What do people actually want from energy?

Walk through a home, sort its energy by service, and swap the furnace for a heat pump on a cold day

F3 of Energy Systems, Step by Step: walk through a Prairie Town home, sort services from carriers and devices, follow a year of energy from what the home buys to what it gets, and watch a heat pump’s COP fall on a cold day.
  • Foundations · L1
  • 20 min guided · about 10 min practice
  • After lesson 1

The idea in one table: a Prairie Town home's year, by service and by carrier (kWh, rounded to the nearest 100)

Service Natural gas Electricity Gasoline Useful
Warmth 16,900 13,500 (furnace, 80%)
Hot water 5,100 3,100 (water heater, about 60%)
Cooling 1,700 not split here
Kitchen and laundry 1,100 900 not split here
Lights, screens and devices 5,400 not split here
Getting around (one car) 16,900 5,100 (about 30%)
All 23,100 8,000 16,900 21,700 of 48,000 bought
  • Services, carriers, devices. People want services (a warm room, a hot shower, a trip); carriers (gas, electricity, gasoline) bring the energy; devices turn one into the other.
  • Warmth is the biggest home service: 16,900 of the house's 31,100 kWh. The one car uses about as much: 16,900 kWh of gasoline.
  • Swap the furnace for a heat pump: the same 13,500 kWh of heat comes from 5,400 kWh of electricity plus 8,100 kWh moved in from the outdoor air (seasonal COP 2.5). Home and car then buy 36,500 kWh.

The heat pump on a cold day (illustrative model)

Day's outdoor average COP Heat needed Electricity
15 °C 4.80 15 kWh 3 kWh
5 °C 3.60 65 kWh 18 kWh
−5 °C 2.88 115 kWh 40 kWh
−15 °C 2.40 165 kWh 69 kWh
−25 °C (below the −23 °C cut-out: backup heater) 1.00 215 kWh 215 kWh

With JavaScript on, you work through this step by step: a tour of the home, a card sort, a guess, the year as a flow diagram, a heat pump swap and a cold day.

Based on real survey averages. The Prairie Town home is illustrative: real 2020 survey averages for homes in Kansas's region, plus one typical car.

Inputs, sources and dates
Input Real or illustrative Source and date
Energy per service Real averages per home using each fuel and end use, combined into one home EIA RECS, Table CE4.6, West North Central (includes Kansas) · 2020 data
Million Btu to kWh 3,412 Btu per kWh EIA · read 3 November 2026
The car Real average: 10,917 miles a year at 22.8 miles per gallon EPA · 2022 data
Device efficiencies Illustrative typical values: furnace 80%, water heater about 60%, car about 30% DOE sources, listed at the end
Cold-day heat need and COP curve Illustrative model, set to give COP 2.4 at −15 °C DOE cold-climate heat pump challenge · 2021

Need a refresher? Lesson 1 (kW and kWh) and toolbox K2 (efficiencies as rates).

What you just did, as diagrams

Energy service
What people actually want: a warm room, hot water, cold food, light, a trip. The service is the goal; energy is the means.
Energy carrier
What brings the energy to the home: natural gas, electricity, gasoline. An energy carrier is bought; a device turns it into the service.
Bought and useful
Of 48,000 kWh the home and car buy, 21,700 kWh becomes useful heat and motion and 17,200 kWh is lost; 9,100 kWh runs appliances and is not split here.
Heat pump
It moves heat in from outdoors: in this home 5,400 kWh of electricity plus 8,100 kWh of outdoor heat give the same 13,500 kWh of warmth.
Cold days
From a 5 °C day to a −15 °C day the heat need rises 2.5 times, but the electricity 3.8 times, because the COP falls from 3.60 to 2.40.
Go deeper · AnalystPrimary energy: count the power plant too

The kWh on a meter are final energy: what the home buys. Primary energy counts the energy before it was converted. For electricity from a gas plant, more fuel energy enters the plant than electrical energy reaches the home.

Suppose the heat pump’s 5,400 kWh came from a gas combined-cycle plant at EIA’s 2025 full-load test heat rate, 7,535 Btu of gas per kWh (F4), through a grid that loses about 5% on the way (U.S. average for 2018–2022, EIA; F6):

\[ \text{sent out} = \frac{5{,}400 \text{ kWh}}{1 - 0.05} \approx 5{,}700 \text{ kWh} \]

\[ \text{gas} = 5{,}700 \text{ kWh} \times \frac{7{,}535}{3{,}412} \approx 12{,}600 \text{ kWh} \]

Heating the home for a year Gas burned
Gas furnace in the basement 16,900 kWh
Heat pump, its electricity all from that plant about 12,600 kWh

That is about 26% less gas, before any wind or solar power enters. Countries count the primary energy of wind, solar and hydro electricity in different ways: F10.

Go deeper · ModelerFrom services to demand: why cold days are hard to model

A demand model starts from the service and divides by the device’s efficiency. On a day with outdoor average \(T\) (°C), the electricity is \(E(T) = Q(T) \div \text{COP}(T)\): heat needed, in kWh, divided by the COP.

Rule In this lesson (illustrative)
Heat needed, \(Q(T)\) 5 kWh for each °C the day is below 18 °C
COP at or above the −23 °C cut-out 144 ÷ (45 − \(T\)): 3.60 at 5 °C, 2.40 at −15 °C
COP below the cut-out 1: the compressor stops and a backup heater runs

Where 144 comes from. It is a chosen number, not physics: the curve is set to give COP 2.4 at −15 °C, DOE’s target (2.4 × 60 = 144).

What it means for a model. Above the cut-out, \(E(T)\) grows faster than \(Q(T)\) as \(T\) falls, so a model that uses one average temperature underestimates the electricity of the coldest days. At the cut-out \(E\) jumps up; below it, \(E\) grows in step with \(Q\).

Analyst lesson A15 prices efficiency and electrification; Modeler lesson M12 puts heat, buildings and transport into one city model.

Go deeperWhat this lesson leaves out
  • The home mixes averages: each end use is the average of the homes that have it, so no single real home matches it.
  • Survey estimates are not meters: RECS models each home's end uses from its bills and features.
  • Useful energy is split only for heat, hot water and the car; cooling, cooking, lighting and devices are left whole.
  • Equipment efficiencies are single typical values; real ones vary with age, sizing and use.
  • The furnace's AFUE counts heat leaving the furnace; this home assumes no duct losses.
  • The gas plant's heat rate is a full-load test value; plants running below full load burn more gas per kWh.
  • The cold day uses one daily average temperature, no defrost cycles and no heat-pump capacity limit above the cut-out.
  • Primary energy here counts only the plant's gas and grid losses, not gas leaks or the energy to drill and pipe it.

Check yourself

Practice, about 10 minutes: four myths, then four questions.

Myth or fact? “What people want is energy.”

  • Myth
  • Fact
Answer

Myth. People want services: a warm room, a hot shower, a trip. Energy is the means, and different carriers and devices can deliver the same service.

Myth or fact? “A heat pump can never deliver more heat than the electricity it uses.”

  • Myth
  • Fact
Answer

Myth. It moves heat from outdoors instead of making it. At a COP of 2.4, DOE's target at −15 °C, 1 kWh of electricity delivers 2.4 kWh of heat.

Myth or fact? “A heat pump's COP is the same on every day.”

  • Myth
  • Fact
Answer

Myth. Pulling heat from colder air is harder: in this lesson's model the COP falls from 3.60 on a 5 °C day to 2.40 on a −15 °C day.

Myth or fact? “Most of the energy in a car's gasoline ends up as heat, not motion.”

  • Myth
  • Fact
Answer

Fact. About 30% moves a typical gasoline car, depending on the drive (DOE); the rest leaves as waste heat from the engine, exhaust and brakes.

Which of these is an energy carrier?

  • A warm room
  • Electricity
  • A furnace
  • A hot shower
Answer

Electricity: it brings energy to the home. A warm room and a hot shower are services; a furnace is a device.

A furnace with an 80% AFUE burns gas holding 20,000 kWh in a winter. Assume no duct losses. How much heat reaches the rooms?

  • 16,000 kWh
  • 20,000 kWh
  • 25,000 kWh
  • 4,000 kWh
Answer

16,000 kWh: 20,000 × 0.80. The other 4,000 kWh leaves, mostly up the exhaust vent.

A heat pump with a seasonal COP of 2.5 delivers 10,000 kWh of heat in a year. How much electricity does it use?

  • 25,000 kWh
  • 4,000 kWh
  • 10,000 kWh
  • 7,500 kWh
Answer

4,000 kWh: 10,000 ÷ 2.5. The other 6,000 kWh of heat is moved in from the outdoor air.

A cold snap doubles a heat-pump home's heat need, but the day stays above the heat pump's cut-out. Its electricity use…

  • doubles too
  • less than doubles
  • more than doubles
Answer

More than doubles: the heat need doubles and, in colder air, the COP falls, so each kWh of heat takes more electricity. Below the cut-out, the backup heater's COP stays at 1.

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
42% Share of U.S. household site energy used for space heating in 2020: 4,026 of 9,481 trillion Btu. EIA, RECS 2020, Table CE3.1 · 2020 data, revised March 2024
57.6 million Btu Natural gas for space heating in a West North Central home that heats with gas: 16,900 kWh. EIA, RECS 2020, Table CE4.6 · 2020 data, revised March 2024
22.8 mpg Average fuel economy of U.S. cars and light trucks in 2022, driven 10,917 miles a year: 16,900 kWh of gasoline. EPA, Greenhouse Gas Equivalencies: Calculations and References · 2022 data, page updated August 2026
about 30% Share of a typical gasoline car's fuel energy that moves it, depending on the drive. DOE, Fact of the Week #1360 · 16 September 2024
COP 2.4 DOE's target for a cold-climate heat pump at −15 °C (5 °F). Its seasonal target, HSPF2 8.5, is a heating-season test rating in Btu of heat per Wh of electricity: 8.5 ÷ 3.412 is a seasonal COP of about 2.5. DOE, Cold-Climate Heat Pump Challenge Specification, Table II-2, p. 6 · version 1.2, September 2021
3,412 Btu The energy in 1 kWh, the factor used to turn RECS's million Btu into kWh. EIA, Energy explained: British thermal units · read 3 November 2026

Next

F4 · How do power plants turn heat, water, wind and sunlight into electricity?

In the real world

  1. Surveys, not meters. EIA's RECS estimates how each surveyed home's energy splits into heating, cooling, hot water and more; most homes have no meter per service.
  2. Ratings, not every day. Heat pumps are rated at set test temperatures, such as 5 °F (−15 °C); a real winter brings days above and below them.
  3. Where it goes next. Services add up to a town's demand (lesson 2) and to the plants that meet it (F4).

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.