What a portable dishwasher really costs to run.

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A portable dishwasher's marketing copy is almost always about water: "uses up to 80% less water than washing by hand." It never mentions the electricity bill, what a bottle of detergent costs per load, or whether the ENERGY STAR sticker is worth the extra money. Those three lines — water, power, detergent — are what a cycle actually costs, and only one of them is large enough to matter.

This is a companion piece to our full cost and water math, which works through the complete case for and against buying one. Here we go narrower on the running-cost side alone: what each line costs, what ENERGY STAR is worth in this category specifically, and when a cheap machine pays for itself.

Start with the specification that's easiest to verify, because it comes straight from a listing rather than a model we built. The Hermitlux countertop model states it uses 5 liters per cycle — about 1.32 US gallons — and the EUHOMY machine is built the same way: a 5-liter tank that you fill by hand, with "No Hookup" printed in the title because that tank is the entire water supply. A machine physically cannot use more water than the tank holds, so 1.32 gallons is close to a hard ceiling for any tank-fed cycle, not just an average.

That figure is exactly why the category's water-saving claims sound almost implausible — a kitchen faucet alone can put out 1.32 gallons in 44 seconds at the 1.8-gallon-per-minute flow rate US code allows, so the machine's entire cycle uses less water than under a minute of open tap.

The catch: the 5-liter figure only holds for tank mode. The COMFEE' six-place model has no tank at all — faucet-only — and the two SPT 18-inch machines here are the same, freestanding portables that connect to the tap rather than carrying their own water supply. Faucet-fed machines aren't rationed by a 5-liter reservoir, so a longer or hotter cycle can plausibly use more, even though none of the three listings publishes an exact gallon figure. Our breakdown of tank versus faucet models covers which machines fall into which camp.

None of the five listings in this comparison publishes a kWh-per-cycle figure, so this line has to be built from physics rather than quoted from a spec sheet. Water is expensive to heat and cheap to move: the pump motor that circulates it draws a modest, steady wattage, but raising the temperature of several liters of water by dozens of degrees takes real energy, and it's the heating element — not the motor, not the control board, not the little fan that dries the dishes afterward — that accounts for nearly all of the electricity a cycle uses.

The padding above the physics floor is deliberate: the EUHOMY listing quotes 60 minutes of hot-air drying followed by up to 73 hours of ventilation — a claim, not a measurement, but the mechanism itself draws power, and heat-drying uses more than the bare minimum to boil the wash water.

The rate is the assumption most worth swapping for your own bill. Residential electricity runs under $0.13/kWh in parts of the South and over $0.30/kWh in California and the Northeast. At $0.30, the annual electricity line becomes about $39 instead of $22 — real on paper, still small against everything else a kitchen spends on. Limescale changes this line too: a heating element caked in mineral deposits takes longer to reach temperature, which is one more reason descaling on a schedule is a running-cost decision, not just a smell-prevention one.

This is the part the marketing never mentions, and it's the biggest of the three lines by a wide margin. Powder or gel detergent for a compact cycle runs roughly 10 to 12 cents a load bought in bulk, and rinse aid — which matters more here than on a full-size machine, since these dishwashers dry with residual heat and a fan rather than a dedicated heating element — adds another 3 to 5 cents. Pods are the wrong product for this category: several machines specify powder or gel because a pod sized for a 12-place built-in often won't fully dissolve in a five-liter, low-volume cycle, wasting detergent and leaving residue.

Put the three lines next to each other and the shape of the bill becomes obvious: detergent and rinse aid outspend electricity and water combined, sometimes by two to one. That's the opposite of what the phrase "portable dishwasher running costs" makes people picture, and it's the reason we've given the line its own section instead of folding it into a single "utilities" number the way some listings imply.

ENERGY STAR is a certification, not an adjective — a manufacturer submits a model for independent testing against a federal procedure, and it only earns the label by beating the minimum energy and water standard by a set margin. Of the five machines here, that has evidently happened for three: the COMFEE' six-place countertop model and both SPT 18-inch portables carry "Energy Star" in their own listing titles. The EUHOMY and the Hermitlux do not, which we read as "not certified or not submitted," not "inefficient" — we have no data either way, so we won't imply one.

Here's the honest part. On a full-size built-in, ENERGY STAR is worth chasing on the electric bill alone, because baseline energy use is large enough that a double-digit percentage cut is real money over the machine's life. A portable is different: our own assumption above puts a non-certified cycle's electricity at about $22 a year. Even a generous 12% reduction — in the range ENERGY STAR has historically required over the federal minimum for dishwashers — is roughly $2.65 a year. Not a number that changes a purchase decision, and treating it as one would be dishonest in the other direction from ignoring it.

Add the three lines from above and a tank-fed cycle costs roughly 23 to 27 cents to run: about 2 cents of water, 8.5 cents of electricity, and 13 to 17 cents of detergent and rinse aid. The honest question isn't whether that's cheap — it obviously is — but what it's cheap compared with, and the answer depends entirely on how the dishes would otherwise get washed.

Both boxes use the same rates and physics as the full water-and-cost model on this site, laid out side by side rather than re-derived. Against a running tap — how a lot of people actually wash — the machine is clearly cheaper. Against a careful, disciplined basin-and-tap-off routine, dishwasher detergent is expensive enough to erase the water and electricity savings, and the machine becomes the pricier way to get the same plate clean.

That split matters more than either number alone: the honest answer to "is a portable dishwasher cheaper than washing by hand" is it depends how you currently wash, and manufacturer claims of 75–80% water savings implicitly compare against the running-tap case, not the basin case.

The EUHOMY tank model in this comparison lists for $249; add local sales tax in most states and it lands close to $270 out the door, which makes it a fair stand-in for "the cheapest real machine worth buying" rather than a $170 novelty. The question a payback calculation actually answers is simple: how many cycles does it take for the money you save on utilities and detergent to add up to the purchase price?

Against the more careful basin comparison, the machine doesn't save money at all — it costs a few cents more per load once detergent is counted, so there's no payback figure to calculate; it runs at a small ongoing loss in pure dollar terms. That's not a reason to skip buying one, but it means the payback clock only starts for people currently washing under an open tap.

None of this prices the roughly ten minutes a cycle saves against hand-washing an equivalent load, which is a real value even when it isn't a dollar one; our guide to machines under $300 weighs that time saving against the sticker price directly. Buy a portable dishwasher for convenience and hygiene — the ability to wash at 140–167 °F, hotter than hands can tolerate — and treat any running-cost saving as a bonus that only shows up if you were previously wasteful with the tap.

The Hermitlux 7-program model and the SPT SD-9263W in white belong in the same table: the Hermitlux is the one listing that states its 5-liter cycle volume directly, which anchors the water-per-cycle section, and the SD-9263W is the SSC's platform at about $15 less without stainless steel.