The washer decides the gas bill
Follow the chain and it is hard to argue with. The dryer removes whatever water the extract left behind. Boiling a pound of water off takes a fixed amount of heat that no equipment choice can change — a shade over a thousand BTU at the temperatures a drum runs at. So the fuel a load takes is essentially the retained water multiplied by a constant, divided by whatever fraction of the fuel actually reaches that water.
Two of those three terms belong to the washer and to the extract, and only one belongs to the dryer. That is why a store that measures its retention and does something about it usually finds more than a store that shops for a more efficient dryer. The page does not assert that, though: it computes both, because which lever is bigger depends on where you are starting. A store already extracting well has little water left to remove and the comparison closes up.
Sensible and latent
The heat splits into two parts and the split is worth understanding. Sensible heat warms the water and the goods up to the temperature at which drying happens; latent heat turns the water into vapour at that temperature. The latent term is much the larger in any normal load, and the page prints the split so you can see it rather than take it on trust. Chasing the sensible term — warmer goods out of the washer, say — moves a small number. Chasing the latent term means removing less water, which means the extract.
The latent heat itself comes from a straight-line fit to the steam tables. That is a property of water, not a claim about equipment, and it is stated on the page so nobody has to wonder where the number came from.
Gas against electric, without a verdict
People want a rule and there is not one, because the answer is two prices multiplied by two efficiencies. Resistance elements put nearly everything they draw into the airstream, so their share reaching the water reads higher; gas loses a real fraction up the stack. But a therm carries 100,000 BTU and a kilowatt-hour carries 3,412, so the arithmetic can land either way depending entirely on what your utility charges.
The page therefore prints the crossover: the electricity price at which the two are level at your gas price, and the gas price at which they are level at your electricity price. Compare it against your own bill and you have your answer for your address this year, which is the only place the question has an answer. Neither figure includes the gas line, the venting, the electrical service or a demand charge, and any of those can be larger than the energy difference.
What the model leaves out
Drying is not linear. A load gives up its free water fast and its last few percent slowly, so any time worked from a flat evaporation rate runs short at the dry end. Overdrying — running past the point where the water is gone and simply heating fabric — is real, common, and invisible to this arithmetic because it depends on how the machine is controlled rather than on how much water was there. Ambient humidity matters too, since a dryer is pushing vapour into whatever air the room supplies.
Treat the output as the floor: the energy the physics demands, marked up by an efficiency you supplied. The gap between that and your metered consumption is the interesting number, and it is the one that tells you whether the problem is the machines, the controls or the extract.
Questions people ask
How much energy does it take to dry a pound of laundry?
Work it from the water, not from the laundry. A pound of dry goods carrying 52 percent moisture holds just over half a pound of water, and each pound of water evaporated needs a bit over a thousand BTU of heat plus the sensible heat to warm it up. Divide that by the fraction of the fuel that actually reaches the water and you have the fuel. The page prints the delivered BTU per pound of water so you can compare it against a meter read.
Is gas or electric drying cheaper?
It depends on two prices you supply and two efficiencies you supply, and the page prints the crossover rather than picking a side. Enter your gas price and it tells you the electricity rate at which the two are level; enter your electricity rate and it tells you the gas price. Anyone who answers that question without knowing your utility bill is guessing. Neither figure includes the gas line, the venting or the electrical service.
What is moisture retention after extraction?
Pounds of water left in the goods per 100 lb of dry goods when the washer finishes. Weigh a load conditioned and dry, wash it, weigh it straight out of the machine, and the difference divided by the dry weight is your figure. It is the number the whole drying bill hangs on and it is measured far less often than it should be, because nothing on a machine displays it.
Will a better extract really cut the gas bill?
It cuts the water the dryer has to boil off, and the page computes what that is worth at your numbers rather than asserting a figure. It also prints the dryer efficiency that would have to be reached to match the same saving, so you can compare the two routes directly. How much a better extract is worth depends on where your retention already sits: high retention leaves a lot of room, low retention leaves little.
Why does the calculator ask for the fabric temperature rather than the air temperature?
Because the water leaves the goods at the temperature the goods are at, and that is what sets the latent heat and the sensible rise. Inlet air runs much hotter than the load for most of a cycle. If you have no measurement, the answer is not very sensitive to it — the latent term moves by only a few percent across a wide temperature range — but the sensible term does follow it directly.