20000mAh Power Bank: How Many Phone Charges Do You Really Get?

"20000mAh" sounds like endless power. Divide by your phone's ~4000mAh battery and you'd expect five full charges. Reality is a bit less generous — but still plenty. Here's why.

The conversion loss nobody mentions

Power banks store energy at 3.7V (the battery cell voltage) but output at 5V (USB voltage). That voltage boost wastes energy as heat. Between conversion loss and circuit inefficiency, you typically get 60–70% of the rated capacity as usable charge.

The math for a 20000mAh bank:

• Rated: 20000mAh at 3.7V
• Usable: roughly 12000–14000mAh at 5V
• iPhone 15 (~3349mAh battery): 3.5–4 full charges
• Samsung S24 (~4000mAh battery): 3–3.5 full charges

What that means in real life

Three to four full charges is a lot. In practice:

• Daily commute: a 20000mAh bank lasts you an entire work week without recharging the bank itself
• Weekend trip: charge your phone nightly for 3–4 days straight
• Heavy day (navigation + camera + hotspot): one full day with power to spare, plus the next morning

When 20000mAh is overkill

If you just need a safety net for the day, 10000mAh (about 1.5–2 charges) is lighter and cheaper. Go 20000mAh when you travel, work long days away from outlets, or share with someone.

One more thing: recharge speed

A big bank that takes 10 hours to refill is annoying. Look for fast input charging (18W+) so the bank itself refills overnight in 4–5 hours instead of all day.

Bottom line: expect 3–4 real-world phone charges from an honest 20000mAh bank — more than enough for most trips. Our mini 20000mAh power bank is built around exactly this use case: maximum charges, minimum pocket space.