The panel-wattage math manufacturers bury in the fine print — plus why your real-world recharge time runs longer than the box implies.
The formula: panel watts × peak sun hours × real-world efficiency = watt-hours per day. Peak sun hours run 4-6 in most of the US on a clear day — not the same as total daylight hours. Real-world efficiency after MPPT losses, angle, and cloud cover lands around 85%, even when the panel and controller are individually rated higher.
Check the unit's max solar input spec before buying panels — it's easy to overshoot. The Jackery Explorer 2000 v2 caps at 400W even though its 2042Wh battery could easily use more; pairing it with an 800W array wastes half the panel's output.
Match your panel wattage to your unit's max solar input spec, not the other way around — buying more panel than the unit can accept just wastes money.