Off-grid solar systems are the go-to if you’re looking to become energy-sufficient, and whether it’s a DIY project or not, you’ll need to know a few basics. This is why we’ve put together a guide for you based on science and real-life data.
In this article, we give you the right tools and take you through the right process that will help you figure out how many solar panels you’ll need to power your RV, tiny house, van, bus, boat, cabin, or even an off-grid house.
Technically, the process of sizing an off grid solar array consists of 3 main parts:
- Calculating your daily energy consumption.
- Calculating your battery bank capacity.
- Sizing your solar array.
If you follow these steps and with the help of a few calculators in this guide, you’ll be able to calculate the number of solar panels you need.
Let’s begin with the first step.
Part 1: How to calculate your daily energy consumption?
Contrary to what some might think, sizing a solar system properly can’t be done based on the square footage of a house or anything like that.
Figuring out how much solar power you need is mainly based on your power consumption; or in general, the power consumption you would like to offset using solar.
There are two ways to do this: the easy way (and we’ll start with that), and the hard way.
The easy way is if you’re already on-grid and you have electric bills showing you how much power you consume each month in kWh (kilowatt-hours).
The hard way is if you’re not grid-tied, or you if you just want the solar panels to power specific appliances.
How to calculate your daily energy consumption using an electric bill?
Electric bill layouts differ from a company to another, but all you have to do is find your power consumption in kWh (kilo-Watt-hours) for a given month. You’ll typically find that information inside a table or a bar graph.
It’s a monthly bill, so simply divide that power usage in kWh from the table by the number of days billed to get your average daily usage.
In the pole graph above, the average daily consumption was already calculated so make sure you don’t divide that any further.
For efficiency purposes, we recommend that you use the value from the month in which you consume the most amount of energy. In the pole graph above, we can see that July is the month for which the highest amount of energy is consumed.
How to calculate your daily energy consumption for specific appliances?
Whether you want to offset all of your energy consumption or just the energy a specific appliance such as an AC, fridge or a space heater uses, the first thing to do is to make a list of all the appliances you want to run off your solar system.
Then you will have to calculate exactly how much energy each appliance will consume per day.
You can do that by figuring out 2 things for each appliance:
- Device wattage
- How many hours you use it per day
To find or define the wattage on an appliance, you’ll have to look for the power consumption label stuck or imprinted on the device. On that label, you’ll have to find the Output rating, which can either be in Watts or Volts and Amps (or milliamps).
For this example, I took a picture of the label stuck to the bottom of my electric water heater. The label shows that my water heater has an output wattage of 1200.
If you can’t find the output wattage, the label will probably provide a voltage and amperage. Below is a picture of my laptop’s output voltage and amperage.
To get the power value in Watts, simply use this equation:
Watts (W) = Volts (V) x Amps (A)
Sometimes the label will give you the amperage in mA (milliamps) rather than A (Amps). To convert milliamps to Amps divide the mA value by 1000.
For example, my laptop has an output wattage of 65W = 20V x 3.25A.
If you can’t find the wattage, voltage, or amperage on the label, or if there’s no label, to begin with, try to google the model number of the device to find that information.
Daily Hours of use
Now that I have my water heater’s and laptop’s output wattage, I can calculate their daily power consumption by using this equation for each of the devices:
Daily power consumption in watt-hours (Wh) = Output wattage in watts (W) x Daily hours of usage (h)
But first you have to estimate the number of hours for which each device is on.
I use my laptop for around 6 hours a day, so the equation would be:
Daily power consumption (Wh) = 65 W x 6 h= 225 Wh
For the electric water heater, from my experience, I know that it takes around 1 hour to heat up the water after it refills. So the equation would be:
Daily power consumption (Wh) = 1200 W x 1 h= 1200 Wh
Now if I wanted to size an off grid solar system that would power my laptop and water heater, it would need to provide 1425 Wh of daily power to feed both of them.
However, that is not the size of the solar array; we are not quite there yet.
We encourage you to do your own calculations and find out the energy consumption you’d like to offset using solar before moving to the next steps.
For this example, we’ll use the energy consumption of both my laptop and electric water heater (1425 Wh)
Now that we have the average daily energy consumption we’d like to offset, we can continue.
Part 2: How many batteries do I need for solar?
There’s one component that makes all the difference when sizing a grid-tied system vs an off-grid system, and that’s the battery bank.
In off-grid, you can’t really define a solar array size until you figure out the capacity of your battery bank.
It is a bit complicated, but don’t worry we have a few calculators that’ll make it quite easy for you.
But let’s dive a little bit into the science behind this.
How to calculate the battery bank capacity for your off-grid system?
Since the only energy available to you is the energy your solar panels produce, you’ll need somewhere to store that energy, so you can use it at the later hours of the day or when there isn’t much sunlight in general.
Sizing a battery bank for your off-grid system generally depends on 5 factors:
- Your energy consumption
- Days of autonomy: The number of days for which you’ll potentially have to rely entirely on your batteries for energy.
- Temperature: The lowest temperature your battery bank will be exposed to.
- The type of batteries: The chemistry of your battery bank.
- The voltage rating: The voltage at which your solar energy system will be rated at.
Above in this article, we’ve already discussed the energy consumption bit, so make sure you have that so you can enter it in the battery capacity calculator below.
Days of Autonomy
This is the number of days for which your battery bank can supply the energy you need without recharging (i.e. days of autonomy or “no sun days”).
This number is totally relative to your location. If you are equipped with a generator you can use to recharge your battery bank, fewer days of autonomy are required.
This number is important because not only you might get frequent power outages, but also because if your batteries’ capacity is smaller than necessary, you’ll have to drain them all the way; doing so repeatedly will cause serious damage.
This is because the deeper you discharge a battery, the shorter its life span will be. The factor associated with this is called Depth of Discharge (DoD), and we will discuss it in a minute.
Now keep in mind that the more days of autonomy, the bigger the battery bank, and the more expensive it is. But it’s better to have a bigger battery bank that will last you a decade, than to have a smaller one that you will have to change every couple of years.
As a rule of thumb, we recommend going with 2 to 5 days of autonomy.
Effect of temperature on battery capacity
Batteries are typically rated at room temperature: i.e. 77°F (25°C). If the temperature goes lower than that, the battery capacity decreases while its lifespan increases. If the temperature is higher, the capacity increases and its lifespan decreases.
We can safely say that batteries operate optimally at room temperature; but if the temperature is different from that, it should be taken into consideration when sizing the battery.
The chart below represents how the capacity changes relative to temperature for 4 different types of batteries.
When calculating the capacity required from our battery bank, we usually use a temperature correction factor (battery temperature multipliers).
An appropriate set of temperature multipliers are already included in the battery capacity calculator, so don’t worry about that; all you have to do is select a temperature value.
Next on the list is the tech behind the battery.
The type of the battery used is important to size our solar panels, because it helps us determine 2 important factors:
- The optimal depth of discharge for that particular battery type
- And the efficiency of the battery
Depth of Discharge (DoD)
Battery capacity is measured in Wh (Watt-hours), or Ah (Amp-hours) for a given voltage, but the usability of that capacity really depends on what’s called a Depth of Discharge, or DoD for short.
The chart above compares the number of charge/discharge cycles relative to the depth of discharge of 2 common batteries, a li-ion battery and lead acid battery.
Both batteries are affected by how deep you discharge them, but we can clearly see that, for a lithium battery you can get almost the same number of cycles at 80% DoD that you would get for only 50% DoD for a lead acid battery.
In other words, the deeper you discharge a battery, the fewer charge/discharge cycles it’ll have left. But some batteries allow you to use more of their capacity without degrading as much.
A 100 Ah typical sealed lead acid battery with only 50% optimal DoD, will only allow you to use 50 Ah of that capacity before it starts seriously degrading.
While a 100 Ah typical Lithium Ion battery with 80% optimal DoD will allow you to use 80 Ah of that capacity, and even more if you choose to trade a few cycles for the extra capacity.
We generally recommend using Lithium batteries.
But the point is:
Knowing and defining a depth of discharge for a certain battery bank, can help you balance the cost of that battery with how often you will need to replace it.
Generally, there always will be losses, but the question is how much losses are generated when you charge and discharge a battery.
The answer is also relative to the chemistry of that particular battery.
Typically, a lead-acid battery generates around 10% to 15% in electricity losses, while Lithium-Ion batteries only generate around 1%.
This means that before you can define a size for your battery bank and solar array, you should know what kind of battery you‘d be using.
PS : this calculator also takes inverter efficiency into account In these calculations.
The voltage rating
We won’t dive too deep into this, as it’s a bit complicated and would make this article too long.
What you should know is:
More voltage doesn’t mean a smaller or cheaper battery, but it can mean an overall cheaper solar energy system.
We recommend you check our article on the main differences between 12V, 24V, and 48V solar energy systems, which will definitely give you a complete understanding of this topic.
Off-grid battery bank capacity calculator
Enter the appropriate values into the calculator to get a battery size in both Wh (Watt-hours) and Ah (Amp-hours).
these are the values to submit:
Energy consumption: Make sure your energy consumption is in Wh (Watt-hours).
Days of Autonomy: We recommend 2 to 5, but you should choose a number adequate to your circumstances.
Temperature: Choose the lowest temperature your batteries will be exposed to.
Battery type: The battery of your choice.
Depth of Discharge: We recommend no more than 50% for Lead-acid, and 80% for Lithium-ion. Keep in mind that the more DoD the smaller the battery size.
System Voltage: Select a battery bank voltage.
As an example we used 1425 Wh as the energy we would like to offset, and we settled on 3 days of autonomy.
As a location, we’ll use Austin, Texas for this example; in the last 50 years, the average lowest temperature for that area is around 14°F (-10°C). We’ll assume that the battery pack will be stored outside, so we’ll use that value as the lowes temperature.
We recommend Lithium batteries so will check Li-ion as a battery type, and we’ll go for 80% DoD. For the voltage, we’ll go with a 24V system.
We’ve entered these values in the battery capacity calculator, and got the following result:
A battery bank consists of a few batteries wired together either in parallel (for higher currents) or series (for higher voltages). For such applications, the individual batteries typically come in 100-400 Ah.
In our example, the battery capacity needed is 396 Ah, and since the available batteries on the market come in 100-400 increments, we find it’s best to go for 4 batteries of 200 Ah each.
This would give us a 24V – 400Ah battery bank, which is able to contain 9600 Wh of energy when fully charged.
And to achieve this, we would wire these batteries in two parallel strings of two in series.
Now that we’ve finally sized our battery bank, we can size our PV array and calculate how many panels we will need.
Part 3: How many solar panels do you need for your off-grid system?
Typically, a 1Kw solar array consists of 3-5 solar panels that each have an output rating of 200-400W.
So after determining the power consumption that you’d like to offset and the capacity of your battery bank, you’ll need to calculate the required solar array wattage.
To calculate the required solar array wattage, one more variable needs to be accounted for, and that’s the number of daily peak sun hours you would have access to.
Once you have your solar array wattage, the number of solar panels you need will depend on their individual Watts output.
Our off-grid solar calculator will do all the work for you, but first, let’s see what the number of peak sun hours means.
What is the number of peak sun hours?
This is the number of hours for which the sun would be shining at its maximum, and it depends on the location. This number is not to be confused with daytime hours.
The width of the yellow square represents the number of peak sun hours, while the curve represents the solar irradiance through daytime hours.
This number is particularly important because it can help us define the solar panel power output we will need to produce a certain amount of energy daily. But we’ll talk about that in a sec.
How many peak sun hours do you get?
Don’t worry, figuring out the peak sun hours you get in your location does not involve generating graphs and calculating surface areas.
The map below shows the annual average of daily peak sun hours in different areas in the United States, supposing that the solar array is fixed and facing south.
Now, once again, these numbers are annual averages, meaning you get fewer peak sun hours in the winter than the yearly average number given (approximately 20% fewer hours).
If you want the best efficiency for your system, we recommend using the peak sun hours you would get in December (the month with the fewest of those).
You can easily get that information using this awesome tool, all you have to do is enter a Zip Code.
We used a Zip Code from Austin, Texas and got the following results:
A person living in Austin, Texas, would use 4.11 as the number of peak sun hours. Let’s use that for our calculations.
Off-grid solar panel calculator
Now that we have our battery bank capacity and the number of peak sun hours, we can easily calculate the required solar array wattage and the number of solar panels using the calculator below.
These are the values the enter or select:
Battery Size: Make sure to enter the battery size is in Wh (Watt-hours).
Peak Sun hours: Enter the peak sun hours number in your location.
The wattage of individual solar panels: Select the power rating of the panels you’d like to use. Keep in mind that the more the wattage, the fewer the panels you’ll need.
Battery depth of discharge (DoD): Select the same DoD you’ve selected when using the battery bank capacity calculator above.
In our example:
- The battery bank rating in Wh is 9600.
- 4.11 peak sun hours available daily.
- 250 W as the output of each of our panels.
- 80% DoD.
Here are the results:
in our example, the solar array has to have at least 1870 Watts of output. One way to configure such an array would be by using 8, 250 W solar panels. This would give us a 2 kW solar array.
Because this is a 24V system, these panels would be wired in 2 parallel strings, each string with 4 solar panels in series.
Going off-grid can be very expensive, so if your solar system costs a little bit over your budget, you can start small and then grow the system gradually.
And budget might not be your only limitation here, you just might not have enough space to install all the panels you need.
In that case, a good idea would be to go back to your appliances list and cross off anything that’s not a priority.
If this system is for your house, maybe consider going with a grid-tied solar system rather than an off-grid one, which costs much less. Check our article on how many solar panels do you need to power your house.