Aquarium heaters cost no different than any other power consuming item. To find the cost of running your aquarium heater simply use the cost of kilowatt per hour. In The US this averages at 12.58 cents.

For convenience sake here is a quick table on aquarium heater operation costs. Depending on the difference in room and tank temperature your heater will be on more or less often.

Heater Size | Cost Per Month | Cost Per Year |

25 Watts | 6 hours a day = $.56 a month
8 hours a day = $.75 a month 10 hours a day = $.94 a month |
6 hours a day = $6.72 a year
8 hours a day = $9 a year 10 hours a day = $11.28 a year |

50 Watts | 6 hours a day = $1.13 a month
8 hours a day = $1.51 a month 10 hours a day = $1.89 a month |
6 hours a day = $13.44 a year
8 hours a day = $18 a year 10 hours a day = $22.56 a year |

100 Watts | 6 hours a day = $2.26 a month
8 hours a day = $3.02 a month 10 hours a day = $3.78 a month |
6 hours a day = $26.88 a year
8 hours a day = $36 a year 10 hours a day = $45.12 a year |

150 Watts | 6 hours a day = $3.39 a month
8 hours a day = $4.53 a month 10 hours a day = $5.67 a month |
6 hours a day = $47.88 a year
8 hours a day = $63.84 a year 10 hours a day = $79.80 a year |

200 Watts | 6 hours a day = $4.52 a month
8 hours a day = $6.04 a month 10 hours a day = $7.58 a month |
6 hours a day = $53.76 a year
8 hours a day = $72 a year 10 hours a day = $90.24 a year |

250 Watts | 6 hours a day = $5.65 a month
8 hours a day = $7.55 a month 10 hours a day = $9.45 a month |
6 hours a day = $67.80 a year
8 hours a day = $90.40 a year 10 hours a day = $113 a year |

300 Watts | 6 hours a day = $6.78 a month
8 hours a day = $9.06 a month 10 hours a day = $11.34 a month |
6 hours a day = $81.36 a year
8 hours a day = $108.48 a year 10 hours a day = $135.60 a year |

As you can see the costs of running an aquarium heater are not too high, but do climb quickly with long hours and high wattage. This means the average cost of running even the best aquarium heater is fairly low for small tanks, where as large tanks can drain more money than expected.