Sizing a solar panel system correctly means balancing your energy consumption, your roof's solar resource, the panel wattage you choose, and the efficiency losses in real-world conditions. An undersized system leaves money on the table; an oversized system has a longer payback period. This guide explains how all four variables interact and how to use the calculator results to request accurate installer quotes.
How Peak Sun Hours Determine the Number of Panels You Need
Peak sun hours are the most misunderstood variable in solar sizing. They are not the number of daylight hours in a day — they are the equivalent number of hours at full-strength solar irradiance (1,000 W/m²). A location with 5 peak sun hours per day receives the same total solar energy as 10 hours at half-intensity. The distinction matters because your panels are rated at 1,000 W/m², and their actual daily output depends on the area under the irradiance curve, which peak sun hours summarize. Phoenix averages 6.5 peak sun hours daily year-round, making it one of the best US solar markets. Seattle averages only about 3.5, meaning a Seattle homeowner needs roughly twice as many panels as a Phoenix homeowner to generate the same annual energy. Miami sits at about 5.3 and Chicago at about 4.2. Local peak sun data comes from NREL's National Solar Radiation Database (NSRDB), and most solar calculators and installers use these values as their baseline. When the calculator presents region-based peak sun hours, those figures are annual averages — your winter production will be significantly lower than summer production, which is why grid-tied systems with net metering are standard: you bank summer credits against winter deficits.
Choosing Panel Wattage: 400W vs. 450W and Why It Matters
Panel wattage determines how much roof space each panel needs relative to the energy it produces. A 400W panel and a 450W panel are roughly the same physical size — about 22 sq ft (2.0 m²) — but the 450W panel produces 12.5% more energy per square foot. If roof space is your binding constraint, higher-wattage panels reduce the panel count needed to meet your load. If roof space is ample, the economics are simpler: buy the wattage tier that gives the best cost per watt installed, which in 2025 is typically 400–430W panels at $0.90–$1.20 per watt for the module alone. Premium 500W panels exist but carry a higher per-watt cost that rarely justifies itself unless roof space is critically limited. Bifacial panels — which generate power from both sides — add 5–20% output boost when installed over light-colored surfaces (white roofs, gravel, or snow-covered ground), making them worth considering for flat or low-pitch roofs in snowy climates. All panels degrade slightly over time at 0.5–0.7% per year, so a 400W panel will produce approximately 348W after 25 years, which is still within the manufacturer's 80–87% production warranty threshold that most tier-1 manufacturers guarantee.
System Losses and Why Real Output Is Less Than Rated
A solar panel's nameplate wattage is measured under standard test conditions — 1,000 W/m² irradiance, 25°C cell temperature, and a specific air mass coefficient. Real-world conditions differ in several ways that reduce actual output. Temperature derating is one of the largest factors: silicon solar cells lose about 0.35–0.45% of output for every degree Celsius above 25°C, and on a hot summer afternoon a rooftop panel can reach 65–75°C, reducing output by 14–22% from rated. Inverter efficiency losses account for another 3–7% as DC power from the panels is converted to AC for home use. Wiring and connection losses add roughly 1–2%. Shading losses — even a small chimney shadow across one string of panels — can reduce output disproportionately in traditional string inverter systems. Dust and soiling on panels in dry climates reduces output by 2–5% if panels are not cleaned periodically. Summing these losses, a system efficiency factor of 0.80–0.85 is realistic for most well-designed residential installations, meaning you should size the system to produce about 18–25% more than your nominal daily consumption target. The calculator applies an 80% default efficiency factor, which is conservative enough to produce a reliable real-world estimate.