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  1. Home
  2. The Home Energy School
  3. Modul 02

Part 1 · The basics

What is electricity?

Three words are enough to understand most of what happens in a fuse box: volt, ampere and watt. We will take them with water as the picture. Then we look at the fuse — what it protects, why it trips, and why it never appears on the line of your bill where the kilowatt-hours are counted.

Rohi Perlsteyn, KronwattPublished Updated

In brief

  • Volts are pressure. Amperes are flow. Watts are the two multiplied together.
  • The fuse protects the cable in the wall. Not what you plug in, and not you.
  • The main fuse measures power, not energy. It trips because too much is on at once.
  • A tripped fuse costs nothing. The same appliances, one at a time, cost the same.
  • A solar panel pushes current one way. A socket wants it back and forth. That is the inverter's job.
On this page
  1. Pressure, flow and power
  2. The fuse protects the cable
  3. Why the main fuse trips
  4. Three phases, in plain words
  5. Direct current and alternating current
  6. Electricity is dangerous, and the law says so
  7. Check yourself

Pressure, flow and power

Think of electricity as water in a pipe. The picture is not perfect, but it holds all the way down into the fuse box, and it makes three words clear at once.

Volt (V) is the pressure. How hard it pushes. In a Swedish home it is 230 V between a phase and neutral — that is what sits in every ordinary wall socket, and it does not change whatever you plug in.

Ampere (A) is the flow. How much is actually running. This changes all the time: a phone charger opens a thin trickle, a kettle opens the tap wide.

Watt (W) is the two multiplied together. Pressure times flow. It is the only formula you need to carry through the whole course.

W = V × A

230 V — the pressure amperes — the flow watts = pressure × flow
The pressure holds still. The flow changes. The power is the two together — and the power is what the wheel feels.

Now we can do something useful with the formula. A fuse is marked in amperes — in flow. That tells you nothing until you multiply it by the pressure.

16 A × 230 V = 3 680 W ≈ 3 700 W

So a 16-ampere fuse lets through about 3 700 W on its phase. That is the whole ceiling. It works just as well the other way round: a 2 000 W kettle draws 2 000 ÷ 230 = 8.7 A. Nearly half the fuse, for a cup of tea.

VThe pressure. 230 V between phase and neutral in every ordinary socket. Constant.
AThe flow. What the appliance is drawing right now. This is what the fuse watches.
WThe power. Pressure times flow. The unit you already met in module 01.

The fuse protects the cable

Here is the most important sentence on this page, and it surprises almost everyone. The fuse is not protecting the appliance. It is protecting the cable inside the wall.

A cable has resistance. When current runs through it, it gets warm — exactly as a radiator gets warm from the same thing. The more current, the warmer. A thick cable takes more current before it gets too hot; a thin one takes less.

That cable is buried in a wall, often next to timber and insulation. You cannot see it, cannot touch it, and cannot hear it when it gets too hot. So something is placed in front of it that gives up first. That is the fuse. It is chosen to match the cable, not to match whatever you happen to plug in.

A fuse is not a limit on how much electricity you may buy. It is a limit on how hot a cable is allowed to get.

Why the main fuse trips

A fuse box holds many small fuses — one per circuit, a room or two each. And then there is the big one: the main fuse, which everything else sits behind. A Swedish house usually has 16, 20 or 25 A.

The main fuse watches power. Power only. It has no idea how many kilowatt-hours you have used this year, this month or today. It asks one single question, over and over: is too much flowing right now?

An example. A house with 20 A per phase, on an evening when three things happen to start on the same phase.

On the same phase Power Current at 230 V If it runs alone
Kettle, 3 minutes 2 000 W 8,7 A 0,1 kWh
Oven, one hour 1 800 W 7,8 A 1,8 kWh
Charging box on one phase, 16 A, two hours 3 680 W 16,0 A 7,4 kWh
All three at once 7 480 W 32,5 A 9,3 kWh
32.5 A on a fuse rated 20 A. It goes. But the kilowatt-hours in the last column are the same whatever you do with the clock.

8,7 + 7,8 + 16,0 = 32,5 A — the fuse is 20 A

And now the important part. Boil the tea first, then switch on the oven, charge the car after dinner. The fuse sits still all evening. The bill comes out exactly the same: 0.1 + 1.8 + 7.4 = 9.3 kWh, either way.

0,1 + 1,8 + 7,4 = 9,3 kWh

Power is how fast it goes. Energy is how much it came to. The main fuse sees only the first. The meter counts only the second. They never talk to each other.

What changing your fuse size actually costs per year

Two real 2026 tariffs, both including VAT. Vattenfall Eldistribution from 1 January 2026: 481 kr/month at 16 A, 674 kr at 20 A, 844 kr at 25 A. Ellevio from 1 June 2026: 450, 590 and 740 kr/month.

(674 − 481) × 12 = 2 316 kr/yr

The step from 16 to 20 A therefore costs about 2 300 kr a year with one company and about 1 680 kr with the other. You do not choose your grid company — the price depends on where the house stands.

Three phases, in plain words

A Swedish house is almost always fed by three phases plus a neutral. Three pipes into the house instead of one. Each phase carries 230 V against neutral, and each phase has its own main fuse.

That is why a subscription is written 3×20 A and not simply 20 A. It means 20 A on each of the three. Together that is a generous ceiling:

3 × 20 A × 230 V = 13 800 W

But — and this is the whole point — the ceiling only holds if the load is spread. The fuses are three separate fuses. The evening in the example above put 32.5 A on one single phase while the other two sat nearly empty. The house had plenty of power left. It was simply in the wrong place.

The same evening, the same house: 3×20 A, everything on one phase

L1 L2 L3 32,5 A 3 A 2 A the fuse: 20 A this is where it goes
The house is allowed 13 800 W. It drew 7 480 W and the fuse went anyway — because all of it stood in the same pipe.

This is why an electrician cares so much about which phase a thing sits on. Moving a charging box or a water heater from a heavily loaded phase to an empty one can solve a problem that otherwise looks like it needs a more expensive subscription. Large appliances — cooker, water heater, heat pump, charging box — are often wired across several phases for exactly that reason. Between two phases the voltage is higher than 230 V; the system is called 400/230 V.

Direct current and alternating current

There are two kinds of current, and the difference is simpler than the names make it sound. Out of the wall socket comes alternating current: the flow changes direction back and forth, fifty times a second. That is what the whole grid delivers, and it is what almost everything you own expects.

Out of a solar panel comes direct current: the flow goes one way only, steadily, like water in a pipe with a pump at one end. A battery is the same — it charges and discharges as direct current. That is why a solar panel cannot be plugged straight into a socket.

Between the two sits a box called an inverter. It turns direct current into alternating current, at the right voltage and the right rhythm, so the house and the grid can accept it. That is its entire job. Solar panels and batteries get a module each later in the course — here you only needed to know why the box exists.

Electricity is dangerous, and the law says so

Short and firm. Fixed electrical work in Sweden must be carried out by a registered electrical installation company. Fixed means anything that lives in the wall: sockets, switches, the fuse box, a charging box, a new circuit. The company must appear in Elsäkerhetsverket's register, and you can look it up before anybody starts.

Replacing a blown plug fuse or pushing a tripped breaker back up is yours to do. Opening a wall is not. This course teaches you to read a fuse box, not to rebuild one.

Check yourself

Six 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 1How much power does a 16 A fuse let through at 230 V?

    • Here amperes and watts have swapped places. An ampere is only the flow — it says nothing until you multiply by the pressure: 16 × 230 = 3 680 W, about 3 700 W.

    • Exactly. 16 A × 230 V = 3 680 W. That is the ceiling on that phase, and it is why a 2 000 W kettle already takes nearly half the fuse.

    • The figure is right, the unit is not. Kilowatt-hours are an amount, and a fuse knows nothing about amounts. It measures a speed: 3 680 W, right now.

  2. Question 2What is the fuse in the fuse box mainly there to protect?

    • Tempting, because "protect" sounds like something valuable is being saved — and appliances often do have small fuses of their own inside. But the one in the fuse box is chosen to match the cable in the wall, which is the part that cannot complain when it gets too hot.

    • Yes. Current heats the cable, the cable lies against timber and insulation, and nobody can see it. The fuse is chosen to give up before the cable gets too hot — which is why you must not size it up on your own.

    • A reasonable guess — but the wrong device. A fuse reacts to a large current, which is to say to a fire risk. The current through a person is small and would never trip a fuse. That is the residual-current device's job, and it is a different thing in the box.

  3. Question 3The kettle, the oven and the charging box — at once, or one after another. What does the bill say?

    • It feels that way, because the meter spins faster. But faster over a shorter time gives the same amount: 0.1 + 1.8 + 7.4 = 9.3 kWh either way. The meter counts amount, not speed.

    • Yes — 9.3 kWh however you arrange them in time. The difference is that all at once puts 32.5 A on a phase fused at 20 A, and then it goes. Tripping costs nothing.

    • The thought is not silly: an oven that cools between two bakes really does cost extra. But in this question each thing runs its normal time, once. The sum is 9.3 kWh, and the main fuse never sees a single kilowatt-hour.

  4. Question 4A house with 3×20 A. How much power may it draw if the load is spread evenly?

    • That is one phase: 20 × 230 = 4 600 W. But there are three, and all three are available at once: 3 × 20 × 230 = 13 800 W. That is the entire point of three phases.

    • This looks like two phases with one held in reserve. But no phase is a reserve — all three are in use all the time, and all three count: 3 × 20 × 230 = 13 800 W.

    • Yes: 3 × 20 A × 230 V = 13 800 W. Note the word spread. The evening in the example drew only 7 480 W and still tripped — because all of it sat on one phase.

  5. Question 5The main fuse goes every time the car charges and somebody switches on the oven. What solves it without paying more every month?

    • Yes. The power was already in the house, it was simply on the wrong phase. Lowering the box's current makes charging slower but not dearer — the car needs the same kilowatt-hours regardless.

    • It works, and sometimes it is the right answer — but the question said without a higher monthly cost. The subscription fee follows the fuse: with one of the companies in the box above, (674 − 481) × 12 = 2 316 kr a year, and not one extra kilowatt-hour.

    • Two separate questions that are easily confused. The contract sets what a kilowatt-hour costs. The fuse sets how many may flow at once. No contract in the world moves a single ampere.

  6. Question 6Why does a house with solar panels need an inverter?

    • Tempting, and the inverter does handle voltage as well. But merely changing voltage is a transformer's job. The reason the box has to exist is the kind of current: the panel gives direct current, the socket wants alternating current.

    • Exactly. The panel pushes current one way only. The grid and the house's appliances expect current that changes direction fifty times a second. The inverter is the bridge, and a battery needs one for precisely the same reason.

    • Understandable, because the inverter is what shows production in the app. But the measuring is a side effect of all the current passing through it. The job is turning direct current into alternating current.

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

Sources

  1. Vattenfall Eldistribution, Säkringsabonnemang 16–63 A, prices from 1 January 2026 including VAT: 5 775 kr/yr at 16 A (481 kr/month), 8 085 kr/yr at 20 A (674 kr/month), 10 125 kr/yr at 25 A (844 kr/month).
  2. Ellevio, Elnätspriser privat – Hus, from 1 June 2026: 450, 590 and 740 kr/month at 16, 20 and 25 A. The two companies' prices differ sharply — which is why the step from 16 to 20 A is given here as roughly 1 680–2 316 kr a year, and not as one number.
  3. Energi- och klimatrådgivningen, the table Så mycket el drar dina apparater och vitvaror, produced with Energimyndigheten: oven 1.8 kWh per hour of use. This is the advisors' own consumer material, not official statistics.
  4. The kettle's 2 000 W is a typical nameplate rating, not an agency figure — neither Energimyndigheten nor Energi- och klimatrådgivningen publishes a kettle figure. The same rating was used in module 01.
  5. Every ampere, watt and kilowatt-hour on this page is calculated here, from W = V × A and from power times time. 230 V to neutral and 400 V between phases are the nominal voltages of the Swedish low-voltage system, and fifty times a second is the grid's nominal frequency — standards, not measurements.
  6. Elsäkerhetsverket is the authority responsible for electrical safety and for the register of electrical installation companies. The requirement that fixed electrical work be done by a registered company follows from Swedish electrical safety legislation.