When to use: Answering "how many solar panels do I need?" or "what size solar system do I need?" from your annual electric usage. This is a preliminary sizing estimate — actual design requires a site-specific solar access/shading study and must go through NEC Article 690 compliance (max system voltage, array current, and OCPD sizing).
This is a preliminary estimate only. Actual system design requires a site-specific solar access/shading study, roof structural review, and NEC Article 690 compliance verification.
This calculator estimates the solar PV system size, panel count, annual production, and roof area needed to offset a target percentage of your annual electric usage. It uses the NREL PVWatts-style derate methodology to account for real-world system losses and is intended as a preliminary sizing tool — actual system design requires a site-specific solar access/shading study and NEC Article 690 compliance verification.
Solar array sizing starts with your annual electric usage in kWh, available from twelve months of utility bills (or estimated as average monthly usage × 12). You then choose a target offset percentage — 100% means sizing the array to produce roughly as much energy as you consume annually, while a lower percentage sizes a smaller array for partial offset.
The required system size in kW DC is calculated as: System (kW) = (Annual kWh × Offset%) / (365 × Peak Sun Hours × Derate Factor). Peak sun hours is the average number of hours per day the sun delivers 1,000 W/m² of irradiance at your location — it is not the same as daylight hours, and it varies by region (roughly 3–4 hours in the cloudy Northeast/Northwest, 5–6+ hours in the sunny Southwest).
Once system size is known, panel count is simply system size (W) ÷ panel wattage, rounded up to a whole number of panels. Because you cannot install a fractional panel, the actual installed system size is usually slightly larger than the theoretical minimum — this calculator reports both the raw calculated size and the actual size after rounding up to whole panels.
A solar array's nameplate DC rating is never fully realized as delivered AC energy. NREL's PVWatts calculator models a typical overall derate factor around 77–82% (this tool defaults to 80%) that accounts for several loss mechanisms: DC-to-AC inverter conversion losses (typically 2–4%), DC wiring resistance losses (1–3%), soiling from dust and debris (2–5% depending on climate and cleaning frequency), and temperature losses from panels running hotter than their 25°C rated test condition (typically 5–10% in hot climates since panel output drops roughly 0.3–0.5% per °C above rated temperature).
Additional losses include module mismatch (small manufacturing variances between panels in a series string, ~1-2%), and system availability/downtime for maintenance (~1-2%). A conservative all-in derate of 75-80% is standard practice for preliminary sizing; a professionally engineered system with premium equipment and clean conditions may achieve a somewhat higher effective derate closer to 82-85%.
Using too optimistic a derate factor will undersize the array relative to actual usage offset needs, while too conservative a factor oversizes the system and adds unnecessary cost. When in doubt, use the PVWatts default of 80% for a realistic residential estimate.
This calculator answers "how big should my array be" — it does not perform the electrical compliance calculations required before a system can be installed and permitted. Once you have a target system size and panel count, the next design steps fall under NEC Article 690 (Solar Photovoltaic Systems).
NEC 690.7 requires correcting the panel string open-circuit voltage (Voc) for the lowest expected ambient temperature at the installation site, since PV voltage rises as temperature drops — exceeding equipment voltage ratings in cold climates is a common design error. Use the site's NEC 690.7 Maximum System Voltage calculator to verify string sizing against inverter and disconnect voltage ratings.
NEC 690.8(A)(1) requires sizing conductor ampacity and overcurrent protective devices (OCPD) from the array's maximum circuit current, with a 1.25 × 1.25 (156%) safety factor applied to the panel's rated short-circuit current. Use the site's NEC 690 PV Array Current & OCPD calculator to size wire and breakers correctly. A licensed solar installer or PE-stamped design is required for permitting in most jurisdictions.
For a typical US home using 10,800 kWh/year (900 kWh/month) with 4.5 peak sun hours/day and an 80% derate factor, you would need roughly 27 standard 400W panels to fully offset usage — about an 8.2 kW DC system. The exact count depends heavily on your local peak sun hours, actual usage, and chosen panel wattage; use the calculator above with your own utility bill data for an accurate estimate.
System size (kW DC) = (Annual kWh usage × target offset%) / (365 × peak sun hours × derate factor). For 100% offset of a 10,800 kWh/year home at 4.5 peak sun hours and 80% derate, that is roughly 8.2 kW DC. Homes in sunnier regions (5.5-6+ peak sun hours, e.g. Arizona or Nevada) need a smaller system for the same usage; homes in cloudier regions (3-3.5 peak sun hours, e.g. the Pacific Northwest) need a larger one.
Using the standard heuristic of about 20 sqft per 400W panel, a 6kW system needs 15 panels × 20 sqft = roughly 300 sqft of usable, unshaded roof area. Higher-efficiency panels (larger wattage per panel) reduce the panel count and total footprint somewhat, but the roof area per kW stays roughly similar since panel physical size scales with wattage.
NREL PVWatts uses a default overall derate factor of about 80-82%, accounting for inverter losses, DC wiring resistance, soiling, temperature de-rating, module mismatch, and system downtime. A well-maintained system with premium equipment in a mild climate may achieve 82-85%; a poorly maintained system in a hot, dusty climate may see derates closer to 72-75%. Use 80% as a reasonable default for preliminary sizing.
Peak sun hours is the average daily hours of 1,000 W/m² equivalent solar irradiance, not daylight hours. It varies from about 3-3.5 hours/day in the cloudy Pacific Northwest and Northeast, to 4.5-5 hours/day across most of the central and southern US, to 5.5-6.5+ hours/day in the desert Southwest (Arizona, New Mexico, Nevada). The default of 4.5 in this calculator is a reasonable national average, but always override it with a value specific to your location for an accurate result — NREL's PVWatts and local solar resource maps provide precise figures by ZIP code.
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