Mike the Poolman


The gas cost (per hour) to run a pool / spa heater

November 5, 2012

(this article was updated in 2020. Click here to read it)

Customers are always asking me, ” Mike, I’d really like to use my heater more but I’m afraid of how much it will cost. How much does the gas cost to run my pool heater” or ” run my spa heater?”
My answer is always the same: ‘size matters, but…’
The but is we can still figure a pretty reliable median number based on the most common applications.

By far the most common heater found in backyards is the largest size allowed in residential applications: 399,000 BTU’s (British Thermal Units). We just refer to it as a 400 or 400,000 (if you have a heater for your pool/spa in Northern California chances are its a 400). Other sizes found are 350,000, 333, 275, 250 and 175. The larger the heater the bigger the campfire and the faster you’ll heat your pot of water.

The tricky part of the equation is knowing how much you pay your local utility company per therm. I live in Folsom and our natural gas is provided by PG&E. I checked my bill and found that we pay 92 cents to $1.40 per therm. I’ll take a high average and call it $1.20.

I know that a 100,000 BTU heater uses 1 therm of natural gas per hour, so a 400,000 heater consumes 4 therms an hour. 4 therms an hour x $1.20 = $4.80 per hour.  So the magic number based on some pretty popular averages is $5 per hour to heat the pool and $5 an hour to heat the spa. Keep in mind, we’re not talking about how long it takes but how much it costs. ‘How long it takes’ has many more factors and that topic is for another blog entry.

One more note: remember, the heater heats at the same rate regardless of whether its heating the pool or the spa. Its either 400,000 BTU’s on or 400,000 BTU’s off.

(this article was updated in 2020. Click here to read it)

Mike the Poolman
Pool Service & Repair in Folsom, CA since 1995

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