- 1 Btu is the amount of energy required to raise 1 lb. of water 1°F; a Btu is used to describe the amount of heat energy in fuels
- 1 gal. of water weighs 8.34 lbs.
- 1 kW of electricity equals 3,412 Btu/hr.
- A 4,500 W element produces an equivalent of 15,354 Btu/hr.
- A 5,500 W element produces an equivalent of 18,766 Btu/hr.

**Heating a 40 gal. tank with a 4,500 W element:**

8.34 lbs. per gal. x 40 gallons x [120°F – 60°F (rise in temperature)] = 20,016 Btu needed

20,016 Btu ÷ 15,354 Btu/hr. (heat produced by a 4,500 W element) = 1.30 hrs.

**Heating an 80-gal. tank with a 5,500 W element:**

80 gal. x 8.34 lbs. per gallon x 60°F (rise in temperature) = 40,032 Btu needed

40,032 Btu ÷ 18,766 Btu/hr. (heat produced by a 5,500 W element) = 2.13 hrs.