TOPK Power Bank 20W USB C Fast Charging 20000mAh Portable Charger PD3.0 QC4.0 - £16.79 With Voucher @ TOPKDirect / Amazon

The faster you drain a battery, the less you actually get out of it. To withdraw the maximum amount of energy, you need to drain it very slowly! And temperature has an effect too.
One might hope that manufacturers of these things would have standardised test conditions so that their claims could be fairly compared. Well, it is a hope… 

added//  To extend the point made by others - mAH needs a voltage specified to give the energy capacity. If you reference that from the native 3.7v of the lithium battery cells, you will get a bigger number of mAH than if delivered at usb’s 5v (let alone other possibilities), and that is even before allowing for some inevitable losses in voltage conversion. 
While it would be more accurate to say this “contains 3.7v cells rated by their manufacturer at 20,000mAH” that ain’t the way marketing people think. 

20000mAH x 3.7v = 74 watt hours (as claimed, scroll way way down on the Amazon listing)
74 watt hours / 5v = 14,800mAH (at 5 volts)

Mystery solved! (edited)

Good to know that too, actually. :-)

ps thanks @MrSwitch i bought as a spare for my usb jackets! i also connect myself / my son to the chargers via usb extension but nice to move around too.

How can you say that when it's been proven to be nowhere close to 20k mAh capacity? Scam of a powerbank

Transmission loss/efficiency charging wirelessly is much worse when charging with a cable, also slower.
Fine if you REALLY need it, but you'll get more life out of the battery and charge quicker with cables. (edited)

How do you know, have you got pics?

Yep, has passthrough so can charge devices while it is also being charged. (edited)

Well it is a 20,000 mah. It's not a 5000 mah phone your charging. It's bound to take a while.

Isn't that 12,000mAh @5V?

Powerbanks' capacity are stated at 3.7V, the internal voltage.

So, if you convert your figures to 3.7v, that makes it 16,300mAh - 82% of the advertised capacity, excluding conversion losses.

Or:
Advertised = 20 Ah x 3.7V = 74Wh
Measured = 62 Wh = 84% of the advertised capacity.

16% loss is pretty good, really.

Your device is close to fully charged. So your device not pulling more power. This not mean that PB can't deliver 20w. Have one of those and it can deliver even little more than 20w. Tested on proper load tester.

That's exactly what I bought it for - heated gilet and it's great.
Slim and not to heavy

Upcoming trip to South Africa where they have power cuts on a daily basis

To be honest, most power banks are rated like that, not just this one.

The slow charging is intentional.
The faster a battery charges, the sooner it dies.

With regards to weight, in general the heavier a battery (for it's size) the better it is. I wouldn't trust any battery that was lighter than I expected it to be.

Powerbanks with slow charging last longer, it's intentional.
The faster a battery charges, the sooner it will die.
They don't expect people to have to charge them more than once a day so presume they'll be charged overnight while you're sleeping.

Tested mine at 5v. 60% of advertised capacity
(edited)

How much you get out of it is determined by what device you are charging.
The battery is less efficent with a fast charging device that creates a lot of heat.
There's also transmission loss.

That's what a lot of people giving 1* reviews don't realise.

These affect ALL power banks, even the most expensive branded ones, though efficiency and transmission loss can be bit better on some more expensive ones but not usually enough to justify the increased cost. (edited)

The 10,000 version I have sometimes requires a button press... Sometimes I have to pull cable out and put it back in.... Janky.

care to explain?

I wouldn't cross the road, too risky. (edited)

None of those options will work. If you see the size of a proper 10W fold-out solar panel, it's about 20x bigger than the surface area of a power bank, so those tiny ones you see in 'solar power banks' are worthless - they might have 500mW max output if you're lucky and you're more likely to damage the internal cells from being in the sun for hours.

Separate accounts I think

When you say weren't what they advertise, do you mean the actual battery capacity?

Point us towards a 20000mAh power bank that actually gives you 20000mAh.
Thanks to transmission loss and efficiency dependant on what you are charging, the list of the above will likely be very small, as in 0.

You could pay more for a more efficient power bank, but it wouldn't be cost effective (pay at least 30% more for one that is 10-20% more efficient = waste of money). (edited)

If you get the 30% voucher, this is about £16.

(no direct links as the seller is currently not allowed here)

It will be 20w total.

Not a chance.
This is 20W output max, you'll need at least 45W (65W would be better).

Checkout a Zendure powerbank. They have a low power mode you can switch on so it doesn't power down the power bank if the current draw is so low.

You're bang on the limit of the 100Wh capacity limit with this one at 20000mah, so you should be fine to take it on the plane in your hand / cabin luggage. mymemory.co.uk/blo…ht/

Well one way of looking at it is I have been on this site for nearly 15 years & this powerbank is definitely the hottest deal of one I have ever seen and it’s by a long way & would say the hukd people are usually correct in deal voting / value etc which is one of the reasons I bought one also as it’s a good price & it’s not black for a change .

Unlikely. They will tell you to return and reorder.

I'm a bit confused, do you mean, yours came as 80% pre-charged already?, or you've since charged it up, etc, but it's only reaching a maximum of 80% instead, or something?

It's 20w max output from this power bank, steam deck standard charger is 45w