I just shared something similar to this in another post. This is a lengthy post and contain estimated numbers for cost of electricity and LP gas, and BTU outputs of gas heaters and Heat Pumps. These numbers are NOT exact and are only estimates and examples. This does not take into consideration, the loss of efficiency for all heaters as ambient air temperatures drop.

You can calculate the approximate amount of time it will take to heat the pool by the following:

Gallons of water x 8.3 (lbs/gallon) = X lbs. of water

X lbs of water x your temperature increase = Y BTUs required

Y BTUs required / BTU output of your heater = Z Run Time in hrs.

Remember to use a decreased BTU output, as gas heaters are usually about 75% efficient. That is, if it's rated for 100,000 BTUs, that's how much fuel it will burn to produce 75,000 BTUs of heat.

Example:

8000 gallon pool x 8.33lbs/gal = 66,640 lbs of water

66,640 x 20 (60F to 80F = 20 degrees increase) = 1,332,800 BTUs required

1,332,800 / 75,000 BTUs = 17.77 hrs to get it up 20 degrees of increased heat.

But this is just to get it up 20 degrees. If you leave the heater on to maintain this temperature, you don't have to use 20 degrees in your calculations, except for the initial heat up period.

With a solar blanket covered pool, you will lose about 20% of the heat added in. An uncovered pool will lose about 50%. What that means is that although you will have to run it along time to get it up to temperature when you activate your heater, the first time, you don't have to keep adding 20 degrees of heat per day. However, if you turn off your heater and only turn it on when you plan on using it, you

__will__ have the 20 degrees (or however many degrees of rise you desire) of heat to produce.

Leaving the heater on everyday means you will only need to add back 20% of 20 degrees, or 4 degrees for a covered pool; or 50% of 20 degrees, or 10 degrees, for an uncovered pool.

Calculating these parameters;

Covered: 66640 lbs x 4 degrees = 266,560 BTUs / 75,000 = 3.55 hrs/day to maintain

Uncovered: 66640 lbs x 10 = 666,400 BTUs / 75,000 = 8.88 hrs/day to maintain

THEN, to calculate what your cost to heat your pool is, you would need to find out what the cost per gallon of LP gas is. One gallon of LP = 100,000 BTUs.

In my example, I used a 100,000 BTU heater, which burns 100,000 BTUs/hr to generate 75,000 BTUs (75% efficiency).

IF a gallon of LP cost $2.50, your daily cost would be:

for spot heating your pool, when you start your heater on "demand", $2.50 x 17.77 hrs/day = $44.43 to get it up 20 degrees.

A covered pool would be $2.50 x 3.55 hrs/day = $8.87/day;

OR for an uncovered pool, $2.50 x 8.88 hrs/day = $22.20/day.

Cut the number of hours in half for a 200,000 BTU heater. The cost to heat would remain the same because although you're cutting the heater run time in half, you're burning twice as much LP per hour. You will save on your pump run time though.

For a more efficient gas heater, which may increase efficiency to 85 - 90%, you would change the BTU output of the heater. All the other numbers would remain the same though (pool volume and pounds of water).

The larger the heater, the more BTU's it will put out, which means the shorter the amount of time it will take to get your water temperature up to your desired temperature. Again, the cost to do so will be the same.

When you compare this to the cost to operate a Heat Pump, you can see where the cost savings of a heat pump are.

A Heat Pump with 100,000 BTUs puts out 100,000 BTUs. However, the cost is based largely upon the efficiency of the unit, even among Heat Pumps.

An Aqua Cal H120,

http://www.aquacal.com/heat-pump-inform ... .html#h120, produces 109,000 BTUs at 5.7 KW of energy. If we estimate the cost of electricity @ $0.11/KW (typical Florida rate), it would cost $0.11/kw x 5.7kw/hr = $0.63/hr.

--> SIDEBAR: COP of a heat pump is configured by using 1 KW = 3,412 BTU. For the H120, 5.7kw x 3412 = 19,448.4 BTUs so, 109,000 BTU output / 19,448.4 BTU input = 5.6 COP

A similar heat pump with a kw input of 6.0 kw x 3412 = 20,472 BTUs, so 109,000/20,472 = 5.3 COP making it less efficient.

The difference would be reflected in the operating cost. The higher COP = greater efficiency.

The initial heat up with this heat pump: 1,332,800 / 109,000 = 12.22 hrs (x $0.63/hr = $7.70 to heat up 20 degrees)

Maintaining with a solar covered pool: 266,560 BTUs / 109,000 = 2.45 hrs/day to maintain(x $0.63/hr = $1.54/day)

Maintaining with an uncovered pool: 666,400 BTUs / 109,000 = 6.11 hrs/day to maintain (x $0.63/hr = $3.85/day)

As you can see, the operating cost difference of LP vs Heat Pump:

Spot heat up: $44.43 vs $7.70 each time ($177.72 vs $30.80 if you heat your pool 4 days/month)

Covered Pool: $8.87 vs $1.54 /day ($266.10 vs $46.20 / month (30 days))

Uncovered Pool: $22.20 vs $3.85 /day ($666.00 vs $115.50 / month (30 days))

A 82.3% savings!

The other factor to consider is the initial cost of equipment and installation.

The Heat Pump is approx $3,200 and the gas heater about $1600 (Google search on Internet prices), factor in the cost of mounting an LP tank and running gas lines (electrical for both would be a wash), the initial savings may come out slightly less for the gas heater than the Heat Pump. Lets say it comes out a $1000 less than the Heat Pump, your annual operating cost savings of heating your pool for 3 months/year (assuming that you get 1.5 months extra at the beginning and at the end of the pool season):

Formula used: (Operating cost of LP - Heat Pump costs) x 3 mo = $cost savings/yr. $1000 initial cost difference / $ cost savings = Return on Investment (in years)

Spot heat up: $177.72 - $30.80 = $146.92/mo x 3 = $440.76 ($1000 / $440.76 = 2.3 years ROI)

Covered Pool: $266.10 - $ 46.20 = $220.00/mo x 3 = $660.00 ($1000 / $660.00 = 1.5 years ROI)

Uncovered Pool: $666.00 - $115.50 = $550.50/mo x 3 = $1651.50 ($1000 / $1651.50 = 0.6 years ROI)

After your ROI period, it's all operating cost savings from there.

Hope this doesn't confuse you too much.

DOH, I did not factor in your desire to connect your gas grill to the LP tank...