Solar PV Basics

How it works

Solar cells absorb photons from sunlight, which sets up an electric field across the cell’s layers and drives current ¹. That current is direct current (DC). Household circuits and the grid run on 60-hertz alternating current ², so every grid-tied system has an inverter that converts DC to AC synchronized with the grid.

From the inverter, power feeds the house panel. Whatever the house is using right then, it uses; any surplus flows out through the meter to the grid. At night or under clouds, the house draws from the grid as usual.

Net metering is the arrangement in which the utility compensates you for power you export ¹. How much credit you get, and whether there’s a size limit, depends on your utility’s policy; DOE recommends asking your utility about availability, system size limits, rates, and how bill credits work before you buy ¹.

In an outage

A grid-tied system without batteries goes dark when the grid does. Grid-tie inverters are required to have anti-islanding protection: when they sense the grid is down, they shut off, so the panels can’t push power onto lines utility crews expect to be dead ³. It’s the same reason generators need transfer switches.

To keep power during outages, a system needs batteries and equipment that can disconnect the house from the grid and run it as an island ^{3 1}. Storage also lets you use daytime solar after dark ¹.

Firefighter safety

Rooftop arrays keep producing DC whenever light hits them, even with the main breaker off. The NEC’s rapid shutdown requirement provides a way for first responders to quickly de-energize the conductors on the roof. Expect it on any new rooftop system.

Before you buy

DOE’s checklist ¹:

• Roof age. Panels last decades; if the roof will need replacing soon, do that first, because the panels will have to come off for it.
• Sun. Shade from trees or chimneys cuts output sharply; a south-facing, unshaded roof or a ground mount does best.
• Utility terms. Interconnection approval and net-metering terms come from your utility.
• Estimate production with NREL’s free PVWatts calculator and compare it with installers’ figures.
• Use certified installers. DOE points to NABCEP certification.

In Wisconsin, a ground-mounted array on a farm or a new structure brings in the permit rules from the Safety and Code module.

What to take from this

Panels make DC; an inverter converts it to grid-matched AC; the house uses what it can, and the surplus flows out through the meter for net-metering credit on your utility’s terms. Without batteries, a grid-tied system shuts off in an outage to protect line crews. Rapid shutdown lets firefighters de-energize roof arrays. Before buying, check roof age, shade, utility terms, and a PVWatts estimate, and use certified installers.