Solar sizing is the bottleneck for most off-grid builds. Undersize and your battery never reaches 100% on cloudy days. Oversize and you spend extra on panels that produce dump load you can not use.

Step 1 - Daily energy demand

Sum every load running on the battery in Wh per day. The Battery Bank Sizer above uses the same number, so the two tools work together.

Step 2 - Peak sun hours

Pick your region. The calculator uses regional averages. If you have a specific PVGIS or NREL number for your exact location, switch to custom and enter it.

Step 3 - Sizing season

Year-round uses the annual average. Summer-only sizes the array smaller (you accept that winter will not cover demand). Winter-capable sizes for the worst month - the array is significantly larger but you produce surplus most of the year.

Step 4 - System losses

Default 25% covers panel temperature (-0.4%/°C above 25°C), dust, wiring resistance, MPPT efficiency (~97%), and tilt/orientation deviation. Bump to 30% if panels mount flat with no tilt or in dusty conditions.

Step 5 - MPPT charge controller

Array W divided by system V gives DC current. Multiply by 1.25 safety, round up to a standard MPPT size. That is the minimum current rating - oversizing the MPPT a little gives headroom for future expansion.