Is electricity cheaper than propane? Propane gas heaters are typically the most costly to run.
It’s not a matter of absolute BTUs that you’ll need but more about how quickly you want to heat up your pool that determines which heater is the best fit. Pool heaters are rated in BTUs per hour with largest residential gas heaters maxing out at 400,000 BTUs/hr. Electric heat pumps typically max out at around 130,000 BTUs/hr.
So, for example, if your pool had an attached spa on it and you wanted to use the heater to both maintain the pool heat AND allow you to quickly heat up a spa when desired, then a 400kBTU/hr gas heater would be the way to go (fast heating). But, if you just have the above ground pool that you wanted to start using a little early and maintain water heat through out the season, then an electric heat pump is a good choice if electricity isn’t too expensive (slow heating).
Either way, a BTU is a BTU no matter how you add it to your pool (fast or slow), so however much energy you use, it’s going to cost you. A BTU raises 1 lb of water 1 degree Fahrenheit. So 26,000 gallons of water weighs 216,840 lbs and will require 216,840 BTUs to raise the temperature 1 deg F (assuming ideal/perfect efficiency and no heat loss). A gallon of propane has 91,330 BTUs in it. So it takes roughly 2.4 gallons of LPG to heat your pool 1 deg F. Depending on how much propane costs you, you could be looking at a hefty heating bill.
In all situations, if you heat your pool, you must use a bubble cover on it because if you don’t, most of the heat you add will be lost to evaporation in the overnight hours when it’s cooler.