WinterWatt

Off-grid sizing for low-sun sites

Most solar calculators size an array on an annual average. An off-grid system does not fail in July. It fails in the darkest week of December, behind a ridge. This one sizes for the worst month and counts the sun your terrain takes away.

Site

Terrain horizon — skyline height in degrees

Load and storage

Array for 100% solar
-kWp
-
Clear-sky in plane-
Real in plane-
Direct sun window-
Max solar altitude-
Sun lost to terrain-
Battery, nominal-
Installed array yields-
Daily balance-

Where the numbers come from

Solar position, angle of incidence and terrain blocking are computed for your coordinates and design day. Clear-sky irradiance uses the Hottel transmittance model with a mid-latitude winter atmosphere.

The cloud cover is yours to supply. Get the real monthly figure for your site from PVGIS and set the clear-sky index so the real figure matches. This page will not invent your weather.

Sizing on one average day is not a substitute for the electrical code where you build.