Worst-Month Sizing Methodology for Reliable Solar Lighting
The single most important principle in solar lighting design is sizing to the worst month — the month with the least usable sunlight relative to the load — because a system that survives December survives the whole year, and one sized on annual averages fails every winter. Nearly every solar lighting failure traces back to this principle being ignored.
This reference explains why averages fail, the method, and how latitude changes everything.
Why averages fail
An annual-average approach looks adequate on paper but leaves a system short during low-sun months. Solar resource varies dramatically by season and latitude: a northern site may receive less than half its summer sunlight in December. A system sized to the average has plenty of margin in June and runs out of stored energy in December — exactly when it can least afford to.
The method
Use monthly solar-resource data (NREL/PVWatts) for the specific location, then size the panel and battery against the month with the lowest insolation relative to the nightly load — usually December in the northern US. For each month, compare the panel's possible harvest to the load; the limiting month sets the panel, and the autonomy requirement sets the battery. It's a deterministic calculation, not a guess.
Latitude changes everything
| Site | Implication |
|---|---|
| Southern (e.g., Florida) | Smaller panel/battery for the same fixture |
| Northern (e.g., Illinois, Minnesota) | Substantially larger panel and battery; steeper tilt |
The same fixture needs a substantially larger panel and battery in Illinois than in Florida. Location-specific worst-month design — not a catalog spec — is the dividing line between solar that works and solar that disappoints. 360 Solar sizes every project to its worst month.
Frequently asked questions
What is worst-month sizing?
Sizing the panel and battery to the month with the least usable sunlight relative to the load — usually December in the northern US. Surviving the worst month means surviving the year.
Why do annual averages fail?
They look adequate but leave the system short in low-sun months; a northern site may get less than half its summer sun in December.
What data does the method use?
Monthly NREL/PVWatts solar-resource data; the limiting month sets the panel and the autonomy requirement sets the battery.
How does latitude change the system?
The same fixture needs a much larger panel and battery (and steeper tilt) up north than in the sun belt.
Is a catalog spec ever adequate?
Only by luck — without location-specific worst-month sizing, a catalog spec risks winter failure.
Request a worst-month sizing study for your site. Get it at 360solarlighting.com/free-quote.