Portable Solar Panel Recharge Calculator
Estimate how long a portable solar panel takes to recharge a power bank or battery pack.
Based on panel wattage, capacity, and available sunlight.
Peak Sun Hours vs Clock Hours
A 20-watt solar panel does not produce 20 watts all day. It hits its rated output only when the sun is roughly perpendicular to the panel surface, near solar noon in clear conditions. The useful concept is peak sun hours: the number of hours per day the sun delivers full rated irradiance (about 1,000 W/m2). This ranges from 3 to 4 hours in northern winters to 5 to 7 hours in desert summer conditions.
In practice, expect 3.5 to 5 peak sun hours for temperate outdoor camping depending on season and cloud cover.
System Efficiency Losses
Charging losses stack up: Solar panel output loses 15 to 25% to heat and angle inefficiency in real conditions. The charge controller or USB converter loses another 5 to 10%. The battery itself has a charging efficiency of 85 to 95% for lithium.
Total real-world efficiency: roughly 70 to 80% of rated panel output reaches the battery.
This calculator uses 80% as the combined system efficiency.
The Calculation
Usable watt-hours input per day = panel_watts x peak_sun_hours x 0.80 Time to full charge (days) = battery_capacity_wh / usable_wh_per_day
For mAh batteries: Wh = mAh x voltage / 1000. Most USB power banks are 3.7V cells (nominal), so 10,000 mAh = 37 Wh.
Practical Tips
Keep the panel pointed at the sun. A 15-degree angle error reduces output by about 3%. A 45-degree error reduces it by 30%. Fold-out panels let you adjust angle throughout the day. Shade from a single branch can cut output by 50 to 80% on most panels due to series-wired cells.