The Complete Off Grid Solar System Sizing Calculator

An off-grid solar system’s size depends on factors such as your daily energy consumption, local sunlight availability, chosen equipment, the appliances that you’re trying to run, and system configuration.

Below is a combination of multiple calculators that consider these variables and allow you to size the essential components for your off-grid solar system:

The solar array.
The battery bank.
The solar charge controller.
The power inverter.

Simply follow the steps and instructions provided below.

PS: For more information, I recommend checking out this detailed guide on sizing and designing an off grid solar system.

I get commissions for purchases made through links in this post.

Step 1: Determine your Daily Energy Consumption

The primary factor determining your off-grid system size is your Daily Energy Consumption, measured in Watt-hours (Wh) or kilowatt-hours (kWh). 1 kWh = 1,000 Wh.

The higher your daily energy usage, the more solar panels and batteries you’ll require. In fact, as you’ll see in the next steps, the sizing of these two components is based on your highest expected daily energy usage (Max. Watt-hours/day).

If you already have a specific number in mind, that’s great! You can move on directly to the second step.

If you don’t, the following calculator will help you list all appliances you plan to use each day, determine their energy consumption, and sum everything up up to estimate your highest daily energy usage.

Energy Consumption Calculator

Appliance:

Select an appliance from the list or enter one manually. If you select an appliance from the list, the calculator will estimate the power usage of the chosen appliance, and if the appliance operates on a duty cycle, the calculator will take that into consideration when calculating its energy consumption.

Power usage (Watts):

This is the "Rated Wattage" of the appliance, which indicates the maximum amount of electrical power (in Watts) it consumes during normal operation at full load.

Daily usage duration (hours):

This is the duration you use the appliance each day. If usage is only for a few minutes, divide the number of minutes by 60 to convert to hours.

Estimated Daily Energy Consumption of the appliance (in Watt-hours):

Your Total Daily Energy Consumption in Watt-hours (Wh):

0 Watt-hours per day (Wh/day)

Your Total Daily Energy Consumption in kiloWatt-hours (kWh):

0 kiloWatt-hours per day (kWh/day)

Step 2: Calculate the Wattage of the Solar Panel Array

The size, or Wattage, of your solar panel array depends not only on your energy needs but also on the amount of sunlight that’s available in your location, measured in Peak Sun Hours.

These “Peak Sun Hours” vary based on two factors:

Geographic location
Panel orientation (Tilt and Azimuth angles).

The calculator below considers your location and panel orientation, and uses historical weather data from The National Renewable Energy Laboratory to determine Peak Sun Hours available to your solar panels.

Using your daily energy usage and Peak Sun Hours, and assuming a system efficiency of 70%, the calculator estimates the Wattage required for your off-grid solar system’s solar array.

Off Grid Solar Panel Array Sizing Calculator

Your Daily Energy Usage (Wh/day):

This is the amount of energy in Wh (watt-hours) that the solar panels should be capable of producing daily. If left blank, the calculator will use the daily energy consumption calculated in the previous step.

Location:

Tilt Angle (degrees):

This is the angle at which the solar array will be tilted (degrees from horizontal). If left blank, a default value of 45 degrees will be used.

Azimuth Angle (degrees):

Define the Azimuth angle (degrees clockwise from true North) for the solar array's direction. For example, 180 degrees indicates a South-facing array.

Required Off Grid Solar Array Size:

W (Watts)

kW (KiloWatts)

Data source: NREL (National Renewable Energy Laboratory), as per NREL's terms.

Step 3: Calculate the capacity of the Solar Battery Bank

In the absence of backup power sources like the grid or a generator, the battery bank should have enough energy capacity (measured in Watt-hours) to sustain operation for several days during periods of low input from the solar array.

This is what’s referred to as “Days of Autonomy”. However, the more autonomy you go for, the larger (and more expensive) the battery bank will be.

Also, to optimize battery life vs. cost, it’s recommended to only use a percentage of your battery bank’s energy capacity and not go beyond a certain “Depth Of Discharge” (DOD) when discharging your battery bank.

This means that you’ll need to oversize the battery bank further if you’re going to follow these recommendations, which vary depending on the type of battery you’ll be using.

Generally, Lithium batteries have an optimal DOD of 80 to 100%, and Lead-Acid batteries an optimal DOD of 30 to 50%.

The calculator below takes these variables, along with factors like operating temperature and system efficiency, into account, and uses your daily energy consumption to calculate the required Energy Capacity of the battery bank.

Solar battery bank sizing calculator

Your Daily Energy Usage (Wh/day):

This is the amount of energy in Wh (Watt-hours) that the battery bank should be capable of supplying daily. If left blank, the calculator will use the daily energy consumption calculated in the 1st step.

Days of autonomy:

This is the number of days you want the battery bank to provide power without solar panel input. Please enter 1 if autonomy is not required.

Depth Of Discharge (DOD):

Please enter the percentage (%) of your battery bank's capacity that you plan on using (DOD). For example, if you only plan on using 50% of your battery bank's capacity, enter 50.

Lowest ambient temperature:

Choose the lowest ambient temperature where the battery bank's going to be stored.

Type (or chemistry) of the batteries you'll be using:

The type of battery you plan on using will determine how temperature efficiency.

Required Battery Capacity in Watt-hours:

- Wh

Required Battery Capacity in kiloWatt-hours:

- kWh

Step 4: Choose the right Solar Charge Controller

Whether you opt for a PWM charge controller or an MPPT charge controller, three specifications must be considered to ensure you choose the right controller your system:

Output Current rating (Amps): This represents the maximum amps the controller can output.
Input Voltage rating (Volts): This indicates the maximum voltage the controller can handle at its input (the solar side).
Output Voltage rating (Volts): This represents the battery bank voltage(s) compatible with the controller.

Assuming you plan on using an MPPT, the following MPPT sizing calculator will tell you what the required specifications are based on the specifications of your system, and will recommend a suitable charge controller based on the specifications.

MPPT Solar Charge Controller Calculator

Solar panel Wattage (W):

Enter the power rating (Wattage) of your solar panel(s).

Solar panel Open-Circuit Voltage (Voc):

Enter the open-circuit voltage (Voc) rating of your solar panel(s). To find this value, refer to the nameplate on your solar panel(s).

Battery bank's nominal Voltage (V):

Select the nominal voltage of your battery bank.

Lowest temperature in daylight:

Select the lowest temperature that you expect your solar panels to be exposed to in daylight.

Number of solar panels wired in series per string:

Enter the number of solar panels wired in series. If you have multiple strings in parallel, enter the number of series-wired solar panels in each string. If all of the solar panels are wired in parallel, enter 1.

Number of solar string (or panels) wired in parallel:

Enter the number of solar panels or strings of panels wired in parallel. If all of the solar panels are wired in series, enter 1.

Required MPPT specifications:

1. The MPPT should be compatible with a battery voltage of:

- Volts (V)

2. The MPPT should have a Maximum Input Voltage rating of more that:

- Volts (V)

3. The MPPT should have a Maximum Output Current rating of more that:

- Amps (A)

MPPTs that match these specifications:

Best Quality:

Best Value:

If you have a small system and plan on using a PWM charge controller, feel free to check out this PWM charge controller calculator instead.

Step 5: Choose the right Power Inverter

Inverters are rated in Watts, indicating the Electrical Power they can supply at their output. Selecting the right inverter requires ensuring it has a sufficiently high Wattage capacity to handle your appliances’ power demands.

But there are two Wattage ratings to consider:

Continuous Power rating: This represents the maximum amount of power the inverter can continuously supply.
Peak/Surge Power rating: This indicates the maximum power the inverter can briefly supply if power demands surge, typically due to an appliance starting up.

The following calculator allows you to list all appliances you want the inverter to be able to simultaneously run, along with their running and surge wattage. It then calculates the required inverter Wattage specifications based on these inputs.

Off Grid Inverter Sizing Calculator

Appliance:

Select an appliance from the list or enter one manually. If you select an appliance from the list, the calculator will estimate both the Continuous and Surge Wattages of the chosen appliance.

Continuous (Running) Power Usage (Watts):

This is the "Rated Wattage" of the appliance, which indicates the maximum amount of electrical power (in Watts) it consumes during normal operation at full load.

Surge (Peak) Power Usage (Watts):

This is typically the "Starting Wattage" of the appliance, representing the initial electrical power (in Watts) required for the appliance to initiate and gain momentum during startup.

The Continuous Power rating of the inverter needs to be greater than:

0 Watts (W)

The Surge Power rating of the inverter needs to be greater than:

0 Watts (W)

Note: Besides the power ratings of the inverter, ensure that the input and output voltage ratings of the inverter align with your requirements.

For instance, if your battery bank operates at 24 Volts, you'll require an inverter with a corresponding input voltage rating of 24 Volts.

And if you live in the U.S., you'll probably require an inverter with an output voltage rating of 120 Volts. Though, in some instances, you may need a split-phase inverter capable of outputting both 120 Volts and 240 Volts to power larger appliances like central AC units and dryers.

Additionally, consider the frequency and waveform of the inverter's output. Low-frequency pure sine wave inverters are generally recommended for their clean, smooth, and efficient outputs, along with their higher surge power capacity.

Step 6: Size your wires and fuses/circuit breakers

After you’re done sizing your off grid solar components and chose the right equipment, the final step to having a properly designed system is to size the wires (conductors) that will connect these components, and the Over-Current Protection Devices (OCPDs) such as fuses and circuit breakers that protect the components and wires.

Each pair of wires must meet these criteria:

The wires need to be thick enough to handle the maximum Amps that may flow through them.
The wires need to be thick enough to limit the Voltage Drop from a component to the next to an acceptable value.

And for the OCPDs (fuses/circuit breakers):

The Amp rating on the fuse/circuit breaker needs to be at least 1.25 times greater than the maximum current (amps) allowed to flow through it.
The Amp rating on the fuse/circuit breaker needs to be low enough that it would blow/trip if the current exceeds acceptable levels.

Since current and voltage vary across different points of the system and each piece of equipment operates differently, sizing each wire pair and OCPD is done separately and differently.

To help you size each of them properly, I’ve made the following calculators and step-by-step guides:

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