Solar Fundamentals: From Sunlight to Socket

Solar panels seem magical — flat boxes on your roof that silently produce electricity for 25+ years. The actual physics is well-understood and surprisingly approachable. This guide walks through how a residential solar system works, from the photoelectric effect in the panel to the AC power coming out of your outlets.

The photoelectric effect, simplified

Solar panels are made of photovoltaic (PV) cells, typically silicon. When a photon of sunlight hits the silicon, it can knock an electron loose from its atom. The cell's structure — built from two layers of silicon with slightly different impurities (called doping) — creates an electric field that pushes those loose electrons in one direction. That flow of electrons is electric current.

A single silicon cell produces about 0.5 volts and 3–8 amps in full sun. Wire 60 or 72 cells in series inside a panel and you get a 30–40V panel producing 8–12 amps. Multiply volts × amps and you get watts — typically 400–540W for modern residential panels under standard test conditions (1,000 W/m² of sunlight at 25°C cell temperature).

Why panels produce less in real life than rated

Standard Test Conditions (STC) are laboratory conditions: 1,000 W/m² irradiance, 25°C cell temperature, specific light spectrum. Real-world conditions rarely match. Three factors reduce output:

Irradiance: A clear summer noon might deliver 1,000 W/m², but most of the day delivers less. Morning and evening sun is 200–600 W/m². Annual average irradiance in the US ranges from 3 kWh/m²/day (Pacific Northwest) to 7 kWh/m²/day (Southwest desert).

Temperature: Panels lose 0.3–0.4% efficiency per °C above 25°C. A panel in Phoenix summer sun at 50°C cell temperature produces 8–10% less than its rated output. Cold, sunny conditions (winter in Colorado) actually produce closer to rated output.

Spectrum and angle: Sunlight's spectrum varies with atmospheric conditions. Panels pointed directly at the sun produce more than panels at an angle. Fixed residential installations are tilted to optimize annual production — typically 30–35° in the continental US.

The components of a grid-tied solar system

A residential grid-tied system has five main components:

Panels (modules): convert sunlight to DC electricity. 15–30 panels for a typical home, totaling 6–10 kW. Mounted on the roof with rails and clamps, or on a ground-mount frame.

Inverter: converts DC from panels to AC for your home and the grid. Three architectures: string inverter (one large unit, lowest cost, single point of failure), microinverters (one small inverter per panel, higher cost, panel-level optimization), and power optimizers + string inverter (compromise, panel-level monitoring with central inversion).

Racking and mounting: rails, clamps, flashing, and attachments that secure panels to the roof or ground. Often overlooked but about 10% of total system cost.

Electrical balance of system: DC and AC disconnects, junction boxes, wiring, conduit, grounding, and the main panel upgrade (often needed to handle backfed solar current).

Metering: a bidirectional utility meter that records both energy consumed from the grid and energy exported to the grid. The utility installs this free when your system is approved.

How net metering works

When your panels produce more than your home consumes, the excess flows backward through your meter to the grid. With 1:1 net metering, the utility credits you at full retail rate for every kWh exported. Those credits offset future consumption — at night, you pull from the grid and use the credits. Annual true-up means you can overproduce in summer and use the credits in winter.

Net metering policy is the single biggest factor in residential solar economics. California's NEM 3.0 (April 2023) cut export credits from ~$0.30/kWh to ~$0.08/kWh, extending payback periods from 6 to 10–12 years. Many states are considering similar reforms. Always check your utility's current policy before sizing a system.

Why solar produces DC but your home uses AC

Photovoltaic cells inherently produce direct current (DC) — electrons flowing in one direction. The utility grid and your home appliances use alternating current (AC) — electrons reversing direction 50 or 60 times per second (depending on country). The inverter bridges this mismatch.

Why does the grid use AC? Historical accident. In the late 1800s "War of the Currents" between Edison (DC) and Tesla/Westinghouse (AC), AC won because it's easier to step voltage up for long-distance transmission and back down for safe home use. Transformers work on AC, not DC. We're stuck with AC distribution, which means solar panels need an inverter to connect.

Why solar is measured in kW and kWh

Power (watts or kilowatts) is instantaneous — how much electricity is flowing right now. A 7.5 kW solar system produces 7.5 kW at peak midday output.

Energy (kilowatt-hours) is power integrated over time. A 7.5 kW system running at peak for 1 hour produces 7.5 kWh. Running at half-peak (3.75 kW) for 4 hours also produces 7.5 kWh.

Your electric bill is measured in kWh because that's what's actually consumed. Your solar system is rated in kW because that's its peak capacity. The conversion between them requires knowing peak sun hours: 7.5 kW × 5 PSH × 365 days × 0.86 (losses) = 11,790 kWh/year.

Why 25 years is the standard solar horizon

Panels are warrantied for 25 years of production at 80–85% of original output. The 0.5% annual degradation rate compounds: at year 25 a panel produces (1 - 0.005)^25 = 88.2% of nameplate. Most panels exceed their warranty — 30+ year lifespans are common — but 25 is the conservative planning horizon.

Inverters don't last as long. String inverters typically need replacement at year 12–15 ($1,500–$3,000). Microinverters and optimizers often come with 25-year warranties and last the full panel life. This is part of why microinverters cost more upfront — they avoid the mid-life inverter replacement.

Putting it together

A 7.5 kW grid-tied solar system in a 5-PSH climate produces about 11,800 kWh/year — enough to fully offset a typical US household's 10,800 kWh/year consumption. With 1:1 net metering and a $0.16/kWh rate, that's $1,888/year in avoided electricity costs. At a net system cost of $17,500 (after incentives), payback is 9.3 years. The remaining 15+ years of the warranty period are essentially free electricity.

For sizing your own system, see the Solar Panel Sizing Calculator. For the full 25-year economics, see the Solar Savings Calculator. And for the science of batteries that store solar energy for nighttime use, see the Battery Buyer's Guide.