EXPLAINER · BEGINNER

How home solar actually works

A plain-English walk through what is actually on the roof, what it does, and how the electricity it produces gets onto the household grid. Written for Illinois homeowners who want to understand the system before comparing installer explanations. The page does not pitch anything, does not link to a quote form mid-paragraph, and does not assume the reader has decided to buy.

Panels, what they do

A residential solar panel is a sheet of silicon photovoltaic cells laminated under tempered glass with a backing sheet and an aluminum frame. When sunlight hits the cells, electrons are knocked loose and routed through a circuit, producing direct-current electricity. A modern residential module is roughly 1.7 by 1.1 meters and produces around 400 to 450 watts at peak sun, depending on efficiency and cell technology. Mainstream monocrystalline panels currently land in the 20 to 22 percent efficiency range, meaning that share of the sunlight hitting the module is converted to electricity. The rest becomes heat, which actually reduces output slightly as panels warm up.

A 7 kilowatt residential array typically uses 16 to 18 panels and covers around 350 to 400 square feet of roof. The panels themselves carry product warranties around 12 to 25 years and performance warranties guaranteeing 80 to 92 percent of nameplate output after 25 years, depending on the manufacturer. Those warranties are only as solid as the company backing them, which matters when the installer goes out of business and the homeowner needs the panel maker to honor the claim directly.

Inverters, DC to AC

Household appliances and the utility grid run on alternating current. The inverter converts the panels' direct current to AC so it can power the house or be exported back. Three architectures dominate residential installs. A string inverter sits in one location and converts the output of all panels in series, which is cost-effective but means a single shaded panel drags down the whole string. Microinverters mount on the back of each panel and convert at the module level, so shading on one panel does not affect the rest, and panel-level production data is available for every module. DC optimizers are a middle path, with optimization at each panel but central inversion downstream.

The choice matters when the roof has partial shade, when the array spans multiple roof planes facing different directions, or when the homeowner wants per-panel monitoring to detect a failed module years into operation. Inverter warranties are typically the shortest in the system, 10 to 25 years depending on technology, and inverter replacement is the most likely service event over the system's life.

Net metering, the utility connection

Once the inverter produces AC, the electricity feeds the household first. Whatever the house does not consume in real time flows back through the utility meter onto the grid. A bi-directional meter records both directions: kilowatt-hours consumed from the utility and kilowatt-hours exported back. The terms of compensation for those exports are set by the state public utility commission and the utility's approved tariff, not by the installer.

"Net metering" in plain language means: if the system exports 100 kilowatt-hours back to the grid in a month and pulls 400 kilowatt-hours from the grid in the same month, the bill is calculated on the net, 300 kilowatt-hours. The headline complication is the rate applied to the export. Under classic full-retail net metering, exported kilowatt-hours are credited at the same rate the homeowner pays for delivered electricity, so the math is close to a one-for-one offset. Under more recent rate structures, including ComEd's Smart Solar Billing for systems with permission to operate dated on or after January 1, 2025, exports are credited at the supply rate (the wholesale-like portion of the bill, currently 9.66¢/kWh through May 2026) rather than the all-in delivered rate near 16¢/kWh. The export credit is real, it is just smaller than the headline number implies, and that gap is one of the most common places sales math overstates payback.

Permits and inspections

Between contract signing and the first kilowatt-hour exported, a typical residential install moves through five steps. The installer pulls a structural permit confirming the roof can carry the array's dead load and wind load, and an electrical permit covering the inverter and interconnection wiring. The installer schedules a mechanical and electrical inspection with the local Authority Having Jurisdiction (the municipal or county building department). After the inspection passes, the utility schedules a witness test of the interconnection and bi-directional meter. Once the utility issues permission to operate, the system is allowed to export to the grid.

Six to twelve weeks from contract to permission to operate is common in northern Illinois, longer in peak season or when the AHJ is backed up. A quote that promises "two weeks to install" is usually quoting the rooftop labor day only, not the permitting and interconnection timeline that controls when the system actually starts producing value.

How much electricity to expect

Illinois sits in a mid-range insolation band, roughly 4.0 to 4.4 peak sun hours per day averaged across the year per the NREL National Solar Radiation Database (NREL was renamed National Laboratory of the Rockies by DOE in December 2025; we use "NREL" as the short form throughout for continuity with the underlying methodology). That is meaningfully sunnier than Seattle, modestly less sunny than Phoenix, and well above the threshold where residential solar can produce a useful share of household consumption. A rough rule of thumb is that one installed kilowatt of south-facing, unshaded array in northern Illinois produces around 1,200 to 1,350 kilowatt-hours per year, so a 7 kilowatt system lands near 9,000 kilowatt-hours per year, close to typical Illinois household consumption.

That figure derates for real-world conditions. East- and west-facing roof planes lose roughly 10 to 20 percent compared with due south. A 30-degree tilt deviates only slightly from the optimum across the year. Mature tree shade over even one panel of a string array can cut whole-string production sharply. The NREL PVWatts public calculator at pvwatts.nlr.gov will run a free production estimate for any address, using the same NSRDB data the research models use, and is the fastest sanity check available outside a paid quote.

Common questions

Does the system keep running during a power outage?

Not by default. A grid-tied solar system is required to shut off when the utility grid is down, a safety feature called anti-islanding that prevents the array from energizing lines that utility crews believe are dead. To keep power during an outage, the system needs a battery (with islanding capability) and an automatic transfer switch, which adds significantly to the install cost.

What about hail, wind, and snow?

Standard residential panels are tested to withstand one-inch hailstones at terminal velocity and wind loads in the 140 mph range. Severe hail events do occasionally damage panels, and the damage is typically covered by homeowner's insurance under the same policy that covers the roof. Snow on the array reduces output to near zero while it sits on the panels and slides off on its own as the panel tilt and dark color warm in sun. Total annual production figures already account for typical Illinois snow loss.

What if the roof needs replacing in five years?

The honest answer is to replace the roof first. Removing and reinstalling a solar array adds several thousand dollars to a re-roof, and the labor is not always available on short notice. A roof with less than ten years of remaining life is usually a reason to wait on solar until the roof is replaced.

Will the system power an EV or heat pump?

It can, but the array has to be sized for the higher load. The mistake to avoid is sizing a system to last year's bill when an EV or heat pump is coming in the next year or two. PVWatts and the household's projected annual consumption together should drive the size, not historical usage alone.