The power bank math nobody does: what a 20,000 mAh brick actually runs

A power bank sold as "20,000 mAh!" and a television that "only uses 60 watts" are two numbers written in different languages, and the argument about whether the brick can run the TV is just translation. A minute with a calculator fixes it permanently. This page is the whole method.

The translation, once and for all

Energy (watt-hours) = capacity (amp-hours) x voltage. Phone cells live at about 3.7 volts, so 20,000 mAh is 20 Ah x 3.7 V = 74 Wh of stored energy. Now subtract reality: every conversion (battery to USB to whatever plugs in after) leaks - budget 80-85 percent of the label for the actual usable pool, so roughly 60 Wh. That number is the fuel tank; every device you name has an hour rating waiting inside it: hours = 60 Wh / device watts. A router drawing 10 W runs six hours. A 60 W television runs about one hour. A 10 W fan on the USB adapter runs six. The arithmetic is not negotiable and no marketing department can legislate against it - which is why the label says mAh and not Wh.

Why "power my TV" needs a different tool

A television on a USB power bank means an inverter brick, a DC-to-AC conversion and then the TV's own supply again - each hop is another ten percent gone, and the flat-screen's "60 W" is its average under content, while at switch-on with the backlight cold it briefly asks for more. The honest line: USB power banks are for phones, lights, routers, fans - devices native to DC or tiny enough not to care about losses. A television and decoder through the night is a small inverter-and-battery job (this desk's generator-versus-inverter article does that sizing by load, on purpose, without quoting prices), not a power bank one.

The two spec traps on the box

• Output watts are a cap, not a promise. A "22.5W" bank running a device that asks for more will run it badly or not at all - check your device's charger label for the real ask, and the bank's output profile (USB-PD is a standard; a no-name "fast charge compatible" sticker is a rumour). A laptop asking for 45-65 W will refuse anything that is not a PD charger with a matching figure and a cable that can carry it.
• Pass-through and the aging curve. "Charge while it charges" is a chemistry tax - the bank gets warm and the usable capacity after a year of daily full cycles is genuinely lower than the number today. Buy capacity for the load you have plus half again, not for the load exactly.

What to actually buy with this method

Write your list - router 10 W, two phones at 5 W each charging overnight, a 15 W fan - total the watts, decide the hours, divide, and the right brick size falls out in watt-hours. The shops will still argue in mAh; you now read the other language, and the argument costs them a sale to you only once.