Introduction: How to Size Your Off-Grid Solar Batteries

Picture of How to Size Your Off-Grid Solar Batteries

If you are designing a solar electricity system and don't have access to the grid, you are going to have to deal with batteries. When you start looking at solar batteries you are going to encounter a little math. Fortunately,  SolarTown is here to guide you through the calculations!

The general goal when designing an off-grid battery bank is to get a system that is big enough to supply all your needs for a few cloudy days, but is also small enough to be charged by your panels. Before you get started here you may want to check out this article. It serves as a good introduction to the solar batteries I'll be talking about:

Alright let's get started! Here are the steps to sizing your off-grid system.

Step 1: Calculating Your Amp-hour Needs

Picture of Calculating Your Amp-hour Needs

1.  Inverter size
To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need.

Example: A room has two 60 watt light bulb and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts

2.  Daily energy use
Next find the energy used in a day. Figure out how long each electronic device will be run in hours during a day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a very rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases.

Example: Light bulbs run for 5 hours a day. Computer runs for 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours

3.  Days of autonomy
Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

4.  Battery bank capacity
Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.

Example:  You want the battery bank to last three days without recharging and that you use 1.8 kwh per day. As 1.8 x 3 x 2 = 10.8kwh, this is the energy we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 10 800/48 = 225 AH. Now if you divide by your battery's rating you find the number of batteries you must use.

Step 2: Don't Overcharge Your Batteries!

Picture of Don't Overcharge Your Batteries!

Once you have sized your battery bank and solar panel array, determining which charge controller to use is comparatively straight forward. All we have to do is find the current through the controller by using power = voltage x current. Take the power produced by the solar panels and divide by the voltage of the batteries.

Example: A solar array is producing 1 kw and charging a battery bank of 24V. The controller size is then 1000/24 = 41.67 amps. Now introduce a safety factor. Multiply the value you have found by 1.25 to account for variable power outputs: 41.67 x 1.25 = 52.09 amps

In our example we would need at least a  52 amp controller. The Flex Max 60 MPPT Charge Controller would fit our specifications.

Step 3:

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Battery Wiring – Putting it all together

Before buying your batteries you need to figure out how many you need. Wiring is going to play a major role in determining this number. The goal is to find a configuration that produces target AH and voltage. There are two methods of wiring components in a circuit: parallel and series. In a series configuration the battery voltages add up while in parallel, current adds up.
Series and parallel connections can be combined to produce the voltage and AH that you require. Just remember:

      Series    →  voltage adds, current  does not
      Parallel  → current  adds, voltage does not

Previously we claimed that you could find the number of batteries you would need by dividing the AH capacity of your system by the AH rating of your batteries. This actually depends on how you wire together your system. Also remember that if a used battery is connected in parallel to a new one, it will degrade the fresher battery decreasing the lifespan of the whole system. Some people say that ideally you should just use a long line of batteries connected in series for your battery bank. Unfortunately this is not always possible due to voltage and AH requirements.

Step 4: You're Done!

Picture of You're Done!

You've now gone through all the steps necessary to size your off-grid battery bank system. If you are looking for more information on solar panel policy and technology, please visit SolarTown!


AhmedA149 (author)2017-11-10

hii.. I have 6 PV panels (280 W , 30V monocrys. multibusbar for each ) and have 2400 W offgrid solar charger MPPT . I ran 2 refrigerators on it ( about 1.5 A for each at 220V mains here in Iraq ) ..but used 4 batteries in series to give 48 V with only 17AH / battery .. what is the minimum AH of each battery should I use please ?? summer it can runs on thoselittle capacity batteries but now it cannot and charge drop rapidly so fast though the batteries are new and fresh .. please advice . best regrds ..

theohphilusb (author)2017-11-08

Hay I have 300watt solar panel and a battery of 200ah. Can it power 8bulbs of 26w and a TV of 75w 12volt.

FamilystoreR (author)2017-10-08

also i have 2 100 ah battries i thinking of using would that ok

FamilystoreR (author)2017-10-08

hi can help me i want solar power my greenhouse which runs a pump of 50w for 24 hours =1200 also i grow light of 600 watts for 8 hours 4,5000 watts so i be looking at inverter of 6000 or more also i cant quite work out what size charge controler i have 50a one on hand would this be suitable .

PMC91 (author)2017-09-11

in your examples, how many solar pannels would I need?

freddiesmchung (author)2017-06-13

If you multiply the batteries capacity by 2 ( 50% DOD ) and 3 days autonomy, isn't you might oversize the amps needed. I believe 70-80 % DOD is good enough for most VRLA Gel type batteries. We should increase the load used by 15% due to losses incurred converting AC to DC. Correct me if I am wrong.

HebaA14 (author)2016-04-18

The number 3 which indicate 50% depth of discharge is come from where ?

John CarloT3 (author)HebaA142017-06-05

Because we shouldn't 100% discharge our batteries.

And as per studies, 50% discharge is the safest level w/o shortening your battery life. Take note that this mostly applies on lead acid types, try to google other %discharge of different batteries.

DonA48 (author)HebaA142016-05-25

The 3 does not indicate the 50% depth of discharge as you say.
It represents your estimated number of days of power use, without the system being recharged by the sun. This example is over a 3 day period of cloudy weather, you may wish to change that to 4 days or 5 days etc. The number 2 is what you would multiply with to represent the 50% discharge. Hope this helps.

mtaturujr (author)2016-03-20

I am in the process of designing my small off grid solar system. Help me out plz. Can a 500 watt pure sine wave inverter run a 0.5hp submersible water pump.

DanM6 (author)mtaturujr2016-09-14

Not enough info on your set-up to answer this question accurately. it takes 745.7 watts to run a 1hp motor continuations, this does not take in to effect start-up energy on said "0.5hp submersible water pump" ... hope this helps you out!

John CarloT3 (author)DanM62017-06-05

I don't know but, you shouldn't use that 1hp=745.7(746w) thing.

That 1hp is mechanical power not electrical. Also that's also the output power you need a slightly higher power for input.

However, you are still correct that his 500w inverter is not enough for his 0.5hp pump. :D

He should check the Electrical Code of his country. My country's electrical code is heavily based on NEC. And as per NEC, full current of 0.5Hp AC motor is:

1phase-115v 9.80amps & 230v 4.9amps while

3phase-115v 4.0amps & 230v 2amps

Peter EkeneE (author)2017-02-06

Hi fellas, i'm Peter.... i want to design a 3kw solar system to run for 8hours per day, what capacity of Battery, inverter and changer controller do i need, any kinda help is welcomed please

Petruch (author)2017-01-16

Great article AND detailed comments. Not much details on " off-grid DC system requires less energy...".

DarshanS35 (author)2017-01-16

and also consider the power consumption of the inverter my friend

ronniereycanete (author)2016-09-19

I have a problem with my solar power system, my panel is 100wts mono, battery 100ah 12vdc, controller charger 20A, inverter is 1500wts DC-AC, total LED BULB load is 50watts, But my system will light for only 3minutes and it will shut down, my thinking is my inverter which is too big for the system,maybe? I need advised. Now I am running it DC supply and it works Good. But I need to know if iwill used the inverter 1500 watts only 3 minutes the duration. The Battery I charge it for 3 days before I energized my system. Any comment?

basically inverter works only within the level of perticular voltage level range. please check voltage range in which the inverter is running is correct or not?

and also check whether ac current going to load is how many ampere?

and charging and discharging cycle of battery is proper or not?

Hello Ronnie, my name is Robert and I might be able to assist you here with your situation. This is perhaps complicated, but math never lies.

The 100 watt mono solar panel with a 20 Amp Charge Controller only deliveries to the battery (12 Volts X 5.7 Amps )= 68.4 watts of output not 100 watts. Think about this for just a minute, you can only draw a maximum of 50 Ah from the battery. Therefore 50 watts for 1 hour = 50 Wh / 12 Volts = 4.166 Amp hours for every hour that light is on. Lastly a 1500 Watt inverter requires about 125 amps of current at full power. You are playing with fire my friend. Be safe and use a 300 to 500 watt inverter with this 100 Ah battery.

ScottC253 (author)briefkase22017-01-06

Hello Robert, your response above seems like you could point me in the right direction. I am looking to put together an off-grid solar system for a small shed. I have been researching on how to go about getting the proper set up to make it as efficient and safe as possible and hope you could further help me. I will be using 2 - 4ft LED shop lights @ 40 watts, which won't be used often as I will only be in there mostly during the day. Most power tools will be DeWalt 20V cordless therefore using at least 2 battery chargers. Also, if possible, I would like to be able to run a 30 gallon air compressor. This is not a necessity as I could reach an outlet with an extension cord but would be very convenient. I have come across this system: Would I need the 200 watt system? And what size battery and size inverter do you recommend?

Hi ronniereycanete, I am not expert in solar systems..But I can suggest one-thing which is about the input voltage of inverter. It may be in out of range.

JoemileoL (author)2016-11-12

i have 5200w of solar panel and i want it run for 12-24 hrs. a day. what is the size of inverter, how many amp of the battery and how many battery i can use you sugets?,..

cesarm001 made it! (author)2016-11-05

Thanks a lot I am now completed my design of two homes one small 1 bedroom and a final with 2 bedrooms and lots of space..... as follows

4 PV @ 200 Watts each (Maybe increase to 4) ??? is all $$$$$

2 Banks of 2 x 12V Batteries (350 Watts) each (in Series) to produce 24V in each bank - Maybe I change for 4 x 6 Volts ???? all deep cycle - New batteries ( I will alternate the banks from time to time to ensure one is a backup ready to go any time - That way I can sleep in peace he he he)

Cooking is all gas so no worries there - Hot water is small unit that provide hot water as you go that way i only use hot water when needed (Bathroom and Kitchen) - The big home I will consider a solar tank on the roof ???? future project - Now is cheap and easy with the small electrical units (See Pic)

2 MPPT Power controllers (One for each bank) to support the PV's power

2 5000 Watts Inverters (Excess but better to be sure than sorry later) - 1 for back up

The power controllers are selectable by breakers to each bank meaning I can switch from one bank to another by turning the the breakers on/off per bank

As option I can also up the system by running the 2 series banks as parallel to get 48 Volts easy ....and

Lots of breakers for each critical connection and meter control panel to check voltages e

Wind power and Grid options for future connections if requires

And small generator for emergency charge if required.

All connections from the Inverter will end up on a breaker box panel.

Thank you very much your info is great

AlvaroC42 (author)2016-07-20

Sir's, If my battery power bank is 48v output and my pure sinewave inverter output is 220v...from which output do l am going to use as voltage draw? Is it from the inverter voltage output or from the battery bank voltage output? Ex. 1800watt-hr/220v(inverter side) or 1800watt-hr/48v(battery side) to get my amp/hr.

KaiE7 (author)AlvaroC422016-10-10

Quite easy: If your inverters efficiency is eg 97 %, you should calculate 1800Wh/220V x 1,1
There is a plus of ten percent (for sure) included.
You also can calculate 2000Wh/48V.
That means that there is a need for approx. 50Ah batterycapacity. It is better to oversize a lil bit.

briefkase2 (author)AlvaroC422016-10-04

Hi Alvaro, my name is Robert and I hope you are checking back here for answers. Do you want to know how many amp hours you are removing from the 48 volt battery bank. So then 220 VAC / 40 Amps AC = 8800 Watts of electrical appliances. Divide by the 48 Volts and you are drawing approximately 200 Amps DC. Let me know the size of the battery bank, I would be interested in pictures as well.

RichardP201 (author)2016-09-02

I put together an off the grid solar system recently in Southern California. I currently have 7 x 100 watt Solar Panels, 4 deep cycle 12 volt 114 amp hour batteries in parallel, and a 3000 watt Pure sine wave inverter. It has been running pretty well, I am considering adding 1 more battery for storage and maybe 1 more panel. If anyone has in advice or sees any issue with my setup, please let me know as this is my first attempt at solar.

briefkase2 (author)RichardP2012016-10-04

Hey Richard, just curious what type of charge controller you have, MPPT or PWM. Can you give the specs on the panels Vmp and Imp.

flywheeler (author)2016-04-12

I have an off-grid system using micro hydro and solar. Making the leap from lead acid to l-ion and am looking at these guys to do the job:

Anyone have experience working with them or know how their batteries stand up?

RobertB411 (author)flywheeler2016-05-11

Did you get set up? We are an American firm that designs and manufactures energy storage. I'd love to talk with you.

R. Bean- 702.208.9873

DanM6 (author)RobertB4112016-09-14

robert, please have someone call me 269 838 9136. I'm doing a start-up solar company. I'm wanting to offer pricing on l-ion's as well as my hybrid L.A. batteries. Thanks Dan.

DavidC784 (author)2016-08-16

years ago I worked out that you could go off grid if you created a massive bank and ran your house like a submarine. that was before solar really kicked in.

as I understand it as I am currently tied to the grid then to go off it in event of a zombie plague for example then at its simplest I would need to recreate 240v to power the inverter first so it can then use the power I generate through my 3.5kw array to charge batteries or run appliances. in theory a simple car inverter and battery would do it.

one thing I have noticed is that most inverters do not generate power until the panel voltage goes above 150v so you have a chunk of lost voltage going spare on cloudy days which could be run through a battery bank if you set one up as a voltage buffer. there's a lot of spare voltage we brits are unable to use. I did wonder if a US 110v inverter could be set up to use this lost power.

OscarR111 (author)DavidC7842016-09-12

wait what? Why you need 240 v to power your inverter?

WankyT (author)2016-06-17

I did the math and I either need forty seven thousand batteries and two panels or vice versa. Is that right?

OscarR111 (author)WankyT2016-09-12

right, but you are never going to charge those batteries with 2 panels...

alisonnpaul (author)2016-05-12

my electricity supply is 99.9% perfect only ever went off for a hour in the last ten years.

But my electricity costs are crazy about £12-£15 a week uk residents only get about 150 days of sun a year though so i'm sceptical on this.

is it possible to be off grid and flip a switch allowing me to go on grid if the batteries were to run dry due to a fortnights bad weather or simply to auto use the grid if not enough power in the batteries

DavidC784 (author)alisonnpaul2016-08-16

it's what you class as sun, panels still work on cloudy days just not as well. our house uses half a KW per hour with everything in the background so phone chargers, a couple of laptops, routers, fridges, freezers etc.

its our first year of grid solar and the hard part is not exporting anything as most is generated during the afternoon but its still worth doing so far, I'd like to add more panels to catch the evening sun as winter time is produces next to nothing so off grid not an option without a second source of power. you can fit switches to go back on grid when your bank goes flat. some have built set ups to use economy 7 to charge a bank at night buts its only effective if you have it already I reckon.

the ideal off grid solution is to drive an electric car, charge it at work for free then run the house off it at night :)

jeanTremblay10 (author)2016-08-04

I got a Sovonics MA-33, the largest solar panel, puts out 33 watts/2.15 amps of 12-volt DC from freinds who have sold their boat . Bought an inverter , a 12v battery ( for lawn mowers and a solar charge controller ( max 7A overcharge 14V=0.5V) but have NOT one clue how to put it all together. Help! If youcan please. I want it to be able to run a submersible pump in my second well to water the gardens .

jlc012. (author)2016-05-16

If all you are looking for is reducing your energy bill, all you need is a solar panel array and bi directional meter installed. Its super cheap compared to what is being described in this article. What this will do is run the meter in reverse when the sun is shinning and reduce your electricity bill, it can even make you money if you put out more than you use, if permitted by your utilities company.

TalonS4 (author)jlc012.2016-06-19

This is talking about off-grid living.

ReyM9 (author)2016-03-08

sir i have a question on sizing the battery for solar system my battery rating is 12v 100Ah and 12v 55Ah. I follow your tutorial. My inverter is 48v see below my calculation and i divide the nominal voltage of my battery so the answer is 533.82Ah i will divide it again in my available battery 533.82/100ah= 5.33 so i will use 6 battery 100ah.. its coorect?
.85 Battery loss
50% DOD
5,445x2days= 10,890wh
10,8901/.85= 12,811.76
12,811.76/.5= 25,623.52
25,623.52/48= 533.82 Ah

billsmile (author)2014-09-09

how calculate watts for the solar panel and charger controller.

mist42nz (author)billsmile2015-03-06

You have to add your load, work out what you maximum use time is. That gives you demand in Wh (Watt hours)

For fixed solar panels, after a few years they'll drop off to 80% efficiency, so leave room to add a couple more then. But generally you get 4 hours charging time, maybe 5 or 6 if you're in a good spot/climate.
So you need to get all your Watthours from first paragraph in that 4 hours.
But in the winter you'll probably only get half as much usable light (at best) so if their is going to be needed winter PV, double that panel again (or use a generator to boost in winter, as you might need powerful things like heaters).

OTOH if you're in a really hot area with mild winters, consider a separate circuit for your airconditioning. Design it so it comes on when the power is there and the day is already sunny, and shut it down in the evening, that way only a small amount of storage is required.

So Watthours x 6 (to get that 24 hour charging... don't forget that you still draw full power to appliances in the charge time!). x2 again for winter.

Then if you're expecting no-sun days, either get a generator for those days or multiply the previous result by the number of days you need. ie if you are only getting 4hrs of good light, to charge batteries to last 3 days , you're going to need a LOT of panels... ie 4hrs to cover 72 hours = 18x initial Watthours, then x2 again for winter ( that's thirty six times your _daily_ load)

For all those panels, you will be able to add the currents together, most panels aren't worth going over pairs (about 60v). So add ALL the currents together, divide by 2 if going to put pairs of panels in series. You'll need at least that much charge control.

Then batteries. MAXIMUM charge speed is C/10. Thats total Capacity divided by 10. Your charge rate is ALL your panels ADDED together . Multiply that by 10, and you need that many Watthours worth of batteries MINIMUM !
eg a Trojan SCS225 a liquid aka flooded 12v battery with deep cycle useful for solar PV, has a rating of around 1.5kWh (which is 1500 Watt hours).

What I found helpful is to split off separate circuits in my house. Lighting on one. Computer, screen, router on another. Laptop and NAS on a third. Looking at one for kettle, and another one for microwave.
This way I can build my system up step by step. It does cost a little more in the long run, but things like my computer run 24x7, so is really efficient for Inverters and panels etc as I they run at about 90% efficiency.
Things like lights are only on for a few hours, so a much smaller charge controller, smaller inverter and a few panels work well there.

The kettle and microwave only really see about 30 minutes use a day, but they can survive nicely on a cheaper "modified sine wave" inverter but it needs to be BIG, around 3kW for the kettle. but with 30 minutes usage that 1500Watthours a day... one (maybe two because of the big current) battery, and 1600Wh in 4 hours... 2 panels max...and a small 20A charge controller. (my computer has 2kW of panels and 90A worth of charge controllers and uses 7200Wh in a day)

10 Led lights, 5 hours, 10 x 14 x 5 = 700Wh, 10Ledx14W = 140W inverter (cheap!), 700Wh/ 4 hours charge time = 175W of panels... one panel. 700Wh meand less than one SCS225 battery.

One thing to consider is the more run time you have, then the more effective changing to PV will be. A kettle takes a while to pay off that $500 panel, at 1.5kWh a day (about 25cents a day). While something using power continuously is going to be more effective to change over, as it then saves you money continuously.

BigAndRed (author)mist42nz2016-02-14

you say about the kettle using so much power and being not cost efective then consider just stop wasting electricty and using a gas cooker to boil the kettle, I use gas now and it very cost efective. Also gas for water heater and bbq. When I go totally off grid then a wood fired heater for cooking, water heater, etc.

mist42nz (author)BigAndRed2016-02-15

Are you being sarcastic? you use the gas, costs extra to go get the gas, and much of the heater goes around the side of the kettle.
submerged elements are highly efficient. What makes the kettle less efficient is people tend to over fill it, and then re-boil the water over and over, and often the water doesn't need to be boiled to 100+ degrees.

Woodfired can be cheaper if you have ready wood, preferrably deadfall. My parents have that set up. Although it is not available in many areas due to the town/city pollution regulations.

kzgqgf (author)2015-11-12

Wait a sec ... is the example above correct? Fine, we need 1800wh per day of "inverted" electricity, but the inverter draws amps off the 12v batteries, not watts. I think we need a Step 2.5 to convert the 110v watts to 12v draw off the batteries as follows:

1800w/110v = 16.4AH draw on batteries by inverter


Step 4: 16.4AH x 3 x 2 = 98.4 AH

But then, no one sensible would use old 60w incandescents over modern 13w CFL's in this application, so realistic requirement is closer to 98.4AH x (13w/60w) = 21.3AH battery bank capacity.

Hope this is correct, else going green is not economical.


MatthewK63 (author)kzgqgf2016-01-12

I think you are confused. Watts are Watts. 110v or 12v it doesn't matter. More voltage means less amperage to produce the wattage. 1800W/110V = 16.4A, 1800W/12V = 150A.

kzgqgf (author)kzgqgf2015-11-12

Sorry, think final calc should be:

49.2 (laptop) + 49.2 x (13/60) (lights) = 59.9AH

kzgqgf (author)kzgqgf2015-11-12

Another reality check: my HP laptop only uses 65w rather than 300w, so total requirement for above is now back to 21.3AH.

hữuh1 (author)2015-11-14

what about the factor, efficiency of battery, 75-85%. it's need more capacity for our own need

kzgqgf (author)hữuh12015-11-15

Maybe the author accounted for that with the "system losses" multiplier of 1.5?

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More by DIY Solar Jon:How to Size Your Off-Grid Solar BatteriesDIY Home Solar: Planning a Solar Array (Beginner's Guide)
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