Where a Countertop Dishwasher Energy Really Goes

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Water is not really what is being compared. Heating the water is almost the whole energy bill of a dishwasher, so the water figure is a proxy for the electricity figure, and once you see that the comparison stops being a matter of opinion and becomes arithmetic you can do in your head.

Three of the countertop machines we carry publish the one number that makes this possible: a built in tank of 5 litres, which they also state as 1.32 gallons. In tank mode that is not an estimate of water use. It is a ceiling. The machine physically cannot use more than it holds, and from that single figure the whole energy sum follows.

The arithmetic, from one published figure

Water takes 4,186 joules per kilogram to raise by one degree Celsius, and a litre weighs a kilogram. That is the only constant you need.

A COMFEE portable mini countertop dishwasher publishes a 1.32 gallon built in tank and a hygiene cycle that heats water to 162 Fahrenheit, which is 72.2 Celsius. Take a cold supply at 15 degrees, which is ordinary. The rise is 57.2 degrees, so the heat required is 5 times 4,186 times 57.2, which is 1,198 kilojoules, and dividing by the 3.6 million joules in a kilowatt hour gives 0.333 kilowatt hours. A normal cycle at perhaps 50 degrees is a 35 degree rise, which is 733 kilojoules, or 0.203 kilowatt hours. A NOVETE portable countertop dishwasher publishes a 5 litre tank and a Baby Care mode at 167 Fahrenheit, 75 Celsius, which is a 60 degree rise and 0.349 kilowatt hours.

So the whole thermal cost of running one of these machines, from the one figure they publish, is between a fifth and a third of a kilowatt hour. That is the number, and nothing else in the machine comes close to it: the circulation pump on a unit this size draws tens of watts for an hour, which is a few hundredths of a kilowatt hour, and the electronics are negligible.

The one real addition is drying, and it is a latent heat rather than a sensible one. To dry a load you have to evaporate the film of water left on it, and evaporation near these temperatures costs about 2,330 joules per gram. A hundred grams of residual water, which is a reasonable amount on a small load, is therefore 233 kilojoules, or 0.065 kilowatt hours. Drying is roughly a fifth of a hygiene cycle, which is a larger share than most people would guess and still a minority of the total.

Why the tank is the specification that matters

The reason a tank cap is so much more useful than a water use estimate is that it is a physical limit rather than a test result. A plumbed machine draws fresh water for each fill, and a cycle has several: a prewash, a main wash, and one or more rinses. Each fill is new water that has to be heated from cold. That is where the gallons in a full size machine go, and it is why the per cycle figure for a plumbed machine depends entirely on how many fills its programme uses, which nobody publishes either.

A tank machine in tank mode recirculates. It heats one tank of water and pumps it round the cycle. So the cap is also, near enough, the energy cap.

And this is where the listings become unhelpful in a specific and avoidable way, because the same machines offer a second mode. A EUHOMY countertop portable dishwasher publishes a 5 litre, 1.32 gallon tank and a faucet connection, and so do both of the others. In faucet mode there is no tank and therefore no cap, and not one of the four listings we carry states how many fills a cycle takes or how much water it draws when plumbed in. The identical machine has a known water figure in one mode and a completely unpublished one in the other, and no listing separates them. If you are buying one of these for the water saving, the tank mode is the mode the saving exists in.

Worth checking one conversion while we are here, because it is the only one in this group that holds up. Five litres divided by 3.78541 litres per US gallon is 1.3209 gallons, so the 5 L and 1.32 gal printed together are genuinely the same quantity. Three separate listings get it right.

The comparison the question is really asking, and the free technique that wins

Against a running tap, the machine wins by a wide margin, and the figure that settles it is a regulation rather than a measurement. United States federal law has capped kitchen faucet flow at 2.2 gallons per minute since 1994. Six minutes of washing up with the tap running is therefore 13.2 gallons, which is 50 litres. Raising 50 litres from 15 degrees to a hand-tolerable 40 degrees takes 50 times 4,186 times 25, which is 5.23 megajoules, or 1.45 kilowatt hours. That is 4.4 times the energy of the hygiene cycle and 10 times the water, for one load of dishes.

Which identifies the real variable, and it is not the machine. It is whether the tap is running. Turn it off and fill a basin instead: 8 litres at 40 degrees is 0.233 kilowatt hours, which beats the hygiene cycle and is close to the normal cycle. A filled basin is a better machine than the machine, on energy, and it costs nothing.

So the honest answer to whether a dishwasher uses less water is: less than a running tap, yes, by about ten times, and that is a comparison against a habit rather than against a method. Less than a basin, no. The dishwasher's genuine advantages over a basin are elsewhere and are real: it reaches 162 or 167 Fahrenheit, which a hand cannot, and the only thing a basin cannot substitute for is temperature. Our own records are careful to describe those as hygiene and baby care cycles rather than as sterilisation, which is the right level of claim.

The feature on this page that best understands all of the above is one that uses no energy at all. That EUHOMY listing publishes 60 minutes of hot air drying followed by up to 73 hours of automatic ventilation. The ventilation is a fan moving room air, so it evaporates the same water using the room's heat rather than a heating element. It substitutes time for energy, which is exactly the right trade in a machine whose whole budget is heat, and it means the 0.065 kilowatt hours of drying in the drawing above is avoidable on that model if you are willing to wait. Our record calls it a convenience feature. It is better than that. Details in our EUHOMY countertop dishwasher review, and in the COMFEE portable mini review and NOVETE countertop review.

Small machine, big machine, and the per dish question

Our existing comparison of a countertop dishwasher against a portable dishwasher gets to the right conclusion without a number. It says portables "use more water and energy per cycle since they clean a bigger load", and that "one full portable cycle can replace two or three countertop cycles, so per-dish the gap narrows for a busy household". Both halves are correct, and the arithmetic above lets you see why and where the line falls.

Energy per cycle is proportional to water, and water on a tank machine is capped at 5 litres. So if a countertop machine does four to six place settings on 5 litres, that is roughly a litre per place setting. A full size machine using, say, 12 litres on twelve place settings is the same litre per setting. The two are a wash per dish, and whichever one you fill properly wins. Run a countertop machine half empty and your litres per dish doubles; run a full size machine half empty and it doubles too, from a bigger starting figure. That is the whole of the per dish argument and it is a loading argument rather than a machine argument.

Which leaves one specification in our set that asserts the conclusion without the figure. A COMFEE countertop dishwasher publishes six place settings, eight cycles including a 45 minute speed cycle, a heavy programme with twelve nozzles, and the claim "Energy Star certified with low water usage". Energy Star certification is a real standard with real published thresholds, and it is the only efficiency credential anywhere in this group, so that is a meaningful thing to carry. What the listing does not do is state the gallons per cycle or the kilowatt hours per year that the certification is based on, which are exactly the two numbers the programme requires a manufacturer to measure. The credential is published and the measurement behind it is not. Our record repeats the claim, as it should, and cannot improve on it. See our COMFEE countertop dishwasher review.

Problems in our own records, found while doing this

Four things turned up that are worth stating plainly.

First, the specification tables on all four of these product records are unusable. The labels are not specification names, they are truncated Amazon bullet headings: one reads "Please Make Sure The Dista", another "6 Place Setting- A Perfect". The values are cut off mid word, as in "Adding water manually with the water filling guide i". One record's labels are rendered in bold mathematical Unicode characters rather than plain text. The features arrays carry the same damage. Every usable figure on this page came out of the summary and pros fields instead, which is why the hygiene temperatures and the tank size are cited and nothing else is.

Second, one of those four records has its pros stored as a single run-on paragraph rather than one claim per line as every other product on the site does, so the list renders as a sentence that reads oddly and runs several unrelated claims together.

Third, and this is the serious one: our roundup of the best countertop dishwashers has nine picks and not one of them is a countertop dishwasher. They are detergent pacs, a drain board, an XL electric griddle, a bamboo laundry countertop, a cutlery holder, a drying rack, a silicone mat, dish cloths and a sponge holder. Its own comparison line also says eight items were compared when nine are listed. Our roundup of the best dishwasher that cleans the best has seven picks, all of them detergents, rinse aid or cleaner, plus a replacement dishwasher door panel. And our portable dishwashers roundup has six picks of which three are descaling tablets and one is a collapsible plastic basin. No argument on this page rests on any of those, deliberately, and I have not linked them.

Fourth, a correction to our own planning note for this article. We had kilowatt hours down as absent from the site. It is not: the term appears in the roundup data of three posts, covering smart plugs with energy monitoring, beverage refrigerators and mini fridges. None of them is a dishwasher, so the gap this post fills is real, but the count was wrong.

The honest limit

Every figure above is heating only, and it is my arithmetic on their published tank size and their published cycle temperatures rather than a measurement of anything. Not one of the four listings publishes a heating element wattage, a gallons per cycle figure, a kilowatt hours per year figure, a number of fills, a cold inlet temperature or a water use in faucet mode. The 15 degree tap and the 100 grams of residual water are both reasonable assumptions of mine, and both move the answer: a 10 degree tap adds about 15 percent to every bar in the drawing, and a 20 degree tap takes about 15 percent off.

Which makes the free moves the ones worth doing, and they are all about the terms the arithmetic says are large. Fill the machine before you run it, because the energy is per cycle and almost independent of how many dishes are inside. Use the normal cycle rather than the hygiene cycle unless you have a reason, because the difference is 0.13 kilowatt hours, which is 64 percent of the normal cycle's entire heat. Let it air dry, by cracking the door or using a ventilation setting if it has one, which removes the drying term completely. And above all, if you wash anything by hand alongside it, fill a basin rather than running the tap, because that single habit is worth more than every other decision on this page put together: 50 litres and 1.45 kilowatt hours against 8 litres and 0.233.