Where "3,600 W" and "4.24 times" come from: Mitsubishi's sheet rates this pair at 12,300 BTU an hour of heat for 850 W of electricity, with 47°F outside and 70°F inside[1]. A kWh is 3,412 BTU[7], so 12,300 BTU an hour is 12,300 ÷ 3,412 = 3.605 kW, about 3,600 W. That is 3,605 ÷ 850 = 4.24 times the electricity, the sheet's own figure for it at 47°F, which it calls the COP[1][5]. When it starts, the wall unit's fan waits for warm air; Mitsubishi calls that Hot-Start[1].
Where "2,750 W from the outside air" comes from: the heat that reaches the room is the electricity put in plus the heat taken from the cold side[5], so the outside air gave 3,600 − 850 = 2,750 W. Over an hour that is 0.85 kWh of electricity, 2.75 kWh from the air and 3.60 kWh of heat in the room.
Where the colder figures come from: for 17°F and 5°F the sheet gives this pair flat out, at maximum capacity, with a COP line for each[1]. At 17°F that is 17,410 BTU an hour, 5,103 W, at a COP of 2.74, so it uses 5,103 ÷ 2.74 = 1,862 W. At 5°F it is 14,690 BTU an hour, 4,305 W, at 2.41, so 4,305 ÷ 2.41 = 1,786 W. The sheet also lists a maximum power input of 1,990 W at 17°F and 1,910 W at 5°F; the heat divided by those would be 2.56 and 2.25, lower than its COP lines. The page goes by the COP lines, which the sheet gives for that very point. Running flat out costs a little on its own: at 17°F the sheet also rates it at a steady pace, 7,560 BTU an hour for 740 W, which is 2.99 times. The chips round to the nearest 10 W: 5,100 W of heat for 1,860 W at 17°F, so 3,240 W from the air, and 4,310 W for 1,790 W at 5°F, so 2,520 W from the air. It is rated to keep heating down to -13°F[1]. A plain electric heater turns each watt into one watt of heat[6]. The Department of Energy says an air-source heat pump can give up to three times the heat of the electricity it uses[3]. That is a figure for heat pumps in general; this one's 4.24 is its rating on a mild 47°F day, and on the sheet it falls to 2.41 at 5°F.
Where "about 57 kWh" comes from: the gallon goes through the same big diesel generator as on our Diesel to Light page. It makes 500 kW on 133.9 liters of diesel an hour[8], which is 35.37 gallons[9], so a gallon makes 500 ÷ 35.37 = 14.1 kWh. About 5% is lost in the wires on the way[10], leaving 13.4 kWh at the house. At 47°F this heat pump turns that into 13.4 × 4.24, about 57 kWh of heat; flat out at 5°F, 13.4 × 2.41, about 32 kWh. The gallon itself holds 137,381 BTU[7], 40.3 kWh, if you burned it for its heat.
Where the numbers come from
- Heating at 47°F, rated: 12,300 BTU an hour for 850 W, COP 4.24. At 17°F, rated: 7,560 BTU an hour for 740 W; at maximum capacity 17,410 BTU an hour, COP 2.74; maximum power input 1,990 W. At 5°F, at maximum capacity: 14,690 BTU an hour, COP 2.41; maximum power input 1,910 W. Rated data is at a fixed compressor speed, with 70°F inside. It heats down to -13°F outside, and its Hot-Start means "no cold air rush at equipment startup": Mitsubishi Electric, submittal sheet MSZ-FS12NA & MUZ-FS12NAH
- Like a refrigerator, heat pumps "use electricity to transfer heat from a cool space to a warm space"; in winter they "move heat from the cool outdoors into your warm house": US Department of Energy, Energy Saver: Heat pump systems
- "In heating mode, liquid refrigerant in the outside coil removes heat from the air and evaporates into a gas"; "the indoor coil releases heat from the refrigerant as it condenses back into a liquid"; a ductless unit needs "only a three-inch hole through the wall"; "up to three times more heat energy" than the electricity it uses: US Department of Energy, Energy Saver: Air-source heat pumps
- "The compressor delivers the vaporized refrigerant under high pressure and temperature to the condenser", where "the cooler air condenses the refrigerant and becomes heated"; in the throttle valve the liquid expands and comes out colder; in the evaporator "the liquid is evaporated by contact with a comparatively warmer space": Britannica, heat pump
- The heat delivered is the heat taken from the cold side plus the work put in (Qh = Qc + W); a heat pump's coefficient of performance is the heat delivered over the work (COP = Qh/W); "heat pumps do not work as well in very cold climates": OpenStax, College Physics 15.5: Heat pumps and refrigerators
- "Electric resistance heating is 100% energy efficient in the sense that all the incoming electric energy is converted to heat": US Department of Energy, Energy Saver: Electric resistance heating
- One kilowatthour is 3,412 Btu; a gallon of diesel holds 137,381 Btu: EIA, British thermal units (Btu)
- Diesel to Light's generator: 500 kW at full load on 133.9 liters of diesel an hour: Caterpillar C18 500 ekW technical spec sheet DM9831-02 (2017)
- One US gallon is exactly 3.785411784 liters: NIST Handbook 44 (2026), Appendix C, tables of units
- About 5% of electricity is lost in transmission and distribution (2018 to 2022): EIA FAQ, how much electricity is lost
The drawing is not to scale. The two units are drawn 1.35 times their size so their parts can be seen. The room, its furniture, the snow, the panel's layout, how fast the drawn refrigerant moves and the sound words are this drawing's own choices. Steps 1 to 4, 6 and 7 are not timed to real life; step 5 plays one hour in 10 seconds.