Knowledge Base : Frequently Asked Questions : Solar Charging FAQ : OK, So What Size Panel Do I need?

OK, So What Size Panel Do I need?

Which Solar Panel Do I Need?

Solar Calculator Notes and Instructions.

First, the notes: We hope this solar calculator will make sizing your panels and batteries a little less painful. Keep in mind that this is only a calculator, and it will directly reflect whatever you, the user, inputs into the fields. If your base calculations are off by even a little bit, the results reflected can be skewed by quite a bit, so view this as a guide, not an absolute.

Solar PanelWith that said, here comes the disclaimer. This calculator is for educational purposes only. is in no way responsible for the results of your calculations, and if you purchase a system based on the results of the Solar Calculator, BatteryStuff will not and cannot be held responsible for returns or exchanges for improperly sized systems. Normal RMA and exchange rules will apply to all solar panel purchases.

See policies for more info.

Now the instructions:

Field #3: This field needs to be DC watt draw only. If you are using an inverter, meaning your device is rated in AC amps and 110 V, you will need to convert that number into DC watts before entering it in the field. Then you will need to add about 10% due to the inefficiency of the inverter. To get there, use the following formulas;

1 Amp AC = 10 Amps DC. (example, 2AC amps =20DC amp)
Add 10% (20.4 amps)
DC amps x 12v = DC watts. (20.4 x12 =244.8 watts)
244.8 would be entered in field # 3

Fields #6 and #12 are for how many hours you expect your equipment to run in a 24 hour period, and your input voltage (12, 24, 36?).

Fields #14 and #18 will determine what size and how many batteries you need. In #14, insert days of backup you would like your battery pack to be good for. This is minus any solar panels, which we will figure in a minute. Field #18 is based on what battery you choose. Say you want to use a 55 AH battery because you like the dimensions, or maybe you like the 21 AH battery due to its terminal configuration. Enter your chosen battery amps there. We don’t like to see any battery discharged more than about 50%, so we will automatically adjust for that.

Still with me? Good, we are almost done. The last two fields, #22 and #25 are easy. How many hours of direct sunlight do you estimate your panel will get. Be realistic. We will then automatically guesstimate for clouds, bad weather etc. Field #25 is just like field #18 in the battery section. Look at our solar page, pick a panel you like and then enter the watts here.

That's it, were done. Not so bad, was it? Click "Next" below to take you to the calculator.


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8 Responses,   5.0 Rating

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  • if i have a rectangural solar panel, how should i align it i.e along its length or along its breadth? will it affect its output? if yes then why?

    April 20, 2013 a 10:31 am
    • The only thing that will affect it is the angle of the panel in regards to the sun. Laying the panel on its length or breadth will not affect it.

      Jeremy Fear
      April 24, 2013 a 9:31 am
    • I have a 12 volt battery that powers my boat trolling motor and I would like to use solar to recharge the battery when the boat is not in use. What do you recommend?

      Jack Piper
      June 9, 2016 a 5:57 am
      • If you are charging them from a discharged state you need to know how many amp you have pulled off the battery in order to know how many you need to put back in. The second factor is time… How quickly do you want to charge the battery back up? Leaving a battery sitting discharged is never a good idea, so it is ideal for them to be recharged fairly quickly… Another words it would not be ideal for it to take a week for the solar panel to recharge the battery. Ideally you would want the battery fully charged by the end of the next day.

        BatteryStuff Tech
        June 9, 2016 a 12:08 pm
      • How many amp of battery i use for 1hp water pump..
        3.3amps of induction motor in 8hr running? Thank you..

        August 2, 2014 a 1:48 am
        • Due to the fact that we don’t have all the information for your system we would need you to contact us at so that we can ask the appropriate questions, to get you the answer that you may need.

          August 15, 2014 a 2:47 pm
        • Looking for off grid solar for a cabin 20×20 that does not have power, to run tv, small frig air conditioning/ heating and lights thanks

          April 26, 2016 a 8:11 am
          • I would suggest reading our article called Solar Systems the Right Way. This will at least give you the basics you need for a solar system, however with most home style solar systems we recommend contacting a local home solar expert in your area for more specific advise.

            BatteryStuff Tech
            April 26, 2016 a 8:49 am
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