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
Related: How to calculate electricity usage of your appliances?
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
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
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
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
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:
- What size wire from the solar panels to the solar charge controller?
- What size fuse or circuit breaker from solar panels to charge controller?
- What size wire from the solar charge controller to the battery?
- What size fuse or circuit breaker from the solar charge controller to the battery?
- Battery to inverter wire size calculator
- What size fuse or circuit breaker between the battery and the inverter?
