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Off Grid Solar Calculator

There are many reasons you might want to go off grid and produce all your own electricity using solar power. Whatever your motivation, you need to determine the size of your solar power system.

A complete home solar system needed to go off grid is made up of many components that all need to be sized to get the power you need. Don’t worry, it’s not as complicated as it sounds. This off grid solar calculator will walk you through the entire process and provide a number of handy online calculators that make it a breeze to determine the size of your off grid solar system

But first, how do off grid solar systems work?

How do off grid solar systems work?

large solar panel setup in field

When you’re on the grid you draw all your electricity from the local utility and pay for the amount you use. Going off grid means you’re severing yourself entirely from the grid and generating the electricity you need by yourself. You never need to worry about monthly bills or random blackouts again.

An off grid solar system starts with the solar panels absorbing incoming solar energy and converting into DC energy. To make the most of your panels, they need to be positioned so that they get the most peak sunlight hours. The total solar energy output of your panel array must be able to generate your entire day’s electricity during those peak sunlight hours.

The energy created by the panels flows through cables into the house. All this energy is stored in a bank of solar batteries that you can draw from as needed throughout the day, even when the sun isn’t shining. A charge controller protects the batteries against overcharging.

The energy stored in the batteries is still DC energy and runs to an inverter which converts it into usable AC energy that is distributed around your house and accessed through electrical outlets.

For smaller systems, a solar generator is a great option. It combines the inverter, charge controller, and batteries into one handy unit.

Elements of the off grid solar calculator

This off grid solar calculator will help you figure out everything needed to size your off grid solar power system:

  • How much battery capacity do you need to store your daily energy needs, plus whatever extra power you want to have available for emergencies?
  • How much space do you have available to install solar panels, and how many panels could you fit there?
  • How many hours of peak sunlight do you get?
  • How much energy do you need to produce during each hour of peak sunlight to meet your daily needs?
  • How many solar panels do you need?
  • What size charge controller do you need to protect your batteries?
  • What size inverter do you need to handle the solar output from your array?

That might seem like a lot of work but it’s not really that hard, and there are also a few online off grid solar calculators that makes it much simpler to do.

How much battery capacity do you need to store your daily energy needs?

solar panels connected to building on top of mountain

Calculating the size of the battery is a matter of determining your solar needs, which involves taking all the appliances you need for your system and finding the output wattage of each appliance (or looking at your maximum daily energy consumption on your electricity bills if you’re currently on the grid).

Determining watt-hours

Most appliances have a label on them that shows the wattage. In some cases, it only gives you volts and amps, but you can calculate the wattage by multiplying those two values together. For example, the average toaster uses 120 Volts and 9 Amps. This gives you a draw of around 1,100 watts (120 x 9 = 1,100).

Off grid solar calculator: household appliance watt usage chart

Once you have the wattage, you need to ask yourself how long you plan on using each appliance over the course of a day.

If you multiply the wattage of an appliance by the number of hours it’s used, you get the watt hours per day that the specific appliance uses. So, if you use that same toaster for an average of ten minutes a day, you get 183 watts per day ((10/60) x 1,100  = 183.33)

Rinse and repeat this process for each appliance and add the watt hours together. Now you’re left with the total number of watt-hours that your solar system has to produce to meet your daily needs.

You can make this all very simple by using an online off grid solar calculator to calculate watt-hours.

This watt hours per day value is also needed if you’re using a solar payback calculator so you might want to keep this value on standby.

Determining battery size

solar panel farm setup

Now that you have the watt-hours per day, we can get to figuring out the size of our battery.

This is the size of battery you need to hold the power you need for one day, but it’s not the size of battery you actually need. What about the days when the sun just isn’t shining? You can still generate some power on cloudy days, but you won’t be getting peak sunlight so you won’t be generating enough to last you through those days.

Let’s try an example:

  • Assume your daily energy needs are 11,000 wH
  • Assume you want to have at least one extra worth of energy stored up in case of a rainy day when you’re not producing optimal power
  • 11,000 x 2 = 22,000
  • Batteries are normally measured in kWh, so divide the wH by 1,000
  • In our example it’s 22,000 ÷ 1,000 = 22 kWh
  • The perfect choice would be the awesome Mammoth PRO battery which can store 23.5 kWh; the Mammoth is a great choice also because it’s 48V and the higher the voltage of your batter the lower the amperage of the charge controller you need, and the lower the amperage the less expensive it is

How much space do you have available to install solar panels and how many panels can you fit there?

Solar panels are typically installed wherever you can get the most sun, usually on a roof but often on a rack secured into the ground. You need to decide where you want yours to go by balancing various considerations:

  • Does it get enough sun?
  • Does it face towards the sun at its zenith?
  • Is any part of it shaded for part of the day by trees or other obstacles?
  • Will the array be in the way?
  • If on the ground, will it be inconvenient or in danger of getting damaged?
  • Will you be happy with how it looks?

These are important things to consider, but the deciding factor will be is there enough space? 

Figure out the maximum area of your chosen space, then calculate how many panels could fit there. The two most popular sizes of panels are 200W and 400W:

  • 200W panels measure approximately 60” x 30”
  • 400W panels measure approximately 80” x 60”

For example, if a nice, big south-facing roof measures 40’ x 20’ you could fit:

  • 400W
  • 6 across
  • 4 up
  • 6 x 4 = 24 panels
  • 200W
  • 8 across
  • 8 up
  • 8 x 8 = 64 panels

How many hours of peak sunlight do you get?

Where you live determines how much sunlight you get each day. There are a few hours each day when the amount of sunlight you get is at its maximum, which is called peak sunlight hours. 

You can look up online what your exact peak sunlight hours are, but the average in the US is about 4.5 hours.

This can be determined by viewing a solar map showing the amount of sunlight per day based on your geographical location. These maps are color coded from a peachy color to a dark red, ranging from low to high exposure.

Off grid solar calculator: calculating with peak sunlight hours map USA

If you want an off grid solar system for your RV and you plan on traveling around the states, read the number of hours of each state you go to and average it out.

The average number of peak sunlight hours is 4.5 in the US.

How much energy do you need to product during each hour of peak sunlight to meet your daily needs?

When sizing a solar power system, we only use peak sunlight hours to calculate how large it needs to be by dividing your total daily energy needs by the number of peak sunlight hours you get.

In our example we need 11,000 wH per day. Assuming the national average in peak sunlight hours of 4.5:

11,000 ÷ 4.5 = 2,444
The total wattage of our solar panels must be at least 2,444 watts

How many solar panels do you need?

solar panel farm

Knowing how much we need to produce in an hour we can easily figure out how many panels we need to do it by dividing the hourly requirement by the size of the panel and rounding up. Using our example of 4,666 watts per hour:

  • 200W
  • 2,444 ÷ 200 = 12
  • 400W
  • 2,444 ÷ 400 = 6

We know we can easily fit enough of either panel into where want to put them, so we could choose either size of panel. It’s a matter of preference.

  • Fewer 400W panels means less chance of anything going wrong
  • 200W panels weigh less so are easier to move and install
  • The cost per W is lower the bigger your panels are so you will save a bit of money going with the 400W panels

What size charge controller do you need to protect your batteries?

A good charge controller does more than just protect the batteries from overcharging, but also optimizes the transfer of energy into the battery.

To determine the minimum amperage of the charge controller you need, take your solar array’s power and divide it by the voltage of the battery you’re charging.

In our example our solar panels produce 2,444 watts and our Mammoth PRO battery is 48 volts:

  • 2,444 ÷ 48 = 50.9
  • You don’t want to be too close to the minimum amperage, so the rule of thumb is to multiply this by 1.25: 50.9 x 1.25 = 63.6 amps
  • Our charge controller must be at least 50.9 amps but closer to 63.6 amps would be best
  • The Outback Power FlexMax FM60 MPPT Charge Controller would be a great choice

What size inverter do you need to handle the solar output from your array?

The inverter converts the DC power produced by the solar panels and stored in the batteries to AC power used by your appliances. Its size should be slightly above the size as your solar array.

In our example, the array was 2,444 W so the inverter must be at least 2,444 W too. The GroWatt SPF 3000TL LVM solar inverter would be a great choice because it can more than handle your array size as well as the 48V of your battery.

Go off grid the easy way with a solar kit


which solar kit is right for you?


There are a few things to figure out when calculating your off grid solar system, but none of them is very complicated. It’s worth doing the calculations yourself because it really helps you to understand how your solar system works, as well as giving you a good idea of the components you need. 

In order to save you some headaches and know for sure that all the components you get are going to work together well, consider opting for a solar kit. The solar kit includes every component you need, thoroughly tried and tested to guarantee that you’ll be getting optimal performance and the most bang for your buck.

Plus, if you buy your solar kit through you have access to solar experts who can go over your calculations to make sure they’re correct and advise you on which solar kit would work best for you.

Off Grid Solar Calculator Conclusion

While manually figuring out the size of your solar system will get you an answer, you could get a more accurate and efficient way of getting the answer with an off grid solar calculator.

There’s a variety of them online but each has the same approach to sizing your system. offers a number of handy online calculators to help make it easy for you to size the right solar system to meet your individual needs:

Previous article Sizing Off Grid Solar Systems

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