1. What Even Is Energy?

Somewhere near you, right now, a fan is spinning or a light is glowing. You pay for that at the end of every month, in something your bill calls “units”. This module is about what those units actually are — and by the end of it, you’ll be able to read an electricity bill the way an engineer does.

We start from absolute zero. No formulas you haven’t met, no assumed knowledge.

The bucket and the tap

Here is the single most common confusion in all of energy, and we’re going to kill it in the first five minutes.

Imagine a bucket being filled from a tap.

• The water collected in the bucket is energy — a total amount of something.
• The speed at which water flows from the tap is power — a rate, an amount per second.

That’s the entire distinction. Energy is the stuff; power is how fast the stuff flows. A big bucket can be filled by a trickling tap if you wait long enough. A gushing tap can fill a small bucket in seconds. Neither the bucket nor the tap tells you about the other — you need both numbers to know what’s going on.

Now swap water for energy:

• Your phone battery holds a certain amount of energy. That’s energy — the bucket.
• Your phone uses that energy at some speed — faster while gaming, slower while idle. That’s power — the tap.

When people say “this heater is powerful”, they mean its tap is wide open: it draws a lot of energy every second. When your bill says you used 250 units, it’s counting the bucket: the total energy your home collected over the month, no matter whether it came in fast or slow.

Keep the bucket and the tap in your head permanently. Half the confusion in energy news — “the plant has a capacity of 100 megawatts” versus “the plant generated 100 million units” — dissolves the moment you ask: is this a bucket number or a tap number?

The units on your bill

Science needs precise units, so here they are, built from zero — four steps up a ladder.

Step 1 — the joule. The joule (J) is the basic unit of energy. It’s small: lifting an apple from the floor to a table takes about one joule. A single drop of petrol holds more than a thousand joules. For anything household-sized, joules pile up into the millions — which is why your bill doesn’t use them.

Step 2 — the watt. The watt (W) is the unit of power: one joule flowing per second. A watt is not an amount of energy — it’s a speed. The moment you see “watt”, think tap, not bucket. Some everyday taps:

Appliance	Typical power draw
LED bulb	9 W
Ceiling fan	75 W
Your own body, at rest	~100 W
Old incandescent bulb	60 W
Fridge (while running)	150–300 W
Geyser / room heater	2,000 W (2 kW)

Yes — you, sitting still, run on about 100 watts, the same as a bright old-fashioned bulb. All of it eventually leaves your body as heat, which is why a crowded room warms up.

Step 3 — the kilowatt. One kilowatt (kW) is simply 1,000 watts. A geyser is a 2 kW machine; a small air-conditioner draws around 1.5 kW. Nothing new here, just a bigger label for bigger taps.

Step 4 — the kilowatt-hour, alias “the unit”. Now the star of your bill. Run a 1 kW appliance for 1 hour and the total energy that flowed is one kilowatt-hour (kWh). That’s the famous unit:

1 unit = 1 kWh = a 1,000 W tap left running for one hour = 3.6 million joules.

Notice what happened: we multiplied a power (kW) by a time (hours) and got an energy (kWh). That’s the bucket-filling equation, and it’s the only formula this module needs:

Energy = power × time

Try it on real life. A 75 W ceiling fan running through a 10-hour summer night: 75 W × 10 h = 750 watt-hours = 0.75 units per night, about 22 units a month. That 2 kW geyser running half an hour every morning: 2 × 0.5 = 1 unit a day. Run the numbers for your own home and the bill stops being a mystery — you can usually name the two or three appliances responsible for most of it. (What one unit costs varies from state to state and slab to slab — typically a few rupees. Why it varies so much is a Module 12 story, and a good one.)

Energy wears seven costumes

Energy shows up in different forms — same quantity, different costumes. Seven costumes cover essentially everything in this course:

1. Kinetic energy — the energy of motion. Wind, a speeding train, a spinning turbine.
2. Potential energy — stored by position. Water held high behind a dam, a stretched bow, a raised hammer.
3. Chemical energy — stored in bonds between atoms. Food, petrol, coal, wood, and every battery.
4. Thermal energy — the random jiggling of atoms; what thermometers measure and what we feel as heat.
5. Electrical energy — carried by charges moving through wires. Not found lying around in nature in useful form — we manufacture it, every second, in power plants.
6. Nuclear energy — locked inside the nucleus of the atom itself. The densest store known.
7. Radiant energy — energy travelling as light. Sunshine is radiant energy that left the Sun eight minutes ago.

Why bother naming them? Because the entire electricity industry is just a conversion business. A power plant is a factory that takes energy in one costume and re-dresses it as electrical energy, because electrical energy is the one costume that can travel hundreds of kilometres through a wire and become any other costume at the far end — motion in a fan, heat in a geyser, light in a bulb.

Nothing is created, nothing destroyed

The deepest rule in this whole subject fits in one line:

Energy is never created and never destroyed. It only changes costume.

That’s the law of conservation of energy, and no experiment in history has ever caught it failing. Every joule around you has a past — a chain of conversions going back, usually, to the Sun.

Trace one chain, backwards from your fan:

Your fan is spinning (kinetic) ← electrical energy came through the wires ← a turbine and generator converted steam’s motion into electricity (Module 2 shows the magnet trick) ← the steam was boiled by burning coal (thermal ← chemical) ← the coal is a 300-million-year-old forest, buried and pressed (chemical) ← those ancient plants captured sunlight (radiant) ← the Sun made that light from nuclear fusion in its core (nuclear).

Read it forward and it’s astonishing: the breeze from your fan is sunlight that fell on a Carboniferous forest, delivered to your room via a furnace, a magnet and two hundred kilometres of wire. Almost every energy chain on Earth starts at the Sun — coal, oil, wind, hydro, biomass are all stored or second-hand sunshine. (Nuclear and geothermal are the interesting exceptions; their story comes in Modules 6 and 9.)

One law, one warning: since energy can’t be created, there is no machine — however clever — that outputs more energy than it takes in. When an advertisement offers you a device that “generates free energy”, conservation of energy is the polite, one-line reason to keep your money.

The heat tax

If energy is never destroyed, why does anyone worry about “wasting” it?

Because conversions are never clean. Every time energy changes costume, a share of it slips away into the one costume nobody asked for: heat. Feel a phone charger, a laptop, a bike engine, a bulb that’s been on a while — warm, every one of them. That warmth is energy that set out to become something useful and didn’t make it.

The fraction that does make it is called efficiency:

• Old incandescent bulb: ~5% becomes light. It is honestly a heater that happens to glow.
• Petrol car engine: ~25–30% becomes motion; the rest leaves via the radiator and exhaust.
• Coal power plant: ~40–45% becomes electricity in a modern one (Module 3 explains the surprisingly deep reason it can’t do much better).
• Electric motor: 90%+ becomes motion — one big reason EVs are interesting (Module 11).

Think of it as a heat tax levied on every conversion, at wildly different rates. And here’s the idea that will follow us through the whole course: a chain of conversions pays the tax at every link. Coal’s chemical energy → steam → turbine → generator → wires → your fan: each arrow takes its cut. Of the energy in coal at the plant, well under a third typically arrives at your ceiling as moving air. The art of energy engineering is, to a first approximation, the art of paying less heat tax.

(“Wasting electricity” now has a precise meaning, by the way: it’s not that joules vanish — they all become stray heat eventually — it’s that you spent high-grade, travel-anywhere electrical energy on nothing, along with the fuel, money and emissions it took to make it.)

How much do we actually use?

Time to put numbers on a whole country.

An average Indian uses roughly 1,400 units of electricity per year (1,395 kWh in FY 2023-24, per the Central Electricity Authority), while the world average sits near 3,700 (2023) — and an average American burns through about nine times India’s figure. Divide it out: an average Indian’s electricity works out to just under 4 units a day — a geyser bath, a day of fans and lights, some fridge time. Much of the rich world takes several times that for granted.

That gap is not a trivia fact; it’s the central tension of India’s energy story, and it cuts both ways:

• Development pressure: every improvement in life — cooling, industry, transport, data — shows up as more units per person. India’s demand will grow for decades; the only real question is what will generate those units.
• The climate stake: if those units come from coal, emissions grow with them; every cheap clean unit changes the trajectory of a sixth of humanity (Module 13 takes this on honestly).

Hold onto the number ~4 units/day. When this course talks about gigawatt power plants, you’ll be able to translate: one gigawatt-hour is a million units — roughly a day’s electricity for about a quarter of a million average Indians.

🔥How much energy fits in a kilogram?Advanced · optional

Line up one kilogram of each major energy store and the numbers are so lopsided they explain half of world history. (MJ = megajoule = one million joules; 3.6 MJ = 1 unit.)

Store (1 kg)	Energy	In “units”
Lithium-ion battery, charged	~0.9 MJ	0.25
Wood, dry	~16 MJ	4.4
Good coal	~24 MJ	6.7
Petrol / diesel	~46 MJ	12.8
Uranium, fully fissioned	~80,000,000 MJ	22 million

Three things worth staring at:

1. Petrol beats batteries ~50:1. Even after accounting for the electric motor’s far better efficiency (90% vs ~27%), petrol still carries an order of magnitude more useful energy per kg. That’s why your petrol tank weighs 30 kg while an EV’s battery weighs 300+ — and why batteries only recently became good enough to fight anyway (Module 11).
2. Coal is mid-table, but it’s cheap and there’s a lot of it. Energy density explains why coal was worth hauling by the trainload for two centuries — and why half of Indian Railways’ freight is still coal (Module 3).
3. Uranium is not on the same chart as everything else. Three-million-to-one against coal. One fuel pellet the size of a pencil eraser ≈ one tonne of coal. This single number is why nuclear plants are refuelled every 18 months while a same-sized coal plant eats several trainloads a day — and why uranium’s waste is measured in tonnes, not mountains (Module 6).

🔥🔥The precise definitions, and where uranium's energy hidesDeep · optional

Power as a derivative. If E(t) is the energy delivered by time t, then power is its rate of change: P = dE/dt, and energy is the accumulation of power: E = ∫ P dt. Your electricity meter is a physical integrator — it multiplies instantaneous power by each tick of time and sums forever. That’s also why a “kWh” is a unit of energy even though “kW” is a rate: the hour supplies the dt. Converting: 1 kWh = 1,000 W × 3,600 s = 3.6 × 10⁶ J.

Where does uranium’s absurd energy come from? From mass itself. Weigh the fragments after a uranium-235 nucleus fissions and they come to slightly less than the original atom — about 0.09% of the mass is missing. Einstein’s E = mc² says that missing mass is the released energy: Δm·c², with c² ≈ 9 × 10¹⁶ m²/s², so a tiny Δm buys an enormous E. Per fission that’s ~200 MeV ≈ 3.2 × 10⁻¹¹ J; multiply by the ~2.6 × 10²⁴ atoms in a kilogram of U-235 and you get ~8 × 10¹³ J ≈ 80 million MJ — the number in the table above.

Chemical fuels also lose mass when they burn — but the fraction is about a billionth, far too small to ever weigh. Chemistry rearranges the outer electrons of atoms; nuclear reactions rearrange the nucleus, where the binding energies are a million times larger. Same law, different floor of the building.

What comes next

You now own the two words the rest of the course is built from: energy (the bucket) and power (the tap), plus the law that no joule is ever free and no conversion is ever clean. Module 2 takes the next step: what electricity actually is, and the one magic trick — a magnet moving past a wire — that produces essentially every unit on your bill.