How long will a 2500 mAh powerbank power a desk fan with 5v , 0.5A and 2.5W?

I use 10000 mah HetoGrow solar power bank for same thing and it works perfectly. I also use it for usb lamp. See pictures below.

Amps x volts = watts. Like most mobile gadgets, your 2500mah powerbank will output power at up to 5 volts too. For simplicity lets assume a constant 5v. Note that 2500mah ~ 2.5 Ah. Note that Ah = Amp hours. So, 2.5 Ah x 5 volts = 12.5 watt hrs.

Theoretically, if you do the maths for both input requirements of the desk fan you get:

12.5Wh/2.5W = 5 hrs OR

2.5Ah/0.5A = 5 hrs

Power bank efficiency

Practically, power banks generate heat when in use. This heat is lost energy. Other factors affect loss of energy. Expected lost energy is typically 15% to 20% for power banks. To know more about this see Solar Power Bank Quality Assurance: Power Bank phone Chargers

Therefore actual duration for your power bank when you apply 15% to 20% lost energy might well be 3.75 hour to 4 hours. This should be about right assuming both fan and power bank are used in standard room temperature undisturbed.

The key assumption here is that the powerbank is REAL mAh. It is pertinent to avoid buying fake power banks, for obvious reasons. See How to Spot a Fake Power Bank.

Dave Ebi is a Most Viewed Power Bank Writer on Quora.  Join HetoGrow on Facebook, Twitter, Instagram G+ or LinkedIn