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The Off-Grid Solar Panel System Calculator helps you size the battery
bank, watts of solar panels and the solar charge controller you need. The
calculator assumes you will need to size your system to get you through average
amount of sun-light in the least sunniest month of the year for your
location. This calculator assumes and adds for inefficiencies in batteries, panels, and wiring.

Battery Bank Sizing

This calculator will help you size the battery bank for your system.

Step 1:

Your Daily Energy Usage:

Example: 14 amps x 240 volts for 8 hours = 27000 wh



Watt Hours per Day:

(Amps x 240 volts x hours)

You can take monthly kwh from electric bill and divide by 30 for daily wh total

Step 2:

How Many Days Should Your System Run without Sun?

How many days of backup power do you want in case of cloudy/rainy days?

Step 3:

Adjust the Effective Capacity of Your Battery Bank Due to Low Temperatures

What is the lowest temperature your battery bank will experience?


(Florida and Bahamas choose 80F)


Select a battery bank voltage:

Note: All calculations assume a 50% discharge to your batteries to optimize battery life and 10% inefficiency


Solar Panel and Solar Charge Controller Sizing

This calculator helps you size the solar panel(s) and charge controller(s) needed for your system.

Step 1:

Determine the Solar Exposure for Your Site

You need to determine the average number of sun-hours per day during the
least sunniest month of the year. This is the ‘Isolation Value’

Select the State-City
Closest to your location (currently only US states are provided)


enter the average sun-hours for your location.

Florida and Bahamas = 5

Step 2:

Sizing Your Solar Panel Power Needed

The total wattage of
Solar Panels that you need is:

Watts, or


This value
takes into account losses due to system inefficiencies.

Step 3:

Determine How Many Solar Panels You Need in Your Array

How many solar panels do you need? That depends on the panel you

Select the wattage of the
panel your interested in, and see the results below:

watts per panel

You will need

panels for a total of


Sizing your Solar Charge Controller

You will need a charge controller that can handle amps

A Flex 80 from Outback can only use 5000 watts of panels at 80 amps, so if your total wattage exceeds 5kw you need to add additional charge controller.