Enter both values to calculate watt hours
Milliamp Hours (mAh)
Volts (V)
Common:
Watt Hours (Wh)
Result will appear here
Understanding Power Bank Capacity
Power bank capacity can be measured in mAh or Wh:
- Energy (Wh) = Capacity (mAh) × Voltage (V) ÷ 1000
- Capacity (mAh) = Energy (Wh) × 1000 ÷ Voltage (V)
For USB and Portable Power
Understanding these relationships helps you:
- Compare power banks - Use Wh for accurate comparison
- Airlines allow 100Wh - Know if your power bank is carry-on safe
- Calculate charges - How many phone charges you'll get
- Size solar chargers - Match panel to power bank capacity
Real-World Solar Examples
20,000 mAh Power Bank
A 20,000 mAh power bank at its internal 3.7V stores 74Wh. At 5V USB output with ~85% conversion efficiency, usable capacity is about 62.9Wh. Airlines limit carry-on batteries to 100Wh.
DIY Solar Battery from 18650 Cells
A Samsung 30Q 18650 cell at 3,000 mAh and 3.6V stores 10.8Wh. A DIY powerwall with 140 cells (14S10P) stores 1,512Wh — enough for a 60W camping setup for over 25 hours.
Phone Charges from Solar Power Bank
A smartphone battery at 5,000 mAh and 3.85V stores 19.25Wh. A 74Wh power bank can charge it 3.8 times theoretically — about 3 times in practice after conversion losses.
When You'll Need This Conversion
- Sizing a Portable Solar Panel for a Power Bank — To charge a 74Wh power bank in one day with 5 sun hours and 80% efficiency: 74 ÷ 0.80 ÷ 5 = 18.5W minimum panel. A 20W panel works in ideal conditions; 30W provides margin for clouds.
- Drone Operations from Solar Setup — A DJI Mavic battery at 5,000 mAh and 15.4V stores 77Wh. Four batteries need 308Wh. A 100W foldable solar panel producing for 5 hours generates 500Wh — enough for remote site surveys.
- Emergency Power Calculations — Daily essentials: phone charge (19.25Wh) + USB LED light (30Wh) + USB fan (24Wh) = 73.25Wh/day. A 30,000 mAh power bank (111Wh) provides ~1.5 days. Pairing with a 20W solar panel extends this indefinitely.
Solar Tips & Common Mistakes
mAh Ratings Are at Internal Voltage, Not USB Output: A "20,000 mAh" power bank at 3.7V internal = 74Wh. At 5V USB output: 14,800 mAh effective — a 26% reduction. This is why power banks charge fewer devices than the mAh number suggests.
Solar Charging Is Slower Than Expected: A 20W portable panel produces about 15W effective in direct sun. Charging a 74Wh bank takes 4.9 hours of direct sunlight. Clouds and angle can double that time. Oversize panels by 50–100%.
Cold Weather Reduces Capacity: Lithium power banks lose 10–20% capacity below 0°C (32°F) and should not be charged below freezing. When solar-charging during winter camping, keep the bank insulated and charge during warmest hours.
Solar Calculators
Related Conversion Calculators
Last updated: January 5, 2026
