A hybrid heating system splits the winter between two energy sources. A heat pump does the everyday work with electricity, and a gas furnace or other fuel-fired backup takes over when outdoor air is too cold for the heat pump to carry the load alone. ACHR News's hybrid heating story makes the case that "Heat pumps do not have to do all the work" for a homeowner to benefit from electrification. That point is sound, but it leaves a bookkeeping problem. Your bill arrives with a kilowatt-hour line and a therm or gallon line, and most people compare them by feel instead of by energy.
Put both lines in one unit
Pick British thermal units as the common unit. One kilowatt-hour equals 3,412 BTU, and one therm of natural gas equals 100,000 BTU. Propane is sold by the gallon, and its heating value depends on the supplier's stated figure, so check your delivery ticket or the supplier's website rather than trusting a generic number. Write the conversion factor at the top of your sheet so every later entry uses the same math.
Converting the bill gives you the energy you bought, not the heat that reached the rooms. A heat pump moves heat rather than making it from fuel, so the heat delivered per kilowatt-hour is higher on mild days and drops as the air gets colder. Treat the converted electricity and the converted gas as input energy, and compare them on that basis. Do not present the result as a measure of comfort or efficiency for the whole house.
Separate the heat pump from the rest of the electric bill
Your electric line covers far more than the heat pump. The refrigerator, water heater, lights, and air handler all draw from the same meter. If you have no submeter, clamp a meter around one supply conductor to the outdoor unit and another to the air handler on a cold day, then log the readings over an hour or two. Use a clamp meter rated for the voltage, and keep your hands out of the service panel. Once you have the average kilowatts while the heat pump runs, multiply by its runtime from the thermostat history or your own run log. That gives you a defensible heat pump share instead of a guess.
Find the switch point in the monthly history
Twelve months of bills show where the backup starts carrying load. Gas use rises in cold months, and that jump is the backup at work. Set the therms for each billing period against the weather for the same period. A public heating degree day figure for your area lets you compare a mild November with a hard January without relying on memory. Where the gas line barely moves through the winter, the heat pump is carrying most of the house. Where gas spikes in the same weeks each year, your switch point sits somewhere near that outdoor temperature band.
Read the fixed charges separately
Gas delivery and electric service charges do not shrink when the furnace stops running. The part that moves is the per-unit energy charge. When you compare one year with another, compare the variable lines, not the bottom-line total. Time-of-use electric rates add another wrinkle. A heat pump that runs overnight may cost less per kilowatt-hour than one running during a peak window, so note the rate period on each billing line before you draw conclusions.
Build a two-column sheet
Make a sheet with one row per billing period. Include columns for kilowatt-hours, the BTU equivalent, therms, the BTU equivalent, heating degree days, and a note for any thermostat or setpoint change. Mark the month you changed the changeover setting or the backup lockout so later readings make sense. Keep the bill copies with the sheet. A table like this shows the switch point faster than any single bill, and it tells you whether the equipment is doing the work you expected it to do.
The bill is a record of the system, not a verdict on it. Convert both meters to one unit, separate the heat pump from the rest of the house, and read the switch point from the months themselves.