A solar power bank sounds perfect: leave it in the sun, phone charged, no outlet needed. The reality is less magical. A typical built-in panel produces only 1.5–2 watts, so fully charging a 20,000 mAh power bank from sunlight alone can take weeks, not hours. I ran the numbers myself with a Python script, and the math is brutally honest.
A 20,000 mAh bank holds 74 Wh (20,000 × 3.7 ÷ 1,000). Assuming 4.5 peak sun hours a day (typical for tropical Indonesia) and 55% system efficiency, a 1.5 W panel yields just 3.71 Wh per day — about 20 days of clear sun for a full charge. A 2 W panel needs about 15 days, a 5 W panel about 6 days. A foldable 21 W panel finishes in about 1.4 days, and a 28 W panel in about 1.1 days. In everyday terms, one sunny day on a 1.5 W panel adds only about 31% of a 12 Wh phone battery, while a 28 W panel can charge that phone nearly 6 times in a day.
So when is solar worth it? As an emergency backup or multi-day outdoor kit, yes — a built-in panel is always there, always trickling in a little power. As a daily city charger, no — a regular USB-C PD power bank charges in hours from a wall outlet, weighs less, and costs less. My rule: check the panel wattage first, not the mAh number. Below 5 W means emergency-only. 10 W and up starts to feel useful. 21–28 W foldable panels are the only realistic option for off-grid trips.
Read the full Indonesian guide with the complete calculation table and buying checklist here: https://www.miftahulrizki.eu.org/2026/11/powerbank-solar-mitos-panel-kecil-vs.html — and if you are shopping for a regular bank first, the same watt-hour math applies, as explained in the full guide: https://www.miftahulrizki.eu.org/2026/11/powerbank-solar-mitos-panel-kecil-vs.html
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