Eschew Obfuscation

Hot Tub Energy: Electric vs Gas

Posted by Max Dunn Tue, 22 Jan 2008 22:38:00 GMT

Nowadays, if you are looking to install a hot tub, your only option is likely to be a hot tub that heats with electricity. The salesman will tell you that they are very well insulated (which they are) and that it will only cost about $30 per month of electricity to heat it (which is possible but optimistic.) Let’s look at the math and physics behind this.

One kilo-watt hour (1 kWh) of electricity can heat about 430 gallons of water by 1 deg F (see below for formulas).

A friend in San Diego noticed that their hot tub went from 105 to 88 over 4 days this winter when it was turned off. On a linear scale, that would be 4.25 deg per day. However, heat loss is exponential, and assuming the average outside temperature was about 50 (day 60 night 40), the rate of loss at 100 degrees would be twice that as at 75 degrees, so the first day it probably lost about 6 degs (then the next day 5 degrees, then 4, then 3).

To make the math easy, let’s assume that the hot tub is about 430 gallons, so it would take 1kWh to heat it 1 degree. So in winter conditions, when the cover is on, it would take about 6 kWh just to keep it hot. Add another 1kWh for the pump and another 2kWh to compensate for the loss of heat while you were using it, and that gets to 9 kWh per day. This is about what my friend observed.

In California, electricity starts out at $0.11 per kWh. So it would cost about $1 per day in electricity (9 * $0.11) for a total of $30 per month. However, that $0.11 rate is only good for about the first 500 kWh per month you use – after that rates quickly move up to $0.36 per kWh. Since the hot tub will take more than half the baseline allotment (270 kWh) most people will be paying the higher rates, up to $100 per month to heat your hot tub!

Using natural gas for heat is much cheaper than electricity. If gas costs $1.20 per therm, a gas hot tub would only cost about $7.50 per month to heat.

So while electric hot tubs are easy to install and easy to find, you could save up to $1000 per year in energy costs by installing a gas hot tub.

Formulas:

• 1 BTU = 1 deg F * 1 lb water
• 1 lb water = 8 gallon
• 1 kWh = 3.6 million joules
• 1 BTU = 1051 joules
• 1 kWh = 3425 BTUs
• 1 kWh = 430 gallon degrees F
• 1 therm = 100,000 BTUs