Estimate how many solar panels fit on your roof, the system size in kW DC, and daily and annual kWh production — with peak sun hours and real-world efficiency derate built in.
What Your Results Mean
Start with your south-facing roof plane (in the Northern Hemisphere). Measure the length and width of the roof section that will hold the array — not the entire roof, just the unshaded, south- or west-facing portion. If you have multiple roof planes, run the calculator once per plane and add the results. The usable fraction accounts for required setbacks from ridges, edges, and penetrations; 75% is a reasonable starting point for a clean rectangular roof with a few plumbing vents.
Panel wattage has climbed steadily — 300W was standard in 2018, 400W is standard now, and 450W+ panels are available from premium manufacturers. Higher wattage means fewer panels for the same system size, which matters when roof space is tight. Panel dimensions matter too: a 400W panel is typically 65 by 39 inches (about 17.6 sq ft). Larger-format panels are harder to handle on steep roofs and may not fit around obstructions.
Peak sun hours are the single most important input after roof size. The National Renewable Energy Laboratory (NREL) publishes free solar resource maps by ZIP code. The US Southwest averages 6+ hours; the Sun Belt gets 5; the Midwest and Mid-Atlantic get 4 to 4.5; the Pacific Northwest and Northeast get 3.5 to 4. Don't guess this number — look it up for your specific location, as a 4 vs 6 hour difference changes annual production by 50%.
Reading the Efficiency Number
The system efficiency input (default 86%) is the DC-to-AC derate factor. It accounts for inverter conversion losses (3–7%), wiring resistance (1–2%), temperature derate on hot roofs (5–10%), dust and soiling (2–5%), and module mismatch (1–2%). Microinverter systems typically run closer to 88–90%; string inverter systems with optimizers run 84–87%; basic string systems run 80–84%. If your roof runs hot (dark shingles, minimal airflow), drop the efficiency to 82%. If you live in a dusty climate, drop to 83%.
What the Calculator Does Not Do
This tool estimates physical fit and energy production. It does not size an inverter, calculate battery storage, model shading from trees or adjacent buildings, or price the system. For a binding quote, a solar installer will run a shade analysis and provide a formal proposal. Use this calculator to sanity-check their numbers, and to understand how much of your roof is actually usable.
The Formula
The calculator follows the standard PV production model used across the solar industry:
Panel area is calculated from the panel length and width you enter. The floor function rounds down because you cannot install a partial panel. The efficiency is applied as a decimal multiplier (86% = 0.86) to convert nameplate DC watts to real-world AC kWh.
Key Variables Explained
- Usable Roof Fraction: The percentage of roof area that can actually hold panels, after setbacks from the ridge, rake edges, hips, valleys, plumbing vents, skylights, and required fire access paths. IRC and local fire codes typically require 3-foot clear paths on at least one side. A clean rectangular hip roof runs 80–85%; a gable with dormers and vents runs 60–70%.
- Peak Sun Hours (PSH): Not the same as daylight hours. PSH counts only the hours when irradiance reaches 1,000 W/m². A location with 12 hours of daylight but intermittent clouds might have only 4 PSH. Use the NREL PVWatts tool for your exact ZIP code.
- System Efficiency (Derate): The conversion factor from nameplate DC rating to actual AC output. NREL's PVWatts uses 0.86 by default. Microinverter systems derate less (0.88–0.90); string inverter systems derate more (0.80–0.84).
Pro Tip
South-facing arrays maximize total kWh. West-facing arrays produce less total energy but more of it in the late afternoon — often the more useful share on time-of-use plans. If evening is when your household actually uses electricity, a west array can be the better fit despite 15% fewer annual kWh.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- Panel count fits usable area only — obstructions, vents and setbacks reduce it
- System derate factor covers inverter, wiring and temperature losses
- Production estimate uses your local sun-hours at the default tilt
- Value figures are estimates — tariffs and net metering rules vary by utility
Worked Example: 2,000 sq ft South-Facing Roof in Phoenix
A homeowner in Phoenix, Arizona has a 40 × 20 foot south-facing roof plane (800 sq ft) on a single-story home. The roof is clean with two plumbing vents, so the usable fraction is 80% (640 usable sq ft). They're quoting 400W panels measuring 65 × 39 inches (17.6 sq ft each). Phoenix averages 6.5 peak sun hours. They specify a string-inverter system with a conservative 84% derate.
Annual production: 78.6 × 365 = 28,696 kWh per year — a large figure for a single home.
For comparison, households in most developed countries use roughly 3,000–12,000 kWh per year depending on climate, heating and appliances — so this system produces several times a typical home's consumption. Where net metering or feed-in arrangements exist, excess production can offset nighttime and winter use; the rules vary by country and utility, so check yours.
If the same roof were in a cloudier city (3.8 peak sun hours), annual production drops to about 16,760 kWh. Location matters more than any other variable in solar production — the same panels can halve or double their output between climates.
