Efficiency Calculator

What efficiency measures

Efficiency answers one question: of everything you put in, how much came out as the thing you wanted? The formula is efficiency = (useful output divided by total input) times 100. Put 1500 watt-hours into a device and get 900 watt-hours of useful work back, and the efficiency is 900 / 1500 = 0.60, or 60 percent. The missing 40 percent left as heat, friction, noise or light nobody asked for. The U.S. Department of Energy uses this same shape of definition for home heating: its FEMP program defines AFUE, the annual fuel utilization efficiency, as the ratio of useful energy output to energy input over an entire heating season (energy.gov, FEMP residential furnaces).

The definition quietly depends on agreeing on what counts as useful. The same device might score 90 percent if its waste heat warms the room on purpose, or 30 percent if the useful output is measured at a different point in the chain. Every efficiency figure below carries its own definition, which is why each source is named alongside it.

Light bulbs: about 10 percent light, 90 percent heat

The Department of Energy's Energy Saver guide states that incandescent bulbs waste about 90 percent of their energy as heat, which leaves roughly 10 percent arriving as light (energy.gov, LED lighting). Run the formula on a 60 W bulb left on for 1000 hours: it draws 60 kWh and delivers about 6 kWh of light. An LED that produces the same light draws about 9 W, uses 9 kWh over the same 1000 hours, and the Department adds that LEDs use at least 75 percent less energy and last 25 times longer than the incandescent bulbs they replace.

The money follows the kilowatt-hours. Saving 51 kWh at the 2024 U.S. residential average price of 16.48 cents per kWh (EIA, Electric Power Monthly, Table 5.3) comes to about $8.40 per bulb per thousand hours. Multiply by the 40 to 50 sockets an average house runs and the savings stop being pocket change.

Cars: over 77 percent versus 12 to 30 percent

The Department of Energy's fueleconomy.gov puts numbers on the two drivetrains: electric vehicles convert over 77 percent of the electrical energy taken from the grid into power at the wheels, while conventional gasoline vehicles convert about 12 to 30 percent of the energy stored in gasoline (fueleconomy.gov, all-electric vehicles). Feed 100 kWh into each side of the comparison: the EV delivers at least 77 kWh to the wheels, the gasoline car between 12 and 30 kWh, and the rest exits through the tailpipe and radiator as heat.

Note what the comparison does and does not say. Both figures are measured at the wheels from each vehicle's own energy source, so they share a definition, and that is what makes the gap meaningful. Comparing an EV's 77 percent against a furnace's 99 percent AFUE would be meaningless: one moves a car, the other warms air in ducts, and the useful outputs differ.

A worked breakdown, counted hour by hour so every watt hour lands somewhere:

The grid: two thirds gone before the meter

The largest efficiency losses in daily life happen before any appliance switches on. The Energy Information Administration's Monthly Energy Review explains that about two thirds of the energy input to the electric power sector is lost in converting heat into electricity, with roughly 5 percent of generated electricity used at the plants and about 7 percent lost in transmission and distribution (EIA, Monthly Energy Review, Section 2). A hundred units of fuel energy therefore reach the meter as roughly 100 x (1/3) x 0.95 x 0.93, about 29 units.

EIA's dedicated estimate for the wires alone puts transmission and distribution losses at about 5 percent of the electricity transmitted and distributed, averaged over 2018 through 2022 (EIA FAQ on T&D losses). The conversion loss is the stubborn part: it is a property of heat engines and thermodynamics, not of careless utilities.

Furnaces and AFUE: efficiency by season, not by moment

A furnace's AFUE is efficiency measured across a whole heating season, and DOE's FEMP tables show what the scale means in fuel. At 70,000 Btu/h output capacity, the best available condensing furnace reaches 99 percent AFUE, an ENERGY STAR model reaches 95 percent in the northern region, and a less efficient model sits at 80 percent (DOE FEMP, residential furnaces).

The same FEMP tables, for the southern region, list annual fuel use of 360 therms for the 80 percent model and 291 therms for the 99 percent model delivering the same heat. The cross-check closes: 360 x 0.80 = 288.0 and 291 x 0.99 = 288.1, so both figures describe about 288 therms of delivered warmth. Over a 21 year lifetime, DOE puts the lifetime energy cost saving of the best available model over the 80 percent model at $947 in that region.

What each wasted watt costs

The calculator above extends the percentage into a bill. Take a device that draws 1500 Wh per hour of running and delivers 900 Wh of useful output, so 0.6 kWh escapes as waste each hour. Run it 4 hours a day and the waste adds up to 0.6 x 4 x 365 = 876 kWh per year. At the 2024 residential average of 16.48 cents per kWh, that is $144.36 a year paid for nothing (EIA, Electric Power Monthly, Table 5.3; EIA's preliminary 2025 average was 17.30 cents).

The same arithmetic runs in reverse when you shop. A device that is 10 percentage points more efficient on the same input saves 0.1 x input x hours x 365 x price per year. Whether that matters depends on the load: it matters for a heater that runs all winter and hardly matters for a clock.

Efficiency numbers worth knowing, and how they mislead

Each figure below is quoted with the definition its source uses, so the rows are not directly comparable to each other:

Four habits produce wrong efficiency numbers. Mixing units in the ratio, such as dividing watts by watt-hours, breaks the formula before it starts; convert both sides to the same unit first. Comparing numbers that use different definitions, like wheel efficiency against AFUE, produces confident nonsense. Quoting the rated efficiency while the device runs at part load overstates real performance, which is why AFUE is measured seasonally. And ignoring standby draw hides a small loss that runs 24 hours a day, every day of the year.

Worked rows: input, output, and the waste between them

Every efficiency claim reduces to the same two numbers. The first table runs the arithmetic on four device types, and the second shows where the missing energy went in one worked case.

Notice the spread. A condensing furnace turns almost all of its fuel into room heat, while a solar panel converts a fifth of the sunlight that lands on it, because the input is free and abundant. Efficiency alone never tells you whether a device is worth owning; cost per useful unit does.

The second table counts every watt hour, so the shares always sum to 100. If a vendor quotes 120 percent efficiency, one of two things happened: the input was measured in the wrong place, or the number includes heat pulled from another source, as heat pumps legitimately do with their own metric, the coefficient of performance.