Planning an off-grid system starts with three questions: how many solar panels, how big a battery bank, and what charge controller. Get them wrong and you either run out of power at night or pay for equipment you never needed.
This calculator turns your daily energy use into a clear starting point. Enter how much power you use, how much sun you get and the battery type, and you will see the panels, battery size and controller rating to plan for.
Solar panel and battery bank calculator
How the solar sizing calculator works
First, your daily use in watt-hours is divided by the peak sun hours and by 0.75, which allows for about 25 percent system losses from wiring, heat, dust and the charge controller. That gives the size of the solar array in watts. We then divide by your chosen panel size and round up to a whole number of panels.
For the battery bank, the calculator multiplies your daily energy by the days of backup, then divides by the usable depth of discharge and by 0.9 for inverter and battery losses. Dividing the result by the bank voltage gives the amp-hours. The charge controller must handle the array power divided by the battery voltage, with a 25 percent margin.
What are peak sun hours?
Peak sun hours are not the same as daylight hours. One peak sun hour means the sun delivers 1,000 watts per square metre for an hour. Most places in the world get roughly 3 to 6 peak sun hours a day, depending on the season and weather. Use a lower number if you need power all year in a cloudy area, and a higher number for a sunny summer-only setup.
Tips for planning an off-grid system
Measure before you guess
Add up your devices with our inverter size calculator, then multiply the watts by the hours you use each device to get daily energy use.
Plan for bad weather
Extra days of battery backup keep the power on through cloudy spells. Two days is a common starting point for a cabin.
Match the inverter to the system
Choose an off-grid or hybrid inverter that supports your battery voltage and has an MPPT charge controller built in if possible. See our solar and off-grid inverter guide.
Frequently asked questions
How many solar panels do I need to run a refrigerator?
A typical refrigerator uses about 1 to 2 kWh a day. With 4 peak sun hours that needs roughly 350W to 700W of solar, which is 2 to 4 panels of 200W.
How big a battery bank do I need for one day of backup?
Multiply your daily energy use by the days of backup and divide by the usable depth of discharge. Our calculator does this for you.
What is the best battery voltage for a solar system?
Use 12V for small systems under about 1000W, 24V for mid-size systems, and 48V for larger ones. Higher voltage means lower current and thinner cables.
Do I need a charge controller?
Yes, unless your inverter has one built in. It stops the battery from overcharging and gets more power from the panels when it uses MPPT.
Please note: Results are planning estimates. Panel output changes with shade, season, angle and temperature. Ask a qualified installer to check any system before you build it.