Why a Power Bank Is 3 Times Short on Energy and 75 on Power

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Not from anything you can carry in a bag, and the interesting part is which number stops you. A battery has two specifications and people instinctively check the wrong one. The energy gap is a factor of 3.29. The power gap is a factor of 75. So the thought that follows naturally, get a bigger pack, closes the smaller of the two problems and leaves the larger one exactly where it was.

Both numbers come out of published figures on our own shelves. A 20,000 milliamp hour pack holds 74 watt-hours and delivers about 48 of them. Boiling the water in a published 1.7 litre kettle takes 158 watt-hours. And that kettle's element wants 1,500 watts, which from a 3.7 volt cell stack is 405 amps.

Turning the headline figure into the right unit

Milliamp hours are a charge, not an energy, and the conversion needs a voltage. A Luvknit 20,000mAh solar power bank with USB-C publishes a 20,000 milliamp hour battery, USB-C and USB-A outputs, a 15 watt USB-C input, a built-in LED torch, multi-protection safety, a built-in solar panel for emergency trickle charging, 0.65 inches of thickness, around 230 grams, and "roughly two to three full charges of a modern phone".

Lithium cells sit at a nominal 3.7 volts, so twenty amp hours is 74 watt-hours. Our own guide to power banks, mAh and real output publishes the usable fraction and gets it right: "Realistically, you get around 60 to 70 percent of the printed capacity as usable output." Take 65 and 48.1 watt-hours leave the pack.

That conversion is worth checking rather than asserting, and a second record checks it. An EIZLXQ 50W 5-port USB-C charger block in a two-pack publishes five ports, two USB-C and three USB-A, a single USB-C port delivering up to 20 watts, IEC 62368-1 and FCC certification, fireproof ABS and PC housing, and the detail that does the work here: it "can take an iPhone from empty to 50 percent in about 35 minutes".

Twenty watts for 35 minutes is 11.67 watt-hours for half a battery, so 23.3 for a full one. Divide 48.1 by 23.3 and the pack is good for 2.06 phone charges, which lands precisely on the low end of its own claim of roughly two to three. Two records, one conversion, and they agree, which is why the 74 watt-hours can be trusted for the harder comparison that follows. Its own drawbacks are a model of the right kind of honesty, including "50W is a shared total, not per port", and the detail is in our review of the EIZLXQ block.

The appliance the question names does not publish the number it needs

An Instant Pot Duo 6 Qt 7-in-1 electric pressure cooker publishes six quarts, seven functions, thirteen smart programs, an 18/8 stainless pot with a tri-ply bottom, a pressure release valve and locking lid, and a claim to cook "up to 70% faster than slow cooking methods". It does not publish a wattage.

Which means the energy for one cook cannot be computed from our own record, and nor can the time to come to pressure that a second person in the vault is asking about. That second question has a one line answer in principle, the liquid mass times its specific heat times the temperature rise, divided by the element power, and the element power is exactly the figure the listing withholds. So the honest position is that we cannot answer it, and the reason is one missing number. Our pages on why a pressure cooker's minimum liquid is not steam and why 5 kilograms of meat needs a 12 litre pressure cooker work the volume side of the same appliance properly, and neither of them needs a wattage, which is how they got written.

What does survive is the preheat, because that only needs the liquid. A 6 quart cooker running a representative 1.5 litres of liquid from 20 to 100 degrees needs 1.5 times 4,186 times 80, which is 502 kilojoules, or 139.5 watt-hours. That is 2.90 times the pack's entire usable energy, before the cooker has started cooking anything. See our review of the Instant Pot Duo 6 Qt for the rest of what it does publish.

The measuring stick that does publish both numbers

One kitchen appliance in this catalogue publishes a volume and a wattage together, which makes it the right yardstick. A Zulay Kitchen glass electric kettle at 1.7 litres and 1500 watts publishes a borosilicate glass body with stainless components, auto shut-off and boil-dry protection, a cool-touch handle, a 360 degree cordless base and a spout scale filter.

Heating 1.7 kilograms of water from 20 to 100 degrees takes 1.7 times 4,186 times 80, which is 569 kilojoules, or 158.1 watt-hours, and at 1,500 watts that is 6.33 minutes of element time before the kettle's own mass and losses are added. Our page on why boiling milk breaks an electric kettle works the flux through that same plate and is the companion to this.

So the energy comparison, finally. 48.1 watt-hours against 158.1: the pack is 3.29 times short of one kettle of water. Bad, but not absurd. Three packs and you have boiled a kettle. More in our review of the Zulay glass kettle.

Now the gap that does not close

Energy is how much. Power is how fast it can leave, and a resistive heating element has no patience at all: it either gets its wattage or it does not heat. So line up every published output figure in this catalogue against 1,500 watts.

A single USB-C port at 20 watts is 75 times short. The whole five-port block at 50 watts shared is 30 times short.

A 140W Smart Auto-Stop USB-C charger, which publishes up to 140 watts, smart full-charge detection that physically cuts the connection when charging completes, and a compact single-port body, is 10.7 times short.

And a Yiicumhh 90 degree Thunderbolt 4 cable rated 240W at 40Gbps, which publishes 240 watts over PD 3.1 with Extended Power Range, a nylon braid rated past 49,000 bends, quadruple shielding and a zinc alloy shell, is 6.25 times short.

The energy gap is 3.29 and the power gap is 75, so power is 22.8 times the worse problem. And the ceiling of the whole route is not the pack at all: the fattest cable we carry tops out at 240 watts, which is still a sixth of what the smallest mains cooking appliance in the catalogue wants. Our page on why a 240 watt cable can charge at 60 watts works through why even that figure is conditional, and the condition is on the listing itself: "Both the device and charger must support these levels to reach full 240W."

At the cell the arithmetic is blunt enough to end the conversation. Fifteen hundred watts from a 3.7 volt stack is 405 amps. Two hundred and forty watts of Power Delivery is five amps at 48 volts. The pack publishes a 15 watt input and, revealingly, no output wattage at all, so the two records that should carry the two numbers this question needs each carry the other one's.

What a fuel carries, and why this is not an engineering problem

A Flintex portable propane tabletop grill publishes 240 square inches of cooking area for four to six servings, foldable legs with a built-in handle, a lid thermometer, an adjustable gas valve, push-to-start ignition, a powder-coated body with enamelled grates and a stainless burner, and operation on standard one pound propane cylinders.

Four hundred and fifty four grams of propane at 46.4 megajoules a kilogram is 21.1 megajoules, which is 5,854 watt-hours of chemical energy. Against the pack's 48.1 usable, that is 121.7 times. At a representative fifty percent transfer into a pot it is 2,927 watt-hours, which is 18.5 kettles of water.

And the reason is specific energy rather than cleverness. Propane carries 12,889 watt-hours per kilogram. The pack carries 74 watt-hours in 230 grams, which is 321.7. A factor of 40.1 before any efficiency is applied, and after it, with fire at about half and a pack at about two thirds into a resistive element, still about 31.

So the honest answer to the question as asked is to stop carrying the energy as electricity. That is not a defeat for the battery, which is extraordinary at what it is for: the pack weighs 230 grams, slips into a bag and runs a phone for a weekend, and the same arithmetic that says it cannot boil a kettle says it can do that twice over. It is a statement about cooking, which is the one domestic job whose power demand sits two orders of magnitude above everything else a person carries. Details in our review of the Flintex tabletop grill, and the honest gap on that record is its own: it publishes a cooking area and no BTU rating, so its fire cannot be set against an element without an assumption.

The honest limit, and what it leaves undone

What is exact here is the conversion chain and every ratio in it. Twenty amp hours times 3.7 volts is 74 watt-hours. One point seven kilograms times 4,186 joules per kilogram kelvin times eighty kelvin is 158.1 watt-hours. Fifteen hundred watts divided by twenty, fifty, 140 and 240 gives 75, 30, 10.7 and 6.25. Fifteen hundred over 3.7 is 405 amps. And the 2.06 phone charges comes from two published figures on two separate records, which is why it is the strongest number on the page: it was derived to test the method rather than to make a point.

What is representative rather than published is a short list. The 3.7 volt nominal cell voltage is a convention and a particular pack might be 3.6 or 3.85, which moves the 74 watt-hours by a few percent in either direction and moves nothing else. The 65 percent usable fraction is the midpoint of the 60 to 70 our own page publishes, and taking the generous end gives 51.8 watt-hours and a 3.05 energy gap rather than 3.29. The 1.5 litres of liquid in a 6 quart cooker is an assumption and scales the 139.5 watt-hours directly. The 46.4 megajoules a kilogram for propane is a standard figure, and the fifty percent transfer into a pot is a conventional estimate that could plausibly be thirty or seventy, which moves the 18.5 kettles between 11 and 26 without touching the 121.7 in stored energy. The one assumption that would change a conclusion is the phone battery, and it is the one the cross-check pinned.

Four things this page deliberately does not compute. It does not give the energy of a complete pressure cooking cycle, or the time for any Instant Pot to come to pressure, because no pressure cooker in this catalogue publishes a wattage and both answers are that figure divided into something. It does not compute how long the pack's solar panel takes to refill it, because no panel area or rating is published, and our own power bank page already declines the same thing for the same reason, correctly: "Recharging a large bank from the panel alone would take many days of perfect sunlight." And it does not work out an inverter and leisure battery setup, which is the answer a lot of people actually want, because this catalogue carries no inverter and no deep cycle battery, and sizing one from products we do not stock would be arithmetic with nothing underneath it. What can be said without any of them is the shape: a 12 volt route needs 124 amps rather than 405, which is why that is where the real solutions live, and why the cable gets thick.

Three numbers to carry. Milliamp hours become watt-hours when you multiply by about 3.7 volts, so a 20,000 pack is 74 watt-hours and delivers about 48. A kettle of water is 158 watt-hours, so the energy shortfall is only about three to one. And the element wants 1,500 watts against a catalogue that tops out at 240, so the shortfall that matters is twenty two times worse than the one everybody checks.