A hot water boiler in your home will use one of two types of energy -- gas or electricity. You can calculate the energy consumption of your hot water boiler with a few pieces of information and good, old-fashioned mathematics.

Find out the size of your boiler, measured in British thermal units. This information will be included in the technical specifications for the unit, and will likely also be on the boiler label.

Multiply the number of BTUs by 0.0002928 to calculate the amount of energy your boiler is using in kilowatt hours (for electric powered hot water boilers). Use the website of your local utility company to find out the current cost per kilowatt hour of energy provided. Multiply this calculation by the cost per kilowatt hour to find out how much energy your hot water boiler is consuming, in dollars.

Multiply the number of BTUs by 0.000009993 to calculate the amount of energy your boiler is using in therms (for natural gas powered hot water boilers). Use the website of your local utility company to find the current cost per therm of energy provided. Multiply this calculation by the cost per therm to find out how much energy your hot water boiler is consuming, in dollars.