Ok, so I've been focused on equipment location and subsequently my heating source in a different thread here. I'm convinced that solar will be a good option for me and i have decided I will definitely go that route. However I don't want to be short sighted and box myself into a corner should I find in the future that I need to supplement the solar heater with an alternative. That would be a bad time to find out I still need a bigger gas line. So I was trying to run some numbers on heat pump vs gas and I'm surprised at what I'm finding. First, I think my 1" gas line will only handle about 150BTU or so given the distance from the meter (~150'). Second, running the cost of operation numbers reveals that they are nearly the same. Which has me questioning my calculations, hoping someone can double check me:
Just using round numbers assuming 100k BTU for both scenarios.
NG cost ~$8.19 / Mcf or $.82 / therm
Electricity Cost ~ $.16 / KWh
Now, that alone should be enough information to reach a conclusion if you know how to convert a therm into KWh (1 therm = 29 KWh, according to google) However that doesn't add up because that would result in an electricity cost being over 5 times higher than gas for the same unit of energy (which isn't reasonable)
So if you calculate it out the hard way:
100k BTU = 29.3 KWh
Assume COP = 6 (best case probably)
Pool Volume = 24000 gal ~ 200k lbs
Delta T = 1 (F)
Time to Delta T = lbs water / BTU x Delta T = 200k/100k x 1 degree = 2 Hrs
Electricity $ = 29.3KWh / COP (6) x $.16 x 2 Hrs = $1.56 (BTW, the rule of thumb for heat pumps is about 5 KWh per 100k BTU, which correlates closely with how I calculated.)
Gas $ = Therm $ x Hours = $.82 x 2 Hrs = $1.64
So, there you have it, a whopping 8 cents per hour advantage for electricity over gas. Lets say you need to run 6 hours a day, thats only $14 a month savings at BEST, and with the disadvantage of slower heating and lower efficiency in cooler ambient temps. Now, I realize I've assumed 100% efficiency of the gas heater which of course isn't appropriate but I've also ignored the drop in COP with ambient air temp so I think those variables probably cancel each other out. In fact, the heat pump could be more expensive to run if temps average below 80. So am I missing something, did I make a mistake? Someone please verify my work, thank you!
Just using round numbers assuming 100k BTU for both scenarios.
NG cost ~$8.19 / Mcf or $.82 / therm
Electricity Cost ~ $.16 / KWh
Now, that alone should be enough information to reach a conclusion if you know how to convert a therm into KWh (1 therm = 29 KWh, according to google) However that doesn't add up because that would result in an electricity cost being over 5 times higher than gas for the same unit of energy (which isn't reasonable)
So if you calculate it out the hard way:
100k BTU = 29.3 KWh
Assume COP = 6 (best case probably)
Pool Volume = 24000 gal ~ 200k lbs
Delta T = 1 (F)
Time to Delta T = lbs water / BTU x Delta T = 200k/100k x 1 degree = 2 Hrs
Electricity $ = 29.3KWh / COP (6) x $.16 x 2 Hrs = $1.56 (BTW, the rule of thumb for heat pumps is about 5 KWh per 100k BTU, which correlates closely with how I calculated.)
Gas $ = Therm $ x Hours = $.82 x 2 Hrs = $1.64
So, there you have it, a whopping 8 cents per hour advantage for electricity over gas. Lets say you need to run 6 hours a day, thats only $14 a month savings at BEST, and with the disadvantage of slower heating and lower efficiency in cooler ambient temps. Now, I realize I've assumed 100% efficiency of the gas heater which of course isn't appropriate but I've also ignored the drop in COP with ambient air temp so I think those variables probably cancel each other out. In fact, the heat pump could be more expensive to run if temps average below 80. So am I missing something, did I make a mistake? Someone please verify my work, thank you!
