If you want year-round off-grid power, size for the worst month—usually winter in temperate regions: shorter days, lower sun, and fewer peak sun hours. This guide explains how to design for that.
Why the Worst Month?
Solar yield is lowest when:
- Days are short (winter).
- Sun angle is low (weaker irradiance).
- Cloud and rain/snow are more frequent.
If you size using summer sun hours, you’ll have plenty in July and run short in December. Sizing with winter (or worst-month) sun hours and your typical daily use keeps you from undersizing. You may “overproduce” in summer; that’s often acceptable and can extend battery life by not deep-cycling as much.
Get Worst-Month Sun Hours
Use a solar map or calculator that shows monthly peak sun hours for your location. Pick the lowest month (often Dec or Jan in the northern hemisphere; Jun–Jul in the south). Use that value in:
Array (W) = Daily use (Wh) ÷ Peak sun hours (worst month) ÷ Efficiency
Round up and add 10–20% margin if you want extra safety.
Tilt and Orientation
- Winter: A steeper tilt (latitude + 10–15°) captures more low winter sun and can improve winter yield.
- Year-round compromise: Tilt ≈ latitude. Fixed tilt is a compromise; the worst month still drives the size.
Shading (trees, buildings) is worse in winter when the sun is low. Keep panels clear of shade in the worst month.
Battery and Days of Autonomy
Winter often has more consecutive cloudy days. Consider 3–5 days of autonomy instead of 1–2 so your battery carries you through stretches with little sun. See days of autonomy.
Summary
Use worst-month peak sun hours and your daily use to size the array; add margin. Consider more days of autonomy for winter. Use the WattSizing calculator with conservative sun hours to get recommendations that work in the low-sun season.
