deeper you go the less cycles you will get out of a set of batteries. I am taking mine down to between 20% and 30 %. Your comments please. I see no mention of any regulation in the comments and questions above? I use a MPPT 40 amp regulator? What do you think of the MPPT from outback or Microcare or any other for that matter.
Willie. Montagu, South Africa.
BatteryStuff Tech Our recommendation is to discharge no deeper than 50%. We mention this in our battery tutorial and other articles in our Knowledge Base. 50% for a 12 volt system should be 12.2v resting voltage. We do not sell MPPT regulators, and therefore cannot say either way if it is effective or not.


Jermaine I am new to learning solar power and I‘ve been doing some research for a school project. I want to design/redesign a solar charger that charges a battery bank thats capable enough to charge a 12 volt car battery but at the same time I want to charge small electronics too. I want to use the battery bank to charge these for later use. How would I go about using solar panels and a battery bank to charge a car battery but not overcharging and burning up small electronic rechargeable batteries? Thanks

BatteryStuff Tech Our solar panels are designed for 12 volt systems, primarily lead acid batteries (like car batteries). To prevent overcharge, please us a solar charge controller. As far as the actual calculations go, please use the calculator to find out how many watts you need for charging. We‘ve also published a helpful tutorial to explain what the fields for the calculator mean. If you know your system voltage, draw amount (amps, watts, etc), and number of sunlight hours, you should be good to go.


Adeyemi T Dear Tech
I have 4X160AH battery with a 48V 5KVA inverter,l would like to know how many solar panels i need to get and the charge controller?
BatteryStuff Tech How many solar panels do you need for what purpose?

Adeyemi T I mean to charge the battery to full capacity. Thanks

BatteryStuff Tech Within how many hours of direct sunlight do you feel comfortable relying on per day? How many days do you need the batteries to be recharged by?

Adeyemi T Lets say 6hr sunlight per day. I want to be using only solar to recharge the batteries

BatteryStuff Tech I do not know how much power you need to recharge the batteries. I will assume because you are using 160 AH batteries that you will need to recharge 160 amps in 6 hours. You can do this for a 48 volt system with a total of ~1,200 Watts of solar panels. That‘s x4 panels of roughly 300 Watts each because our solar panels are rated for 12 volt systems.

Adeyemi T I have a 2X24V each,370Watts solar panel , how long will it take to charge the battery?

BatteryStuff Tech What are the capacities of the batteries? What is the output voltage of the solar panels?

Adeyemi T 160AH

BatteryStuff Tech 370 Watts will charge a 48 volt system at a rate of 7 amps per hour. Therefore, it will take 23 hours of direct sunlight to charge 160 Amp Hours.










Micah I had a question about the Solar Panel array. If I‘m sizing a system, and I want just one solar panel for the system with no additional panels in parallel then I would: under item 25 (selecting panel size) I would insert a panel wattage until item 31(rounded number of solar panels) approaches 1?
Thanks in advance for clarifying the situation!
My background: I‘m doing an undergraduate project where I‘m building a PV powered Vapor Compression Refrigeration system. I‘ve estimated that the power consumption is about 300 watts per day, or 12.5 per hour 24 hours a day. It will be running on a 12V system, and its days of backup should be between 13 days. I‘m also limiting the system to 1 battery only. Where I‘m from, using TMY3 data from our local airport, the average peak sun hours per day is around 5 hours. With this, I‘m finding I need a panel of 130 watts.
Note: I‘m only using this calculator as a reference for sizing the system so that I have a ball park of what to expect as I‘m going along in the design and building process.

BatteryStuff Tech Line 31 will always round up. The actual number that you need to equal to 1 will be line 29. The is the Raw number. For instance, if you select a panel in line 25, and line 29 gives you 1.5, then it‘s telling you really need 1 1/2 solar panels, but since you can‘t have half of a panel, line 31 will round it to 2 to be on the safe side. Have Line 29 equal to 1 and you‘ve selected an appropriate size panel.


Abdullah Hasan Al Ghaya Gentlemen: Greetings I have loads electrical capacity of 15,000 watts which is about lambs ,fans and conditioners and fans Laptops and small mills Please help me choose a system capacity of the solar system works? As I hope that you know all the machines and equipment that will be used in the system in terms of quantity and number .. Thanks

BatteryStuff Tech This information is not adequate enough to help you size a system. Please provide the following: Amount of draw from the system PER HOUR. The number of hours the system will be running. And the number of sunlight hours you need to recharge for.


Iqbal Singh i need information about a small household project. i m having a 1000watts A.C load and the A.C voltage is 220V. i need this load to be run continuously for 10 hrs a day without any backup. can u please do the calculations for the number of batteries and 100 watts solar panels i would need to install. invertor efficiency is about 90% and sun is available for 6 hrs a day. i want this system to fully rely on solar panels and no other source of power.

BatteryStuff Tech Are you unable to insert these numbers into the calculator for yourself? If you need help understanding what the fields mean, please refer to our calculator tutorial.


Jon Hi
I am completely new to all these conversions and solar panels etc. I am trying to complete a very small diy project where I want to run a small aquarium air pump for 18hours a day. The pump is rated at 3.5w 240v ac. Are my calculations right in that this could be done with a small 30w panel. Bearing in mind I need to use an inverter as I can‘t source an adequate 12v dc pump .
BatteryStuff Tech Yes, a 30 watt solar panel will do just fine for your application, even when taking into consideration the inefficiency rate of an inverter.


Balakrishna I had a confusion with below items can some please explain
19 Actual # batteries wired in parallel Raw number number (gives 2.13 for my specifications)
20 Batteries wired in series Relates to system voltage number (gives 4.00 for my specifications)
21 Rounded number of Batteries Always rounded up number (gives 1 for my specifications)
BatteryStuff Tech 19 – This means that based on the battery capacity you need and the battery AH you chose to use for your battery system, then you will need a little more than 2 batteries to achieve the capacity you need. Realistically, you can‘t have a fraction of a battery, so the rounded up field should display 3 in total.
20 – 4 means you have a 48 volt system.
21 – This is incorrect, as you have at least 4 batteries in series, and within that system, you need 3 in parallel, this should come to 12 batteries total. The calculator, I believe, was developed for 12 volt systems, so the numbers may be thrown off by the increase in system voltage.


Craig I have a hunting cabin and I just need lights. I have 3 50 watt dc lights that I need to be on for about 6 hours at night for at most 3 days in a row I am in Minnesota with about 6 hours of sunlight in early November. I have a deep cycle battery 12 volt 130ah and a 18 watt solar charger. Will this work? Or what would be the cheapest fix.

BatteryStuff Tech A 50 watt light bulb will draw 4 amps at 12.5 volts DC (Watts = Amps x Volts). 3 bulbs will draw a total of 12 amps per hour. For 6 hours, that‘s 70 amps. For 3 days, that‘s 210 amps total draw. A 130 Amp Hour battery will not cut it, especially considering an 18 watt solar panel will only charge at a rate of 1.25 amps per hour. Recharging at a rate of 7 amps a day, while the draw amount is 70 amps a day is not going to be enough. You will either need a 145 Watt Panel to recharge in a single day, or give yourself more days to recharge the batteries. Or you can switch to lower Wattage bulbs, or a combination of all three suggestions.


Rocket Hi, this may sound a little stupid, but i am trying to get my head around 12v power Inverters and deep cycle Batteries. i tried to use your calculations, but came up with more questions that answers.
If I need 8amps (2500watts)per hour under full load (1 to 2 hrs daily) for cooking and about 12amps for lighting and radio 24/7.
I will be operating a 2500/5000w modified sine inverter, with a 140AH battery. Will this battery be capable of running this system? I was also looking at a 40watt solar panel through a 30amp regulator for recharging and a 2kva genset for use during the cloudy days, or will I need a larger AH battery and larger panel?
There is an average of 10 hours daylight (tropical climate),If the calculation is correct, I will need 4 × 40watt panels.
It may be the numbers I entered,
line # and figures
3 = 100 (8amps x 12 rounded up)
6 = 7 (60 / 8 average)
12 = 12v system
14 = 1 backup (genset)
18 = 1 × 140AH battery
22 = 10 daylight (conservative 9closer to 12)
25 = 40 watt paneltold me I needed 4 × 40watt panels in line 31.
cheers for any advise,

BatteryStuff Tech The 140 AH Battery system is good and will provide enough power for 1 single day safely. 2 days will discharge the battery all the way. But four 40 watt panels is sound for the amount of charge you need to put back into the batteries on a daily basis. I do recommend the results of the calculator.


Jasir thank you for your help. what does the 18th line mean? and how can i calculate or find this?. still i‘m getting the wrong rounding at both battery and solar panel numbers, so i sent screenshot of the calculator to your email: tech@batterystuff.com please check and help me about the wrong fields. my estimated watt demand is the device consumption from device data sheet not calculated by the formula DC Amps x 12 and system voltage is 48VDC.

BatteryStuff Tech Line 18 is the capacity of the battery you choose to input. Line 15 will tell you the raw Amp Hour storage you need, but you choose the battery. For instance, if line 15 says 100, you can use x10 10AH batteries, or x5 20 AH, or x1 100 AH battery. You input this because the battery you use is up to you and what you decide to purchase. Once you decide on a battery, the next lines will tell you if you need more than one. Back to the example, if you put 10 AH, line 21 will tell you need 10 batteries.
This calculator is designed for 12 volt systems, but the principles will work for any system. Use our other calculators for more info on battery selection and amperage/inverter calculation for 48 volt systems.


Jasir my watt demand is 300w/h but i make 400 to increase the backup power is that is right sir? line 14: # of days backup power required, is that means the needed backup in whole week when we say 24*7= 168 hours and I have direct sun 10 hours daily which means 10*7 = 70hours sun light in week, so i need a backup of 16870 = 98hours or 4.08 days is that is right sir?. line 19 Actual # batteries wired in parallel and rounded at line 21 to 2 so where is the problem in here. the same manner in solar panel, line 29 Number of solar panels in parallel 2.409 is calculated and rounded to 10 at line 31.

BatteryStuff Tech Line 14 refers to number of days. The number should be 7, not 168. Backup power refers to the battery system, assuming no solar whatsoever. This number is entirely up to you and has no influence on the solar demand.
This section of the calculator allows you to figure out how many batteries it would take to give you the runtime necessary in an emergency, at the same time taking into consideration a safe 50% battery discharge rule. The number of batteries you need depends on the capacity of the batteries you choose to enter into field 18.
If line 29 gives you 2.409, the line 31 should be 3. I don‘t know where the number 10 came from. But if any previous field was entered incorrectly, it could throw off the rest of the calculator.


Irfan Ul Haq sir my load is 750 watts two coolerado units, i have ten 150 watts solar panels and eight batteries of 100AH each an inverter charge controller ,sun light is available for 6 hours.is it adequate system

BatteryStuff Tech I recommend you input your numbers into our solar calculator and find the answer yourself. There is some information missing, such as the duration of the load and the time of backup power desired. Without this information, I cannot verify if you have enough reserve or charging power for your load.

Irfan Ul Haq Sir thanks for your prompt reply,duration of the load would be 6 hrs and backup power also 6 hrs.

BatteryStuff Tech Thank you for the information. According to my calculations, the battery bank of 800 AH will give you 6 hours and 24 minutes of uninterrupted runtime when you need the backup power. Any longer duration and you‘ll drain beyond 50% capacity. The solar system consisting of 1,500 watts total is nice. This will ensure full charge in 5 hours after the batteries have been discharged by the drain. Daily drain is roughly 360 amps, while total recharge amount in 6 hours of direct sunlight is roughly 650 amps. If your numbers are correct, the system should be adequate.




Suntamer Most of the comments address small loads over long periods of time. I am trying to powers a gate opener which operates five times a day for one minute cycles each time using 5.5 amps 110 volts and 500 watts at the peak of operation. I need a battery to support this. Currently I am using a 20 watt 12 volt panel with a 33 amp hour battery. This gives me about one cycle per per day then its done. Living in the tropics I get almost 10 hours of strong sunlight. I just can‘t get it figured. To complicate issues, the gate opener draws 0.05 amps and 5.5 watts when not operating to power proximity sensors. Do I need a serious battery bank for this power surge.

BatteryStuff Tech If your numbers are correct, then a single day of gate use will consume 28.7 amps from the battery. (5.5 amps x 5 times a day + .05 amps x 24 hours constant). For a battery only rated for 33 AH, this is not good. The battery is being nearly depleted on a daily basis. Especially considering a 20 watt solar panels will only charge at a rate of 1.3 amps per hour. If you get 10 hours of direct sunlight, that‘s still only 13 amps. Every day of use, the battery is drained more than being recharged.
A larger battery, at least 60 AH, will only discharge 50% from the daily draw. But I suggest adding another 20 watt solar panel to the system as well.

Catprog the problem is the 5.5amp is 110V not 12V.
For a 1 minute cycle * 5 times = 5 minutes = 1/12 hour
1/12 * 500 = 42 watt hours + 5.5*24 = 174 watt hours
174/12 = 14.5AH/day
Looking at the figures I would go up to 16AH/day for the inverter. Possibly 20AH for a big day.
I agree though 60AH battery and 40 watt of solar as a minimum

Suntamer Thanks for your response and looking at my numbers. I will upgrade and let you know the results of your advice.

Suntamer Upgrading as suggested in your previous reply. I am using an 85 Watt panel providing about 6 amps. what is the maximum distance between the panel and the battery (79Amph). I have read suggestions of 30 feet but I need to go 60 feet. Any thoughts on the gauge of connecting wire for this distance.

BatteryStuff Tech We include 16 gauge wire for our panels, up to 127 Watts. We recommend trying to keep the distance of the wire to 50 feet or less. More will work, but you will see a drop of efficiency the further you go.





Tarek very useful…..thanks to the creator….

BatteryStuff Tech Thank you for the feedback. I‘m glad it was helpful.


Don Filled out the calculator properly, I think, but if I put 2 days backup power, the rounded number of batteries comes up as 0. For 3 days, it comes up as 1. I am confused, shouldn‘t I always need at least one battery?

BatteryStuff Tech Hmm, without knowing any of the other fields you entered I cannot replicate the issue or even possible know how to fix it. But you‘re right, it‘s not recommended to have 0 batteries if you want ANY backup power. :)
The calculator works for me when I use it. Can you possibly take a screen shot of the calculator filled out and send it to tech@batterystuff.com?


Amy I have a attic exhaust fan that has a 38 volt motor. What is the optimum wattage for it to run? Thank you in advance.

BatteryStuff Tech Volts is only half of the equation. You will need to find out the amp draw of the motor. Voltage multiplied by amperage equals total watts.


Jesus I‘m trying to run a 2 amp(max draw)DC bilge pump 24/7, and have a 3 cloudy day reserve. Just got a 60W panel, and a new 115 Ah 12V deep cycle battery. Do I really need (3) 60W panels in series with (3) 115 Ah batteries to run a little tiny bilge pump constantly. I did calculations on my own before I found this site and bought the parts. Not sure how I came up so short. I get 7 hours a day of direct sunlight on the panel.

BatteryStuff Tech The solar calculator was correct. We do recommend three 115 AH batteries and three 60 Watt Panels (in parallel, not series). It‘s actually less than three of each recommended, but the calculator always rounds up to the nearest whole unit. No partial batteries or panels.
Based on what you currently have: To keep a safe discharge level of 50%, we recommend you use a single 115 AH battery under a 2 amp load for no more than 31 hours and 57 minutes. Having 3 batteries will ensure at least 3 days of backup power should the sun forbid to shine. A single 60 watt panel will deliver about 4.5 amps per hour (a total of 31.5 amps in seven hours). This would still not cover the 48 amps draw taken in a single day, hence the need for more than one panel. Our calculator is very conservative. It‘s better to size a little larger to make up for unexpected weather or extra need for longer duration of power.


Enriv I have two10 W solar panel and two12 V small 12AH/20HR Battery bank. What is the best wiring diagram for my panels? is it parallel or in series to compensate a 5 hrs fullcharging.

BatteryStuff Tech As long as both the batteries and the panels are wired similarly (in seres or parallel) it won‘t make a difference in terms of time to charge. You‘re looking at the same amount of power in the end.


Abdulla i need to figure out how many panels it will take to charge a 165ah, 12 V battery ?

BatteryStuff Tech Please read through the Walkthrough, which explains how to use the solar calculator. To answer your question, you would need to know the amount of draw on the battery, and how many hours of sunlight you feel comfortable relying on.


Phil Hi, I got a bit confused with the answers provided in blocks 2931 of the calculator. It says in block 29 I need just over 7 panels in parallel. Block 30 says I need just 1 if in series. But block 31 says I need 8 total panels. ??? My plan was to just have a panel to keep a 90 aH deep cell battery charged in series. Well, 1 vs 7 is going to save me a lot of money. Can you clarify? Thanks!

James Ville Line 29 is the RAW of solar panels that will (total Watts) be enough for your system based on your calculations. This number may have decimals. Line 30 indicates the voltage system. 1 is for 12 volts. 2 is for 24 volts, etc. If your batteries are in parallel, so too must the panels be in parallel. Only one string of panels is needed. Nothing should be wired in a series with another set. The last line rounds up the number in line 29. If your RAW number was ~7.38, it‘s going to round up to 8. You can‘t have a partial panel. Line 31 is the true number of panels needed, in accordance to the Watt rating recommended.
If your batteries are connected in series for more than 12 volts, then there is was an error in your numbers. The system voltage you enter (line 12) will determine the number in line 30.


Sean OBrien hi

Sean OBrien HI
I just bought an energizer(84020 12V AllInOne JumpStart System with BuiltIn Air Compressor and Power Inverter) as I am in sales and constantly on the road. I wanted to charge the battery with solar since I am in solar sales and I like to walk the walk. Its a 12v 18AH SLA but is expecting AC as the charge. I was wondering how many watt panel might supply an adequate recharge if I strap it to the roof of my FJ as I have latops phones and other sales tools plugged in to the unit. I ma guessing Ill need a small inverter and charge controller as well. Thanks in advance! 
Bill Very very helpful. I used it to size a backup panels and batteries for my refrigerator after a Hurricane. In 2005 locally some lost power for 3 weeks, this helped me calculate what I needed for refrigeration for me. The only question I have is could take a battery to 50%? Is that in Volts or AH?

James Ville 50% discharge has to do with capacity. Well, it actually has to do when chemistry. As the battery discharges, the amount of available electrolyte becomes less and less as it becomes diluted with water, a byproduct of the chemical reaction that creates electricity. The internal plates also become covered with lead sulfate, which also hinder further chemical reaction.
You cannot accurately measure available AH unless you run a load and determine runtime under that load. But you can measure volts. a 50% discharged 12 volt battery will not read 6 volts. 12.2 volts is closer to 50% discharged. Anything less is severe and will rapidly accelerate sulfation and decrease battery life.


Abdullah Al Ghaya thank you too much about good knowldge .

Abdullah Al Ghaya i am very thanks

Mike what size solar trickle charger would I need for at jeep wrangler. It is the tow vehicle for my class a motorhome

Jerryb Hello u woyld need a solar panel at 14.5 volt at 5 amps with a blocking diode So battery will not back feed to solar panel solar panel are light Activated resisters

James Trickle Chargers generally keep batteries maintained when not in use for prolonged periods of time. For a full sized automotive battery, I would recommend at least a 15 watt panel. Battery MINDer makes the SCC015, a very nice unit that also desulfates and restores batteries.


Back Pain Relief I‘m not an expert when it comes to this. Didn‘t even know this was possible. Useful read, appreciate your posting this.