The following post explains through calculations how to select and interface the right solar panel, inverter and charger controller combinations for acquiring optimal results in the form of free electricity.

Contents

## Calculating Solar Panel, Inverter and Battery Charger Specifications

For the sake of convenience, let's believe you possess a a 100 watt appliance or load that you would like to operate, free of charge through solar power, for around ten hours every night.

In order to exactly determine the dimensions of the solar panel, batteries, charge controller and inverter the following mentioned parameters will need to be strictly calculated and configured.

## Estimating Load Wattage

1) First you will need to estimate how much watts of electricity you may require for the specified load.

Let's say you have a 100 watt load that needs to be operated for approximately 10 hours, in that case the total power required could be estimated simply by multiplying the load with hours, as given under

**100 Watts x 10 hours = 1,000 Watt hours**. This becomes the absolute power necessary from the panel.

## Determining Approximate Solar Panel Dimension

2) Next, we need to determine the approximate dimensions of the solar panel for satisfying the above estimated load requirement. If we assume a roughly ten hour daily optimal sunshine, the specifications for the solar panel could be simply and quickly calculated as explained in the following expression:

**1,000 Watt hours / 10 hours sunlight = 100 Watt solar panel.**

However, generally you may notice that mostly during the summer days or weeks we normally get around 15 hours of reasonable amount of sunshine, but the winter season produces only roughly 4-5 hours of effective sunshine.

Contemplating the above scenario, you too might agree and recommend considering the worst possible sunshine hour into calculation so that even on the weakest of sunshines your load keeps running optimally.

Therefore taking into account the 4 to 5 hours sunshine per day consideration, we calculate the true power for the solar panel which would enable your load to keep running throughout the year .

**1,000 Watt hours / 5 hours sunlight = 200 Watt solar panel.**

## Calculating Battery AH

3) Once you have calculated the solar panel as per the above calculations, it's time to calculate the AH rating for the batteries that might be required for operating the specified load under all conditions. If the selected battery is rated at 12V, in that case:

**Dividing 1,000 Watt hours by 12 Volts = 83 Amp Hours of reserve battery power.**

Let's upgrade this value a little more with a 20% added tolerance, which finally gives a rounded up figure of around 100 AH. Hence, a 100AH 12V battery is what you may finally require for the inverter.

## Evaluating Charger Controller Specifications

4) Now, to figure out how big your solar charge controller would need to be for the above calculated parameters, you might need to take your solar panel current or the Amperage specs into consideration, which may be simply gotten by dividing the panel's wattage rating with its voltage rating (Ohms law remember?)

**100 / 12 = 8.3 Amps.**

We have so far applied a "plus tolerance" to all the previous parameters, so let's show some generosity to the Amp spec of the panel also, and instead of sticking to the 8.3 amps limit, you might be happy raising the level to around 10 Amps? That looks good, right?

## Assessing Inverter Specifications

5) Finally we boil down to the inverter specifications, and determine the reasonably exact capacity that would keep the unit compatible with the above discussed results, and keep the load running without issues, whenever required.

Well, calculating the inverter specs doesn't look difficult at this point of the discussion.

Since we already know the maximum load wattage which is 100 Watts, implies that we simply choose an inverter which might be capable of handling a 100 watt comfortably.

That implies, we simply need to get an inverter rated at 100 watts,.... OK, you may be thinking of adding some tolerance to this candidate also, not an issue, instead of 100 watts you can opt for a 125 watt inverter, allowing all the gadgets to happily "shake-hands" and your house powered round the clock forever, free of cost.

Anonymous says

hi sir i have a question that i have a 55w solar panel 50ah lead acid batery i dont have a controler so for how long the batery will be full charged? and also i can run 2w 5 bulbs,40w tvand dvd for how many hours? and also what is the suitable charge controller needed? thanks

Swagatam says

Without a charger controller it would be dangerous to charge the battery, or drive any gadget…. not recommended.

OM KUWAR says

frds…without controller dangerous for battery .if full charge battery …batterys supply retun to solar panel …and anothr things you can use the formula i = p÷v …55w÷12

achilles hector says

Dear Mr. Majumdar

Can you help me in installing a solar panel at home?

I have the following data:

Solar panel wattage: 216 watts

Electricity required in our home: 106 watts

Hours of use: it doesn't matter to me as long the hours wont go below 10 hours.

Hours of sunlight (based on the normal daylight ): Max.- 7 hrs.; Min.- 6 hrs

Can you fill in the blanks? :

battery: voltage:___; amp-hours:___

inverter: wattage:___; voltage: 220

Can I ask if you can make me a solar charge controller out of the data above?

And can you add a function to the solar charge controller? : If the sun has enough light,

the inverter will get the source from the solar panel while the battery is being charged.

Then the moment the cloud dims, in which the solar panel can't produce enough voltage,

the relay will change the inverter's source, which is from solar panel to battery. With this function I can protect the appliances from low voltage and use the solar panel during daytime.

thanks a lot for your kind consideration!!!!

yours truly

Eshkariel Tapiador

Swagatam says

Dear Achiless,

I'll try to do it and respond soon.

achilles hector says

Good day Swags!!!

How's my circuit?

Swagatam says

Good day Eshkariel,

you can try calculating the parameters with the help of the data given in the above article. It isn't very difficult if you see.

I'll try to design the solar charger that you have asked and will let you know soon, pls remind me if i forget, though:)

Swagatam says

Good Morning Eshkariel,

Your calculations look OK to me.

You can try the inverter that's been covered in this article although it's not a pure sine wave, just a modified one.

https://homemade-circuits.com/2013/04/how-to-modify-square-wave-inverter-into.html

transformer rating could be = required wattage divided by available voltage, this will give the amp rating for the trafo.

Battery could be rated at = required wattage divided by voltage,this will give the AH of the battery, but make it twice at least for a reasonable back up time.

eshkariel tapiador says

Don't forget the circuit…

Swagatam says

eshkariel, I think it's already published in my blog, you can refer to this post for more info:

https://homemade-circuits.com/2013/04/solar-water-heater-with-battery-charger.html

kanu kamliya says

hello sir I want to run 100 watt inverter on solar panel so how many watts panel require for it???

Swagatam says

hello kanu, you can try a 25 watt panel and a 25 AH battery

kanu kamliya says

hello sir I am student from Anand,Gujarat, I

want to make 100 watt solar

inverter,so how many watts solar panel and

battery require for it?

Swagatam says

please see the above comment.

NANDULA SUBRAHMANYAM says

Hello sir. Can you suggest me what are all the components to be bought by me including their grades as I am not much aware of electrical components

Swagatam says

Hi Nandula, please specify your requirement specifications….only then I would be able to suggest.

NANDULA SUBRAHMANYAM says

My requirement is making a speed restrictor for vehicles.

Swagatam says

you can try the following circuit

https://homemade-circuits.com/2013/12/vehicle-speed-limit-alarm-circuit.html

Unknown says

I hv a 1000 wt invetor and a 240 solar panel and battery but if I try to connect the invetor it just says faulty Wat cn be the problem with it

Swagatam says

your specs are not sufficient for judging the fault…

Andy Fisher says

I have a solar company getting ready to do my solar. I will have 28 lg 315 watt panels. They are giving me a 7600 watt inverter. I asked why my inverter is less wattage than my panels. He said you divide the wattage of the panels by the inverter and get 1.17. He said as long as it's. Between .75 and 1.35. Is this right??

Swagatam says

The technician is probably correct. the inverter wattage should be much less than the panel wattage otherwise the solar panel could be forced to run at an inefficient zone causing huge drop in inverter voltage

owor paskr says

Hello mr Swagatam.

I have 8 solar panels of 200watts each. I would like to ask which inverter size is the most suitable for these panels when wired all together.

Can I use the inverter which come with inbuilt charge controller?

Which battery sizes and how many pieces do I need to make the wiring.

Which wiring method would give me the most efficient power outputs.

Can I use the solar power output out of these 8 panels to run a Panasonic home theater of 13200watts?

Thanks in advance

PASCAL

Swagatam says

Hello Pascal,

the most suitable inverter that could be used with the mentioned 200 x 8 = 1600 watt panel is around a 1300 watt inverter, 13200watt is out of question, since it's way out of the range of the panel.

connecting the panels in parallel would be a more convenient option….

the battery voltage will need to be matched with the panel output if no MPPT is used in the middle, and therefore will need to be wired accordingly.

JITENDRA PATEL says

Sir , I read all posts today.in your blog for calculation , in point #2 , you calculate 1000 wh/ 5 hr sunlight= 200 w but in point #4 , you wrote , 100 w panel /12 v = 8.33 Amp charge controller.why you took 100 w instead of 200 w ???

Swagatam says

Jitendra, 200 watt is for countries with weaker average sunlight, and 100 watt is for countries for stronger average sunlight…

the example is given for a 100watt panel, anybody can easily convert the results for 200watt also….

there's nothing too difficult or confusing about the calculations.

JITENDRA PATEL says

Sir , second question .your answer to Ms. Achilles Hector should be direct I.e. Battery V = 24 , Amp Hr = 44+@ 24 v , Inverter w = 216+ w , but you replied her to follow calculation as per yr blog.I know you can't calculate for everybody but how can we cross check whether our calculation is right until and unless you or any other reader give her the answer.I request you and atleast one reader to confirm my answer.

Anonymous says

Sir, i have 8 panels 250W each in a parallel connection, and 8 batteries 150 Amp. 12V In a Series parallel connection 48V. my Inverter is 48V – 220V pure sine. can i connect it together.

Swagatam says

If the panel voltage is within 60V then it would be fine, however if the inverter load is too high then the batteries might not charge properly….in other words the inverter load and the battery charging consumption must be less than 250 watt combined

Laptop Arcade Anura says

Hello sir I try build a solar system for my home.to run 5hours on battery backup.3X45w led tv's,20X12w led bulbs

1X95w refrigerator at night only. I have this item for solar system (1X200W mono panel,1X max solar input >50 12~24v 20Amp charging control unit,1X 2000W inverter,which type battery I need for 5 hours battery backup.and how many hours get it battery full charge.please help sorry for my bad English.

Swagatam says

Please provide the total wattage of the solar panel, and the total wattage of the load, only then I would be able to suggest you the approximate results

Laptop Arcade Anura says

2 solar Panel 100W 17W 5.68A

max load 100W per hour.need 5 hour's battery backup for night time only.

Max says

Hello Sir

Total Home load is 3600W expected to run for 10H daily

I have 12pcs 300w 36v solar panels

I also have 5000w inverter

I also have 6pcs 24v 250AH batteries

I needed to know the size of charge controller to use since I have 36v from Panels and 24v for batteries and inverter

Swag says

5kva inverter for for a 3.6kva load is OK.

Now 3600 x 10 = 36000 gives us the total power requirement for the load.

you have 12 x 300 = 3600 watt solar panel output, but since the average sunshine can be expected to be around 6 hours a day, the power output is not sufficient for 36000 watt power requirement.

your solar panel would provide just 3600 x 6 = 21600 watts of average power, whereas the requirement is 36000 watts.

for a 3.6kva load the inverter operating at 24V would require 3600/24 = 150 amps.

your battery Ah rating is 250 x 6 = 1500 Ah, so it can be comfortably discharged at 150amp rate, that’s OK, so the battery specs is fine.

your charger should be able to charge your battery at the rate 150 amp for at least 10 hours, which implies that your charger controller must be rated to output 28V at 150 amps….however again since the sunshine could be just for 6 hours, this will fall short. To compensate this you will have to upgrade the solar panel appropriately, or you can can use a solar tracker that will enable the solar panels to absorb sunshine for 10 hours optimally, and your existing devices can be used without any modifications or upgrades.

Swagatam says

for a 100 watt load to run for 5 hours you will need a minimum 500 watt hour battery, if it's a 12V batt then it would need to be 500/12 = 41 or 40 AH rated.

you can use your 100watt panel to charge the battery withing 8 hours,,,preferably using an MPPT charger circuit.

SinnSabs says

Hello Sir.

One Question only I want to clarify is the matching of Solar Voltage module to the Charge controller module.

Is 12 volts Charge controller can handle a 18 Volts solar panel?

or If we buy 12 volts charge controller, I will also buy a 12 volts Solar panel?

Swagatam says

Hello Sinn,

You will have to check the specification details of the controller, which will tell you the maximum input it may be designed to handle.

However in most cases a controller for a 12V battery would be rated to handle 18V easily.

No, a 12V controller will definitely not correctly work with a 12V input and will require require a minimum of 15V as the input.

as stated earlier an 18V input in most cases would be quite normal and recommended

Unknown says

I have 900 watts inverter what rating battry and solar panel should use

Swagatam says

what is your inverter's battery specification?

Anonymous says

Hello Swagatam

Please I have a 1kv inverter and 12v 75amp battery and a 12volt battery charger but when ever I charge the battery it doesn't charge again to start the inverter.what should I do please

And also what capacity of solar panels can I use and run the inverter with a load of 850watt

Swagatam says

Hello, have you confirmed whether your battery is getting fully charged or not?, and whether your battery is working good or not?

you will need a 1kW solar panel for running a 850 watt load

ola korede says

hello sir am having a 3000va inverter and four 200AH/12v batteries connected serially but in term of usage consumption stays on just two when the remaining two remains full even after the former is low. what can I do to make the remaining two full battery work simultaneously or switch over former is low

Swag says

OLa, that can be difficult to solve if the batts are in series….because the batts which are getting discharged faster could be the defective ones and while getting discharged these may be causing a high resistance for the good batteries and preventing their current from reaching the inverter…you can try shuffling the positions of the batts withing the series or simply change the defective ones with new.

ola korede says

pls do you think the wiring can be responsible for the battery not working properly

Swag says

not the wiring, most probabaly it could be due to defective batts.

Gurvinder says

Good day

I would like your help in the following scenario. I have been to many websites and each shows a different outcome and formula. I really appreciate your guidance

Total WH: 20000WHR per day (12 hours during night) 18kVa load

Panel : 310W x 80

Can you advise size of inverter and battery (off grid only)

Swag says

Hi, your panel output is 310 x 80 = 24,800 watts, multiplying this with minimum 6 hours sunshine gives 148,800 watt hour.

to operate a 18kva load you will require a 180kwh battery.

to charge a 180kwh battery, you will need at least 200kva power for minimum 6 hours.

your solar panel is 148.8kwh, which seems to be a little short for the battery.

you must upgrade to to at least 200kwh panel

inverter power handling capacity must be 20kva

David Ifeagbason says

Good day sir…Thanks for your post. I want to achieve a 60kva solar power and i have a 310watts panel..Am looking at using 24 solar panels and i have a 20kva inverter how many battries do i need to achieve this

Swag says

Hi David, you can calculate the parameters with the help of the explanation presented in the above article, you can also refer to the examples which I have discussed in the comment section.