Off-Grid Solar System Sizing Calculator

Why size for worst-month sun hours instead of annual average?

A grid-tied system can borrow from the grid on a cloudy week and export surplus on a sunny one, netting out over a year. An off-grid system has nowhere to borrow from, so it must generate enough even during your location's worst month (typically December-January in most of the US) or you'll run the batteries down with no backup.

How many days of autonomy should I plan for?

2-3 days covers typical multi-day cloudy stretches in most climates; remote cabins or critical medical-equipment applications sometimes plan for 5+ days. More autonomy days means a much bigger, more expensive battery bank, so this is a real cost-vs-risk tradeoff, not a 'more is always better' decision.

Should I add a generator to an off-grid system?

Almost universally yes for anything beyond a small seasonal cabin — a backup generator lets you size the battery bank for typical conditions rather than worst-case-ever conditions, since the generator covers the rare extended low-sun stretch instead of requiring the battery to handle it alone. This usually cuts total system cost significantly.

Why is the inverter sized above peak load, not equal to it?

The 1.25x multiplier gives headroom for motor startup surges (well pumps, refrigerator compressors) and avoids running the inverter at its absolute continuous limit constantly, which shortens its lifespan. Check the specific inverter's continuous vs surge rating against your largest motor's startup draw too.