Narration script

Welcome to SOL 101, Energy, Light and the Solar Resource. This is unit one, and it is about the vocabulary of energy. That may sound basic, but almost every confusing statement you will ever hear about solar panels comes from mixing up two ideas: energy, and power. By the end of this lecture you will never mix them up again.

Here is what we are going after. You will be able to define energy and power, convert between joules, kilowatt-hours and electron-volts, explain irradiance, watt-peak and efficiency, and estimate the size of things using powers of ten.

Why start here? Because a solar factory runs on these numbers. The flash tester at the end of a module line reports power, in watts. Factory output is counted in gigawatts per year. A polysilicon plant is judged by kilowatt-hours per kilogram. If the units are wrong, every later calculation is wrong.

Let's begin with energy. Energy is the capacity to do work: to push something, heat something, or drive charge through a circuit. Its unit is the joule, which is roughly the energy needed to lift an apple one metre. The key law is conservation: energy is never created or destroyed, only converted. So a solar cell does not make energy. It converts the energy of light into electrical energy, and whatever it does not convert ends up as heat or reflected light.

You will use three energy units. The joule, for physics. The kilowatt-hour, which is three point six million joules, for electricity bills and factory energy use. And the electron-volt, a tiny unit, about one point six times ten to the minus nineteen joules, which is perfect for a single photon or a single electron. Silicon's band gap, the most important number in this curriculum, is one point one two electron-volts.

Now power. Power is energy per unit time: joules per second, which we call watts. A sixty watt lamp uses sixty joules every second. For electricity, power is voltage times current. Voltage is the push per unit of charge, current is the flow of charge, and when you multiply them you get energy per second. A modern solar cell at its best operating point delivers about zero point six volts at around thirteen amps, so about eight watts.

Sunlight is spread over area, so we talk about irradiance: power per square metre. Bright midday sunlight on a surface facing the Sun is roughly one thousand watts per square metre, which is why that number became the standard test condition. If you add up irradiance over a whole day you get irradiation, energy per area, in kilowatt-hours per square metre.

This figure is the single most useful picture in the unit. The curve is an illustrative clear day. Its height at any moment is power per area. The shaded area under the curve is the energy for the whole day. Power is the height; energy is the area. Five kilowatt-hours per square metre per day is the same energy as five hours of full one thousand watt sunshine, and that is where the informal phrase peak sun hours comes from.

Next, watt-peak. A module's nameplate rating in watt-peak is its maximum power under standard test conditions: one thousand watts per square metre, the A M one point five global spectrum, and a cell temperature of twenty-five degrees Celsius. It is a power under defined conditions, not a promise of what you get outdoors. Efficiency is simply output power divided by the sunlight power landing on the area.

Let's do the worked example together. A module measures one point one three four by two point two seven eight metres, so its area is about two point five eight square metres. At one thousand watts per square metre, two thousand five hundred and eighty three watts of sunlight land on it. If it is rated five hundred and eighty watts, its efficiency is five eighty divided by twenty five eighty three, about twenty two and a half percent. The second example scales down to one cell: a hundred and eighty-two millimetre square cell at twenty-five percent efficiency gives about eight watts, at around fourteen amps. Those big currents are why cells are cut in half, and why interconnect ribbons matter.

Energy is power times time. The most common error in energy writing is confusing kilowatts, a rate, with kilowatt-hours, an amount. Our five hundred and eighty watt module, in a place with five peak sun hours, ideally delivers zero point five eight kilowatts times five hours, about two point nine kilowatt-hours per day, and real systems deliver somewhat less.

Physicists like to estimate before they calculate, using powers of ten. This chart spans nineteen orders of magnitude, from a tiny indicator LED on the left to all the sunlight that hits the Earth on the right, about one point seven times ten to the seventeen watts. Compare that with humanity's average energy use, roughly twenty terawatts. The planet receives around ten thousand times more solar power than we use. The resource is not the constraint. Conversion, materials and manufacturing are, and that is exactly what this curriculum is about.

A few misconceptions to drop now. A four hundred watt panel does not make four hundred watts outdoors, except under conditions close to the standard test. Kilowatts and kilowatt-hours are not interchangeable. Higher efficiency means more power per square metre, not more energy from any panel regardless of size. And solar cells do not create energy; they convert it.

What does this mean on the factory floor? Every module is flash tested and binned by its power at standard test conditions. Capacity is stated in gigawatts per year; the industry shipped roughly seven hundred gigawatts of modules in twenty twenty-five. And the energy intensity of each step drives cost and carbon footprint. Employers expect quantitative, first-principles reasoning, and unit discipline is the first habit.

To finish, try the review questions on the page before opening the answers. Compute the power of a cell from its voltage and current, convert kilowatt-hours to joules, and sort a list of quantities into power and energy. In the next unit we will ask a deeper question: if energy is always conserved, why can't we convert all of it into electricity? The answer is heat, and the second law of thermodynamics.