How much does it cost to heat a pool in Phoenix?

I am in phoenix Arizona. I have a 12-14,000 gallon in ground pebble tech pool and a 400,000 btu natural gas heater. Would like to know costs to heat pool of in phoenix, with a natural gas heater. Please state approximate pool size, in gallons and if you heat your pool a few times a month or all month long. Obviously winter would cost more than summer.