Estimate monthly electricity cost to run a hot tub.
Almost all of a hot tub's electricity goes on holding temperature rather than on using it, because the water sits at 38 C around the clock whether anyone gets in or not. Heat loss is proportional to the temperature difference with the outside air and to the surface area of the shell and cover, and surface area scales with volume to the two-thirds power — so a tub twice the size loses only about sixty percent more heat. The cover is where the difference between a cheap tub and an expensive one shows up: a tight, dry, well-fitted cover over a foam-filled cabinet can hold temperature on three or four kilowatt-hours a day, while a waterlogged cover on a poorly insulated shell can take three times that. Filtration adds a small, near-constant amount on top.
Standby heat loss of a heated tub
Surface area = (litres / 1000)^(2/3) x 1.6; loss in watts = surface area x U value x (set temp - ambient); daily kWh = loss x 24 / 1000 + 0.6 for filtration; monthly cost = daily kWh x rate x 30.437
Marginally, and only for gaps of several days. Loss is proportional to the temperature difference, so dropping from 38 C to 30 C cuts standby loss by roughly thirty percent — but reheating costs most of that back, so it only pays over a long absence.
Because heat rises and water evaporates upward. A waterlogged or ill-fitting cover is the most common cause of a hot tub bill doubling, and replacing one is far cheaper than any other efficiency measure.
Less than people expect. Opening the cover for an hour and running the jets costs perhaps half a kilowatt-hour to a kilowatt-hour of extra reheating — the standing loss over the other twenty-three hours dominates the bill.