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  2. The Home Energy School
  3. Modul 01

Part 1 · The basics

What is energy?

We start from nothing. No prior knowledge, no physics. By the end of this module you will know what a kilowatt-hour is, why it is not the same thing as a kilowatt, and why your house is fundamentally a bucket with holes in it.

Rohi Perlsteyn, KronwattPublished Updated

In brief

  • Power is how fast it goes. Energy is how much it came to. That is the whole difference.
  • A kilowatt-hour is one kilowatt for one hour. Your bill measures only that.
  • Heat is energy too. In a Swedish house most of it goes to heat, not to appliances.
  • The radiator does not warm the house up. It replaces heat that has already leaked out.
On this page
  1. Energy is not the same thing as electricity
  2. Power is speed, energy is distance
  3. What is one kilowatt-hour good for?
  4. The house is a leaking bucket
  5. Where does the energy go?
  6. Check yourself

Energy is not the same thing as electricity

The first word we need to sort out is also the one most often muddled. Electricity is one kind of energy. Energy is far more than electricity.

The firewood in the shed is energy. The oil in the tank is energy. The sunlight falling on your roof is energy. And the warmth in the room you are sitting in right now is energy. They do not look alike, but they are all measured in the same unit, and they can be traded for one another.

That is why this course is about a house's energy and not just its electricity. A Swedish house uses far more heat than it uses electricity for appliances — and that heat can come from electricity, from wood, from district heating, or from the air outside. It is the same question every time: how much energy, and what does it cost?

Power is speed, energy is distance

Now comes the one thing you must keep straight for the whole course. Two words that sound alike and mean entirely different things.

Watt (W) is power. It is a speed: how fast energy is being used right now. An old light bulb was 60 W. A kettle is about 2 000 W. A phone charger is 20 W.

Watt-hour (Wh) is energy. It is an amount: how much actually happened. And a thousand watt-hours is one kilowatt-hour — 1 kWh. That is the unit on your bill, and the only unit you really pay for.

Power is km/h. Energy is the number of kilometres you actually drove.

And just as with the car, you get one from the other in the same way: speed times time.

kWh = kW × hours

kWThe speed. How fast the appliance draws energy. 2 000 W is the same as 2 kW — you are only moving the decimal point three places.
kWhThe amount. What the meter counts, and the only thing the retailer invoices per unit.

An example that shows why it matters. A 2 000 W kettle is a thirsty appliance — but you use it for three minutes. A 400 W underfloor heating loop is a modest appliance — but it runs around the clock.

Appliance Power Time Energy
Kettle 2 000 W 3 min 0,1 kWh
Bathroom underfloor heating 400 W 24 h 9,6 kWh
LED lamp 8 W 5 h 0,04 kWh
The thirsty appliance costs almost nothing. The modest one costs a hundred times more — because it never switches off.

What is one kilowatt-hour good for?

A unit you cannot feel is a hard one to be careful with. So before we move on: here is what a single kilowatt-hour actually buys. Every figure is calculated, not looked up — the arithmetic is in the box below the table.

1 kWh is enough for Roughly
Boiling water in a kettle10 times
Keeping an LED lamp lit125 hours
Driving an electric car6 kilometres
Taking a shower2 minutes
Heating an ordinary house on a cold January dayabout a quarter of an hour
 The rows about heat are the short ones. That is the entire point of this course.
How we worked out the shower

A shower head delivers roughly 12 litres a minute. The water arrives at about 10 °C and has to reach 40 °C — a lift of 30 degrees. Heating one litre of water by one degree takes 1.16 watt-hours.

12 × 30 × 1,16 = 418 Wh per minute

418 watt-hours a minute means one kilowatt-hour is gone in about 2.4 minutes. A ten-minute shower is therefore about 4.2 kWh — more than forty kettles.

Energimyndigheten gives 1.25 to 1.5 kWh for a five-minute shower, calculated at 10 litres a minute. Our arithmetic gives more, because we assumed a stronger shower head. Put your own figures in — the head and the temperature are what decide.

The house is a leaking bucket

Now comes the picture the rest of the course rests on. It is simple, and it changes how you see your house.

Heat only ever moves one way: from warm towards cold. Always, every second. You cannot stop it. You can only slow it down.

Your house is at 21 °C. Outside it is 1 °C. So heat is leaving right now — through every wall, every window, every crack. It never pauses.

And here is the turn. Your radiator is not warming the house up. The house reached 21 °C hours ago. What the radiator is doing now is replacing heat that is escaping, exactly as fast as it escapes.

heat in the level stays still heat out, all the time 21 °C 1 °C outside
A warm house is not a full bucket. It is a bucket you are constantly refilling.

Your entire heating bill is two things, and only two: how fast the house leaks, and for how long. That is why a week in January costs more than a month in May. And it is why insulation, windows and draught-proofing are the same question as heat pumps and solar panels — they simply attack different sides of the same bucket.

Where does the energy go?

Let us put numbers on the bucket. This is what an average Swedish house used during 2024, according to Energimyndigheten's official statistics. These are averages across two million houses — your own may sit a long way from them.

One year in an average Swedish house, kWh

  • Heating and hot water14 200

  • Household electricity — everything else5 654

Energimyndigheten, Energistatistik för småhus 2024 (published 10 June 2025): 14.2 MWh per house for heating and hot water, and 5 654 kWh of household electricity. The average of 90.5 kWh per square metre implies a heated area of about 157 m².

Nearly three quarters goes to heat. That is why a course about a house's energy has to be more about walls and heat pumps than about how you charge your phone. And it is why the last row of the table above was so short.

Check yourself

Five questions. Getting one wrong is the useful part — the explanation is written for exactly that answer. Nothing is timed, nothing is saved, and nobody sees what you answer.

  1. Question 1A 2 000 W kettle runs for 30 minutes. How much energy has it used?

    • Exactly. 2 000 W is 2 kW, and half an hour is 0.5 hours: 2 × 0.5 = 1 kWh. Notice the power never changed — it was the time that did the work.

    • That is what it would have used over a whole hour. Half the time gives half the energy: 2 × 0.5 = 1 kWh.

    • Here watts and kilowatts have swapped places. 2 000 W is 2 kW — same figure, different unit. And 2 kW for half an hour is 1 kWh. A thousand-fold slip is worth catching.

  2. Question 2Which of these costs the most money over a year?

    • A strong appliance, but it is almost always idle: 1.2 kW × 52 hours = 62 kWh a year. The modest underfloor loop draws fifty times that.

    • Yes — and not by a little. 0.4 kW × 8 760 hours = 3 504 kWh in a year. That is more than half the household electricity of an ordinary house, from a single loop in one bathroom.

    • The strongest of the three, and still not the priciest: 2 kW × 0.33 h × 365 = 243 kWh. Time decides, not power.

  3. Question 3You turn the thermostat down from 21 °C to 19 °C while you are away. What happens to the heat leaking out?

    • Exactly. Heat runs from warm towards cold, and the speed depends on how steep the drop is. At 1 °C outside the drop shrinks from 20 degrees to 18 — roughly a tenth less leakage, without buying anything.

    • Right direction, wrong reason — and the reason is what matters later in the course. The walls, windows and cracks are exactly as they were. The only thing that changed is the difference between inside and outside.

    • The opposite, in fact. A cooler house has a smaller drop down to the outdoor temperature, so heat runs out more slowly. It is why an unheated summer house uses no energy at all in January.

  4. Question 4What is the radiator doing in a house that has already reached 21 °C?

    • If it were, the room would get hotter and hotter. It does not — the thermometer holds still. So the radiator is replacing exactly the heat that leaks out, no more and no less.

    • Yes. And that insight is the whole course in one sentence: your heating bill is the leak, added up. Everything from here is about making the leak smaller, or replacing it more cheaply.

    • It cycles on and off, but it can never stay off for long. Heat does not stop leaking just because the room is at the right temperature — it leaks every second, all winter.

  5. Question 5In an ordinary Swedish house, what does the biggest share of the energy go to?

    • They are the most visible and the least heavy. All household electricity together — lights, TV, fridge, laundry, chargers — averages 5 654 kWh a year. Heating and hot water are two and a half times that.

    • Yes — about 72 per cent in an average Swedish house. It is why this course spends three modules on heat and walls before it even mentions solar panels.

    • The hob is strong but used briefly — a few hundred kilowatt-hours a year. It sits inside household electricity, and household electricity is the smaller of the two bars in the figure above.

Your answers live only in your browser, and vanish when you leave the page.

Sources

  1. Energimyndigheten, Energistatistik för småhus, reference year 2024, published 10 June 2025 (official statistics EN0102): 14.2 MWh per house for heating and hot water, 90.5 kWh/m², 5 654 kWh of household electricity. The figures are purchased energy and exclude the heat a heat pump lifts from air or ground.
  2. Trafikanalys, Körsträckor 2022: a passenger car covers 1 126 mil a year. At roughly 1.56 kWh per mil that is about 2 000 kWh including charging losses — the figure behind the electric-car row.
  3. The January day: an average house uses 14 200 kWh a year on heat and hot water, and a cold January day carries a little over 0.6 per cent of that — about 90 kWh. One kilowatt-hour then lasts a quarter of an hour.
  4. The shower, the kettle and the lamp are calculated on this page, from power and time. Water's heat capacity, 1.16 Wh per litre per degree, is a physical constant.