Solar Calculators

Rooftop PV system math — without the sales pressure. Our solar calculators handle panel count from roof area, system sizing in kW DC, daily and annual kWh production, and dollar value for any roof and location.

Solar is the fastest-growing trade in residential construction, and the math that sizes a system is well-established — yet most homeowners only ever see it through a salesperson's proprietary tool. Order a system too large and you've spent money on panels that produce more than your net-metering cap allows. Order too few and you've left free roof real estate underutilized. Inverter sizing, DC-to-AC derate, peak sun hours, and usable roof fraction all matter, and every one of them is a number an experienced solar designer carries in their head — or looks up in the NREL PVWatts database.

The calculators in this section apply the same production model the National Renewable Energy Laboratory uses in PVWatts, with the standard 0.86 derate factor and ZIP-code-specific sun hours you look up yourself. We don't sell panels, we don't take a commission, and we don't have a "schedule your free consultation" form. Run the numbers here first, then take them to a local installer for a binding quote that includes a shade analysis.

Whether you're sizing a 4 kW array on a garage, a 10 kW system on a south-facing roof, or comparing south vs west orientation on a time-of-use rate plan, the tools below give you the production and dollar numbers you need before you talk to an installer. Each calculator includes the formula, a worked example, and a FAQ covering the questions homeowners ask most often.

Residential solar in the United States is governed by NEC Article 690 (Solar PV Systems), the International Fire Code (access pathways and setbacks), and local utility interconnection rules. Below is a quick reference for the standards we use across our solar calculators, plus links to deeper resources.

Production Model Standards

• NREL PVWatts: The National Renewable Energy Laboratory's free production estimator. The industry standard for first-pass system sizing. Our calculator uses the same derate factor (0.86 default) and peak sun hours methodology.
• Peak Sun Hours (PSH): The equivalent hours per day when solar irradiance reaches 1,000 W/m². US range: 3.5 (Pacific Northwest) to 6.5+ (Southwest). Look up your ZIP at the NREL solar maps.
• DC-to-AC derate: Conversion factor from nameplate DC watts to real-world AC kWh. NREL default = 0.86. Microinverter systems = 0.88–0.90. String systems = 0.80–0.84.

Code & Safety References

• NEC Article 690: Solar PV Systems. Covers conductor sizing, overcurrent protection, grounding, rapid shutdown (690.12), and arc-fault circuit interruption. The 2023 NEC added requirements for PV systems on buildings with rapid-shutdown devices.
• IRC R324: International Residential Code section on solar PV systems. Covers roof load, wind uplift, fire classification, and required access pathways. Most local codes adopt IRC by reference.
• UL 1703 / UL 61730: Safety standard for PV modules. UL 61730 replaced UL 1703 in 2020; all panels sold in the US must carry the listing.
• UL 1741 / IEEE 1547: Inverter safety and grid interconnection standard. Smart inverters must meet UL 1741-SB for the 2023 grid-support requirements.

System Sizing Constants

• Standard panel: 400W, 65 × 39 inches, ~17.6 sq ft (as of 2026)
• DC/AC ratio: 1.15–1.25 (array kW ÷ inverter kW) — industry standard for string inverter systems
• Roof setback: 3 ft minimum from ridge, rake, hip, and valley per IRC R324; local fire codes may require 4 ft paths
• Usable fraction: 60–85% of roof plane area after setbacks, vents, and shading
• Temperature derate: Panels lose 0.3–0.5% efficiency per °C above 25°C cell temperature (STC). Hot roofs run 50–65°C, so expect 8–15% loss on dark shingles.

Conversion Factors

• 1 kW DC = 1,000 W DC (nameplate rating)
• 1 kWh = 1 kW produced for 1 hour
• 1 sq ft = 0.0929 sq m
• 1 sq m = 10.764 sq ft
• 1 inch = 2.54 cm; 1 foot = 0.3048 m
• Annual kWh = system kW × peak sun hours × 365 × efficiency