Off Grid Solar Calculator

What an off grid solar calculator actually sizes

An off grid system has no utility connection to fall back on, so every part of it has to be sized for your real energy use. If the panels are too small, the batteries never fully recharge. If the battery bank is too small, the lights go out after one cloudy afternoon. If the inverter is too small, it shuts down the moment a pump or a microwave starts. This calculator sizes all four core parts in one pass: the solar array, the battery bank, the inverter, and the charge controller.

You give it your daily energy use, the peak sun hours where the system will sit, the number of cloudy days you want covered, and a few equipment choices. It returns the panel wattage and panel count, the battery capacity in kilowatt-hours and amp-hours, the inverter rating, and the charge controller current. The numbers are planning estimates, so treat them as the starting point for a shopping list rather than a finished wiring diagram.

The formulas behind the results

The 1.25 factor in both lines is a common safety margin. It keeps the equipment below its limit during normal use and leaves room for cold, bright days when panels produce more current than their label suggests.

How to size an off grid system, step by step

The steps below follow the same order the calculator uses. Working through them by hand is a good way to check any result you get.

Step 1: Find your daily load. List every appliance, its wattage, and the hours it runs per day. Multiply watts by hours to get watt-hours, add them all, and divide by 1,000 for kilowatt-hours. Be honest about winter use, since lights and heating fans run longer when days are short.

Step 2: Use worst-month sun hours. A system sized for June will struggle in December. Pick the lowest month you plan to live in the property.

Step 3: Choose your days of backup. Look at how many gray days in a row your location gets. Two to three days is typical for a cabin with no generator.

Step 4: Pick battery chemistry and voltage. Lithium needs a smaller bank than lead-acid for the same load. Higher voltage keeps current, wire size, and controller cost under control.

Step 5: Size the inverter from your peak load. Count only the appliances that can run together, then add the safety margin.

Typical daily energy use for common off grid loads

These are rough figures to help you build a first load list. Check the label on your own appliances, because efficiency varies widely between models.

Heating and cooling appliances such as electric heaters, air conditioners, electric kettles, and dryers can each use more than the rest of the house combined. Off grid designs usually switch these to propane, wood, or solar thermal instead of sizing batteries and panels around them.

Worked examples

Common sizing mistakes to avoid

More CalcMora tools for planning a solar setup

Once you know what size system you need, a few other calculators help with the surrounding decisions. If you are also weighing a grid-tied system, the solar panel savings calculator shows payback period and 25-year return from your electric bill. Long cable runs between the array and the batteries lose power, and the voltage drop calculator helps you check whether your wire is thick enough. Running a NAS or small homelab off grid adds a steady load, which the server power cost calculator converts into daily watt-hours. To learn how many hours of usable light a site gets, the sunrise sunset calculator gives exact daylight times by city. If your off grid plan involves a motorhome, the RV loan calculator covers the financing side. And because cloud cover drives how many backup days you need, the rain probability calculator is a quick way to check the weather for a planned build weekend.