How the solar charging calculator works
The derate stack
Panel labels are flash-test numbers at perfect angle and temperature. Real delivery: −10–15% for heat, −10–20% for angle error, −30–50% for clouds — the realistic band is 50–75% of label, and flat-on-a-backpack setups sit at the bottom. The calculator stacks panel derate and conditions honestly.
Harvest math for trip planning
Wh per day = real watts × sun hours × charging efficiency. A 40 W panel in 5 good hours harvests ~100 Wh — two phone charges or a third of a laptop bank. That framing (phone-charges-per-day) is how trip solar decisions should be made: the panel extends days, it does not replace outlets.
The pairing rule
Match panel wattage to bank size: ~1 day of recharge per panel-to-bank ratio of 0.5 W/Wh (a 40 W panel refills a 100 Wh bank in ~2 days). Bigger banks need proportionally bigger panels or acceptance of slow duty. Direct-to-device charging often beats through-bank charging (one less conversion loss) — check the panel's USB-C PD output.