How solar panels work

A solar panel does something that still sounds a little like magic: light goes in, electricity comes out, with no moving parts and nothing burned. It is also, by most measures, a mediocre converter — a typical panel turns only about a fifth of the sunlight landing on it into electricity. The rest is reflected or becomes heat.

And yet solar has become one of the best financial decisions a homeowner can make, in a way a 20%-efficient machine rarely is. The reason is that the input is free and enormous: even a modest roof receives far more raw sunlight in a year than a home actually needs in electricity. You do not need to capture it efficiently. You just need to capture enough of it.

This guide covers both halves: the physics of how sunlight becomes usable electricity inside your walls, and a calculator to estimate how much a system your size would actually generate — including how much of it could run the heat pump from the earlier guides.

01Light, one photon at a time

Sunlight arrives as photons — small packets of light energy. A solar cell is a thin sandwich of silicon treated so that one layer has a surplus of electrons and the other a shortage. When a photon with enough energy strikes the junction between them, it knocks an electron loose. Wired correctly, that loose electron has only one way to go: out through a circuit, as current.

A solar cell burns nothing and has no moving parts. It converts light directly into a flow of electrons — the same physics in every panel, from a pocket calculator to a solar farm.

02From sunlight to a socket

One cell produces a trickle — a fraction of a volt. Panels wire many cells together for a useful voltage, and the electricity that leaves a panel is direct current, DC: it flows one way, like a battery. Almost everything in your home runs on alternating current, AC, which is why a second piece of equipment — the inverter — sits between the roof and the wall socket. Select each stage to see what changes.

03What actually determines how much you get

Panel efficiency is fixed once you buy it. What varies day to day — and what you can partly control — is how much usable sunlight actually reaches the cells:

• Orientation & angle. In the northern hemisphere, south-facing panels tilted roughly to your latitude catch the most sun over a year — but splitting panels east/west often loses surprisingly little while spreading output across more of the day.
• Shading. A single shaded cell can cut a whole panel's output far more than its small area suggests, because cells are wired in series — one weak link limits the string.
• Temperature. Counterintuitively, panels lose efficiency as they get hot — typically 0.3–0.5% for every degree above 25 °C. A cold, sunny day can outperform a scorching one.
• Weather & season. Diffuse light on a cloudy day still produces meaningful power, just far less than direct sun — and winter's low sun angle and short days are why output swings hard across the year.

04Size a system for your roof

Enter a system size and your rough sun exposure to estimate what it would generate in a year — and how much of that you would realistically use yourself versus send back to the grid.

05Self-consumption, export, and batteries

Electricity you generate and use in the same moment is worth the most — it replaces electricity you would otherwise buy at the full retail price. Electricity you export to the grid is typically paid at a much lower feed-in rate, sometimes a fraction of the retail price. That gap is the entire argument for shifting consumption toward daylight hours: running the dishwasher, charging an EV, or letting a heat pump's thermal store heat up while the sun is out.

A home battery closes some of that gap by storing midday surplus for the evening, at a real cost — batteries are expensive, and every charge–discharge cycle loses a bit of energy. For many homes, shifting a few big loads to daytime captures most of the same benefit for a fraction of the price of a battery.

06The short version

• A solar cell converts light directly to DC electricity — no moving parts, nothing burned.
• Panels are only around 20% efficient, and that's fine — sunlight is free and abundant.
• An inverter converts DC to the AC your home actually uses, and continuously tracks the panels for maximum output.
• Heat, shading, angle and weather move output more than most people expect — cold and sunny beats hot and hazy.
• Self-consumed electricity is worth far more than exported electricity — timing your usage matters as much as system size.

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