There is no simple answer to these questions. Before we can answer them, we need to know about the beginning water temperature, heater power, heat loss, surface area, heat retention, and final use temperature. Cost of operation depends on insulation, surface area, and energy cost.
Electric heaters come in three sizes: 1.5 kW (120V,15 Amp); 6 kW (240V,30 Amp); and 11.5 kW (240V,60 Amp). Each heater has a different rise rate. The following table compares rise rates:
TYPICAL WATER TEMP. RISE RATES (IN DEGREES F./HR.)
SPA/TUB MODEL Max 1.5kW 6.0kW 11.5kW NAT'L GAS*
Gallons 120V 240V 240V 100,000BTU
Opu 4' dia. x 35"- 241 2.6 10.5 20.1 48.1
Opu 5' dia. x 35" 387 1.6 6.6 12.6 30.0
Opu 6' dia. x 35" 567 1.1 4.5 8.5 20.4
Rub 4' dia. x 44" 303 2.0 8.0 15.5 36.7
Rub 5' dia. x 44" 486 1.25 5.0 9.6 22.9
Rub 6' dia. x 44" 712 0.9 3.4 6.5 15.6
Rub 7' dia. x 44" 982 0.6 2.4 4.6 11.3
Rub 8' dia. x 44" 1294 ---- 1.9 3.6 8.6
*NOTE: For Propane, derate 12%
Also see rise rates for our larger tubs: https://www.greatnorthernhottubs.com/our-hot-tub-weights-and-measures.php
Note that the rise rate on our 241 gallon hot tub is 20.1° per hour with an 11.5 kW heater, 10.5° per hour with a 6 kW heater, and only 2.6° per hour with the 1.5 kW heater.
Let's assume that the starting water temperature is 50° and we want to raise the temperature 54°, to 104°. Look up the heater you'll be using on the chart. Divide 54° by that number. The result is the number of hours it will take that heater to raise the water temperature 54°. You'll find the 1.5 kW heater may take several days for the initial heat up. That is too long. We don't recommend this small heater. It seems that a 11.5 kW heater would be best, however they need a 60 amp circuit and can only be used where 150-200 amp service is available. Usually commercial establishments with extremely large spas and hot tubs use 11.5 kW heaters. If you want this size heater, you need a 150-200 amp electrical service. Our Advice: Get the right sized heater! The best choice for most home spas and hot tubs is a 6 kW 240V heater. Beware of 120V only plug-in spas. They have 1500 watt heaters which won't heat your spa adequately-imagine using a hair drier to heat up your hot tub! If you are leaving the water at a constant temeperature it's more economical to use an electric 6kW heater. While the 6kW heater is ideal for most tubs, you may want to reevaluate this if you are looking at a 8' diameter or larger/extra deep systems.
So, which is the most economical, energy-efficient tub? The deep, narrow hot tub (400-600 gallons) or the shallow wide spa (250 gallons). Many people think "Of course! The spa. There's more water to heat in a hot tub!" Not true! Hot tubs have greater thermal mass (gallons of water) than spas. They take longer to heat up but retain heat longer. They take less energy to maintain use temperature. We don't pay to heat water! We pay to replace the heat that sneaks out the top, sides, and bottom of the spa or tub. A typical five-foot diameter Rubadub Tub® has roughly 94 square feet of surface area. A spa that seats the same number of people has at least 140 square feet of surface area. A round hot tub has less surface area than a square spa, so less heat escapes from the top. The large volume of water in a Rubadub Tub® allows it to hold heat considerably longer than a comparable spa. For example, consider an outdoor installation in the wintertime. If you lose electric power, the Rubadub Tub® retains enough heat to keep it from freezing for five to six days. A spa would lose its heat and freeze within a couple of days. It's really a matter of thermal mass-the larger the thermal mass, the slower it cools.
Note! Even though the water in the hot tub won't freeze quickly in the above situation, the pipes and equipment will! If you lose electric power, you must act quickly to prevent damage to your equipment. Drain the hot tub or spa during extended power outages in winter!
Most of the time, you shouldn't. Hot tubs and spas are energy-efficient. Remember what we said about heater size and raise rates. You're better off if you leave the heater on all the time, especially if your tub is outside. The thermostat maintains the proper use temperature. Your spa or hot tub will always be ready when you want to get in. No waiting!
Well, yes and no. Gas heaters aren't often used in home spas and tubs for three reasons- up front heater cost, outdoor winter use restrictions, and indoors, cost of modifications to chimneys and flues. Gas heaters are possible for home units, but they cost considerably more than electric heaters. You could pay $600-1,600 for the heater alone. You shouldn't use gas outdoors in the wintertime. The gas heater must be in a heated environment. Some dealers will install gas heaters outside; but, every winter these heaters freeze and cause damage to the heater and tub. Neither tub nor spa warranty nor homeowner's insurance covers damage from freezing. Gas heaters belong indoors, just like your home furnace. Many homes require modification to chimneys or flues to accommodate a gas heater. Installing a gas heater can mean new venting through the house up to the roof. These modifications can cost another $1,000 or more. If you really want a gas heater, contact a building inspector or a heating and air conditioning contractor who can tell you if your existing flue or chimney will be adequate and the requirements for vent and combustion air.
A propane gas hot tub heater, commonly used in cabins and some rural homes, costs about the same as electricity. Use propane in remote areas where electricity is not available, or if you installed your tub in a cabin and can't wait for the slower rise rates of electric heaters. Large propane heaters offer fast rise times of 20-30 degrees per hour depending on heater size and the number of gallons being heated. If the cabin has its hot tub heater installed outdoors or in a non-heated porch area, you won't be able to use your tub year round because of the same problems you have with any gas heater - heat exchanger freeze-ups, pilot lights blowing out, and stiff controls that don't operate properly.
Our Advice: If you can recover the cost of the heater and installation in 5-7 years and install the heater indoors, get the gas heater. However, if it's going to take 10 to 20 years to recover installation costs and you're only going to live in the house for 5 years, you've just lost the money. For most people, a 6 kW electric heater is the most practical.