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No internet, no account required.
NEC 690.7
Validated against the standard.
PDF export
Pro-only. Generate professional reports.
$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC 690.7 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
Solar array sizing answers the first question of any PV job: how many kilowatts of panels do you need? This app sizes the array from the customer's annual energy use, the local sun hours, and a realistic system derate, so the design meets the target offset without guessing.
How it works
Array kW = annual kWh ÷ (peak sun hours × 365 × derate)
The derate (commonly around 0.77–0.80) accounts for inverter losses, wiring, soiling, and temperature. Divide the array kW by the panel wattage to get the number of modules.
Worked example
A home using 12,000 kWh/year at 4.5 sun hours: array = 12,000 ÷ (4.5 × 365 × 0.78) = 9.4 kW. With 400 W panels that is about 24 modules.
Related calculators
Match the inverter with the inverter sizing calculator and estimate output with the production estimate calculator.
Solar panel sizing FAQ
What is a peak sun hour?
A peak sun hour is one hour of 1,000 W/m² of sunlight. Your location's daily average — often 3 to 6 — is what turns array kilowatts into expected daily energy.
Why apply a derate factor?
Real systems lose energy to inverter conversion, wiring, soiling, shading, and heat. A derate around 0.77 to 0.80 turns nameplate kW into realistic production.
Does it size by bill or by roof?
You can size to a target annual kWh (to offset a bill) and the app reports the array size and module count; you then confirm the roof has the area for it.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Solar Panel Sizing on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.