How much of the fuel you pay for becomes heat or work? Enter energy in and out in kWh, BTU or therms and get the efficiency, the losses in dollars and a check against typical furnaces.
What does 80 AFUE mean for your fuel bill?
How much of the fuel you pay for becomes heat or work? Enter energy in and out in kWh, BTU or therms and get the efficiency, the losses in dollars and a check against typical furnaces.
An AFUE of 80 means that over a heating season 80 of every 100 units of fuel energy leave the furnace as heat for the house, and the rest goes up the vent. The calculator below is set to the useful output mode and filled in with 100 therms of gas going in and an efficiency of 80 %, ready to show the useful heat, the loss and the same figures in kWh.
Three things the rating does not tell you. It stops at the furnace cabinet, so heat lost from ducts in an attic or crawl space comes on top of it. It is measured on the higher heating value of the fuel, which is why a European boiler label for similar equipment can read above 100 %. And it describes the equipment, not your bill: the fuel you buy is your heat demand divided by the rating, which the energy input mode works out.
To see how 80 AFUE compares with typical furnaces, choose a type under Compare with; below the usual band the result also shows the fuel a better unit would save, and a price per therm turns that into dollars.
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Thermal efficiency, before and after you put numbers on it
A furnace that burns 1,000 therms and puts 600 therms of heat into the house is running at 60.0 %, and the other 400 therms went up the flue. This calculator does that division in any of three directions: efficiency from two readings, useful output from an input and a rating, or the input you have to buy for a given amount of heat. It takes kWh, BTU, therms or megajoules, compares the result with typical furnaces, heat pumps, motors and engines, and prices the losses if you give it a fuel cost.
Going by the sticker
- An 80 % and a 95 % furnace look like a 15-point gap, and nobody says what that is in fuel
- A European boiler label reading 104 % looks like a better machine than any AFUE rating
- A heat pump at 300 % looks like a typo next to a 95 % furnace
- A reading above 100 % from a gas appliance gets believed
With the ratio worked out
- 80 to 95 % means 15.8 % less fuel for the same heat
- The European figure is on a different heating value and lands near 94 % on the AFUE basis
- 300 % is a coefficient of performance of 3, heat moved in rather than made
- A fuel-burning device above 100 % is flagged as a bad reading, not praised
What the form wants from you, in order
Seven fields in total, but each mode shows only two of the three energy boxes, so a typical run is four entries.
- What to work out - efficiency when you have two readings, useful output when you know what goes in and the rating, energy input when you know the heat you need.
- Energy unit - one unit for every energy figure. Gas bills come in therms, oil and propane in gallons that you convert to BTU (about 138,500 BTU per gallon of heating oil and 91,452 BTU per gallon of propane), electricity in kWh.
- Energy in - the fuel or electricity the device consumed over the period you are looking at.
- Useful energy out - the heat delivered or the work done, from a heat meter, a test report or a manufacturer's output rating.
- Efficiency - the rating in percent. AFUE for furnaces and boilers, coefficient of performance times 100 for a heat pump.
- Compare with (optional) - a device type, so the result is read against its usual band instead of on its own.
- Price per unit (optional) - what one unit of the input costs you, which turns the losses into dollars. Then read the results.
How the three modes do the arithmetic
All three are the same relation rearranged. Efficiency is useful energy divided by energy in, and it only means something when both are in the same unit, which is why the form asks for one unit for everything.
useful out = energy in × efficiency / 100
energy in = useful out / (efficiency / 100)
A heat pump runs the same formula and simply lands above 100. One kWh of electricity in and 3.5 kWh of heat out gives 350.0 %, a coefficient of performance of 3.50, and 2.50 kWh drawn from the outdoor air. At $0.15 per kWh, each useful kWh of heat costs about $0.04.
What a higher rating saves on the same heat
The table holds the heat demand fixed at 600 therms a year of useful heat, an example figure for a house rather than an average, and asks how much fuel each AFUE band needs to deliver it. The saving column is measured against the oldest furnace in the list. The numbers scale directly: double the demand and every figure doubles.
| Furnace | AFUE | Fuel for 600 therms of heat | Saved against 56 % |
|---|---|---|---|
| Old, standing pilot, low end | 56 % | 1,071.43 therms | - |
| Old, standing pilot, high end | 70 % | 857.14 therms | 214.29 therms |
| Non-condensing | 80 % | 750.00 therms | 321.43 therms |
| Condensing, entry | 90 % | 666.67 therms | 404.76 therms |
| Condensing, common | 95 % | 631.58 therms | 439.85 therms |
| Condensing, top of the range | 98.5 % | 609.14 therms | 462.29 therms |
Two things fall out of it. The steps shrink: the jump from 56 to 70 % saves 214.29 therms, while the last step from 95 to 98.5 % saves only 22.44. And the familiar replacement, 80 to 95 %, cuts fuel by 118.42 therms, which is 15.8 % of what the 80 % furnace burned, not the 15 points on the label.
Efficiency of heating equipment, motors, engines and power plants
The furnace bands are the Department of Energy's: 56 to 70 % for older systems, 80 to 83 % for mid-efficiency and up to 98.5 % for high-efficiency condensing units. The heat pump bands come from the same department's guidance on geothermal systems. Power plant figures are worked out from EIA's average tested heat rates, dividing 3,412 BTU (one kWh) by the BTU a plant burns per kWh.
| Device | Efficiency | What the figure means |
|---|---|---|
| Electric resistance heater | 100 % | every kWh ends up as heat in the room |
| Geothermal heat pump | 300 to 600 % | COP 3 to 6, heat moved in from the ground |
| Air-source heat pump, central ducted | 175 to 250 % | over a season; mild days run higher |
| Condensing furnace or boiler | 90 to 98.5 % | AFUE, higher heating value basis |
| Premium electric motor, 5 to 200 hp | 89.5 to 95.4 % | six-pole ratings at full load; bigger motors lose less |
| Non-condensing furnace | 80 to 83 % | AFUE |
| Older furnace with a standing pilot | 56 to 70 % | AFUE |
| Natural gas combined-cycle power plant | about 45.2 % | heat rate about 7,548 BTU per kWh |
| Heavy-duty diesel engine, best point | about 42 % | research engines with waste heat recovery reach 55 % |
| Gasoline car engine, best point | 30 to 40 % | peak brake efficiency, not what the car averages |
| Coal steam power plant | about 34.1 % | heat rate about 10,018 BTU per kWh |
| Simple-cycle gas turbine plant | about 31.0 % | heat rate about 10,999 BTU per kWh |
| Whole gasoline car, fuel to road | 12 to 30 % | idling, gears and accessories take the rest |
Where a single percentage misleads
AFUE stops at the furnace cabinet. It counts what leaves up the flue, not what leaks from ducts in an unheated attic or crawl space, which DOE-cited figures put as high as 35 % of the furnace's output. A 95 % furnace losing a fifth of its heat in the ducts delivers 76.0 % of the fuel energy to the rooms, less than an 83 % furnace with its ducts inside the heated space and sealed.
Heating value decides whether a boiler can show more than 100. Natural gas carries about 10.8 % more energy on the higher heating value, which includes the heat locked in water vapor, than on the lower one; for fuel oil the gap is about 6.8 %. European ratings use the lower value, so a condensing boiler at 104 % there is roughly 93.9 % in AFUE terms. Same boiler, different yardstick.
An engine's best point is not its average. A gasoline engine near 40 % at its sweet spot sits in a car that turns only 12 to 30 % of the fuel into motion down the road. Put a whole trip into this calculator and the result will be the lower figure, and that is not a defect of the engine.
Often asked about furnaces, engines and percentages
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Calculator verified by the LiczGrupa.pl team
Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.
Reviewed by: Natalia Skrzek